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 Table of Contents  
Year : 2020  |  Volume : 10  |  Issue : 2  |  Page : 75-86

Ultrasound in obstetric anesthesia and critical care

1 Department of Anaesthesia and Critical Care, Government Medical College, Amritsar, Punjab, India
2 Department of Anaesthesia, Washington University, Saint Louis, MO, USA
3 Department of Anaesthesiology, Pain Medicine and Critical Care, AIIMS, New Delhi, India

Date of Submission28-Jun-2020
Date of Acceptance07-Jul-2020
Date of Web Publication20-Aug-2020

Correspondence Address:
Dr. Ranjana Khetarpal
9, Garden Colony, Amritsar, Punjab - 143 001
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/joacc.JOACC_56_20

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Ultrasound is a safe and inexpensive imaging modality, which can be used as a diagnostic and monitoring tool in anesthesiology, along with therapeutic management of emergency and critical illness scenarios. This current review considers an array of recent applications of ultrasound in obstetric anesthesia, encouraging its use as standard practice for diagnostic and therapeutic management. The rapid advancement of clinical applications of ultrasound in obstetric anesthesia needs an appraisal for its uses and limitations. This review presents ultrasound application under various scenarios such as difficult neuraxial blockade and airway management, gastric antrum assessment, difficult intravascular access, transverse abdominal plane block for pain management, lung ultrasound, and transthoracic echocardiography and assessment of intracranial hypertension and usage in emergency and critical care in obstetrics. Each application is presented with recent advances along with limitations to its use in clinical practice. Ultrasound is becoming a versatile tool nowadays for obstetric anesthesiologists for providing the best care possible.

Keywords: Airway ultrasound, gastric and lung ultrasound, intracranial pressure, neuraxial blocks, point-of-care ultrasound, transthoracic echocardiography, vascular access

How to cite this article:
Khetarpal R, Kaur P, Borle A, Trikha A. Ultrasound in obstetric anesthesia and critical care. J Obstet Anaesth Crit Care 2020;10:75-86

How to cite this URL:
Khetarpal R, Kaur P, Borle A, Trikha A. Ultrasound in obstetric anesthesia and critical care. J Obstet Anaesth Crit Care [serial online] 2020 [cited 2021 Apr 18];10:75-86. Available from: https://www.joacc.com/text.asp?2020/10/2/75/292748

  Introduction Top

Pregnancy is a state associated with various changes in the body due to the effect of hormones and the gravid uterus. The tissue edema along with weight gain leads to difficulty in identifying the epidural and subarachnoid spaces and exaggerated lordosis with the rotation of pelvis to the axis of spinal column leads to difficulty in flexion. To prevent the needle trauma to the spinal cord, correct identification of the space is desirable, as spinal cord ends at L1-2 level in adults but with variation from T12 lower half to L5 upper half.[1]

Perioperative pulmonary aspiration is an uncommon event, though remains one of the most feared and serious complications related to obstetric anesthesia. The pulmonary aspiration incidence in parturients is greater than that of the general population[2] owing to the delayed gastric emptying. Thus, anesthesiologists should have accurate information about the gastric contents for management in crisis requiring immediate action.[3] Ultrasound is a safe, easily accessible, and inexpensive imaging modality. Ultrasonographic measurement of the cross-sectional area of gastric antrum has been regarded as a validated method in women during labor and before elective cesarean delivery.[4]

Airway and tissue edema in pregnant women leads to difficult intubation which, in case of any life-threatening circumstances, poses a risk of poor airway management. Ultrasound is used to locate cricothyroid membrane in cases of difficult airway in emergency situations.[5] Traditional vascular access is another difficult task in pregnancy and with more prevalence of morbid obesity, alternative techniques for successful vascular access are needed.[6] Ultrasound-guided central vascular access is a widely used procedure in anesthesiology. Another major burden in obstetric anesthesia is the pain after cesarean delivery. Transverse abdominal plane (TAP) block with a local anesthetic infiltration is a good option, wherein various combinations of local anesthetic with or without adjuvants can be used. Anesthesiologists require accurate and quick diagnostic tools for clinical management and monitoring for both elective and emergency circumstances. Point of care ultrasound (POCUS) is relevant for obstetric anesthesia and nowadays, has expanded its role for the perioperative management. The POCUS has been defined as “diagnostic or procedural ultrasound guidance which is performed at bedside by a clinician for obtaining rapid evaluation and assessment or even treatment of the patient.”[7] Many subspecialties of anesthesiology such as critical care, obstetrics, pediatrics, and regional anesthesia can be linked with the use of POCUS.[8] POCUS expands various applications such as transthoracic echocardiography (TTE) or focused cardiac ultrasound (FoCUS), lung and airway ultrasound, brain, abdominal, and gastric evaluation, deep vein thrombosis assessment, intracranial tension measurement, ultrasound-guided regional anesthetic procedures, vascular access, surgical airway access, drainage of pericardial and pleural effusions, etc. Use of POCUS in emergency obstetric anesthesia includes assessment of difficult airways by identification of cricothyroid membrane, to avoid rapid sequence induction if empty stomach is confirmed, for shock assessment during pregnancy, or for identification of insertion site for a rapid spinal anesthesia. It is gaining popularity in critical care management in obstetrics. When coupled with proper training skills, clinical ultrasound application can lead to lower morbidity and mortality in parturients.

  Search Methodology Top

This review on ultrasound use in obstetric anesthesia was carried out using databases from PubMed and Web of Science. Terms such as “point of care ultrasound,” “obstetric anaesthesia,” and “ultrasound in obstetrics” were used for search. The research was restricted to articles in English language and done in January 2020 and studies which did not include ultrasound in their study were not taken into account.

  Neuraxial Blocks Top

Assessment of landmarks and anatomy using the traditional method of palpation is difficult in obstetric population due to anatomical and physiological changes. Incidence of a difficult neuraxial blockade is reported to be 4%.[9] Positioning the patient can be challenging due to gravid uterus which leads to difficult neuraxial block.[10] Imaging modalities like X-rays or fluoroscopy are not suitable in parturients. For the estimation of vertebral level for central neuraxial blocks in case of cesarean section or labor analgesia, Tuffier's line/Intercristal line (a virtual transverse line joining the tops of the posterior iliac crests) is taken as an anatomical landmark which passes through the L4 vertebra.[11] During pregnancy, intercristal line is positioned higher than L4 vertebral level.[12] Therefore, it is vital to correctly identify vertebral levels to avoid trauma to the spinal cord. Misidentification of vertebral level can lead to major complications including medicolegal implications, besides affecting the level of neuraxial blockade. Inaccuracy in identifying landmarks properly was pinpointed in various studies by Broadbent et al.[13] and Srinivasan et al.[14] The space was correctly identified in Broadbent et al. study in only 29% of the cases and error was made in 51% of the cases assuming that interspinous space to be lower than what it actually was. The study done by Srinivasan et al. demonstrated the inaccuracy of landmark technique for parturients undergoing elective lower segment cesarean section. In this study, 45.5% of investigators who performed spinal guided by Tuffier's line, performed the procedure at or above the level of L2-L3 intervertebral space. Another study done by Lee et al.[15] concluded that the level identified clinically was one of the space above that identified by ultrasound in >40% of the parturients. Ultrasound imaging has been shown to be more accurate than a landmark method for localization of lumbar interspinous spaces.[16]

It was demonstrated in a prospective observational cohort study[17] that serious complications occur in 1:3000 pregnant women undergoing anesthesia. The most commonly encountered are high level of neuraxial block and respiratory arrest. In a study done by Stendell et al., unrecognizable landmarks were found to be the most frequent predictor for difficult neuraxial blockade among 73,579 patients, in consideration with the operator experience. There was a higher incidence of inadvertent dural puncture, backache, paraesthesia, and neurological sequelae in association with difficult neuraxial block.[9] Furness et al. demonstrated that ultrasound predicted correct intervertebral level more accurately than palpation.[18] Based on two meta-analyses, a publication in 2016 reported that ultrasound done prior to giving the block resulted in the reduced number of attempts compared to the block given by traditional palpation method[19] and ultrasound usage had no effect on postdural puncture headache. When anatomical landmarks are not palpable, use of ultrasound reduces the total attempts needed.[20] A meta-analysis done by Shaikh et al.[21] and Perlas et al.[22] showed promising outcomes with ultrasound-guided spinal/epidural techniques and concluded that ultrasound improves the safety of neuraxial anesthetic technique as failure rate, occurrence of traumatic procedures, needle redirections, and number of attempts were all reported to be less. Watson et al. found 76% accuracy of ultrasound for identification of intervertebral spaces.[23] Ansari et al. demonstrated the reduced number of attempts and improved efficacy of neuraxial block with higher patient satisfaction with preprocedural ultrasound.[24] Another study[25] found reduced VAS scores and adverse effects with the use of preprocedural ultrasound.

Epidural analgesia is considered as the most effective method for labor pain relief,[26] but it becomes difficult in parturients due to gestational changes and preeclampsia, hypertension, thrombocytopenia associated with pregnancy act as contributory factor for epidural hematoma[27] and inaccurate needle insertion point which can lead to neurological injury.[28] Epidural space is narrow and deep in pregnant women, thus leading to reduced safety zone for puncture of ligamentum flavum due to the steep angle of interspinal space.[29] Unintended epidural vein injury as a complication in lumbar epidural catheter placement is seen more in the pregnant patients[30] and rate of complications increase with multiple attempts and multiple puncture sites. National Institute of Health and Clinical Excellence (2008) recommends the ultrasound usage for epidural procedures.[31] Ultrasound can be used for the determination of skin to epidural distance (SED) for epidural insertion point and estimation of the angle of the needle,[32] which correlates well with the actual puncture depth in obese parturients.[33] Preprocedural ultrasound predicts accurate SED and intervertebral levels[32] and can also be used to estimate changes in distance according to changes in the position of the parturients, i.e., sitting or left lateral position.

Ultrasound-guided neuraxial blockade is useful in obese pregnant women[34] as it reduces the number of epidural and spinal attempts and prevents accidental dural punctures. The number of catheter advancement attempts are also reduced with ultrasound which reduces the rate of complications. Ultrasound may be difficult to perform technically[35] and images may be unclear due to attenuation of ultrasound beam by greater skin to epidural space distance due to high content of adipose tissue, increased number of reflective surfaces, and impaired acoustic window. This can be avoided by compression of adipose tissue but at the cost of under-estimation of the depth of epidural space. Sahin et al. reported that procedure time was cut down and attempts were lesser along with higher success rate of neuraxial block in obese parturients.[20] First attempt success rate[20] in ultrasound-guided spinal anesthesia for obese elective lower segment cesarean section was 92% compared to 44%. Ultrasound [Figure 1]a, [Figure 1]b, [Figure 1]c, [Figure 1]d can be used to identify midline and vertebral levels, measure the epidural space extent, direction of epidural needle in cases of scoliosis, or previous spine surgery or spinal cord abnormalities. It helps in shortening the duration of procedures, increasing patient satisfaction, and facilitates difficult epidurals. Ultrasound images can be stored or printed for records. It facilitates the learning curve and acts as a superior teaching tool.

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Ultrasound also has disadvantages in terms of time taken to perform the procedure which can be prolonged when the patient is experiencing painful labor. It was observed in a study that ultrasound lengthened the procedure time by 75 s in 75 laboring women in comparison to neuraxial block given by using landmark technique.[24] Moreover, ultrasound assessment is preprocedural and changes in posture can alter the insertion point. A real-time neuraxial block placement might be useful but not fully. The gap between the scan and to actual performance of the procedure should be kept as less as possible to minimize the alterations in the parameters of intervertebral spaces as real-time performance of spinal/epidural is not possible. Breach et al. concluded that only additional 4.5 min were needed for the preparation and procedure.[36] But, at the same time, this also reduced the 1 min time for carrying out the actual procedure at the end and also reduced the number of failed attempts and traumatic complications in emergency sections.

In a study done by Onur Balaban et al., an epidural procedure in ultrasound group took 93 s and in control group (without ultrasound use) took 88 s.[37] It was observed that prepuncture ultrasound did not alter number of attempts, level, and time needed for catheterization among pregnant women with normal weight. A study done by Ansari et al. showed no significant difference in the total number of attempts of central neuraxial blocks which might be due to inclusion of patients with easily palpable spines.[24] Lack of training and expertise can limit the use of ultrasound as a rescue option when encountered with difficult neuraxial block. Arzola et al. observed that practitioners avoided the use of ultrasound for neuraxial blocks due to lack of technical expertise.[19]

  Transverse Abdominal Plane Block Top

Pain after delivery is a common complaint of parturient, which requires a balanced analgesic technique with reduced risks and complications. Opioids (morphine) are generally used as a part of multimodal analgesia regimen, but their use has disadvantages in the form of nausea/vomiting, itching, drowsiness along with respiratory depression.[38] Other analgesic modalities are needed to decrease their requirement. Infiltration of local anesthetic at the incision site is used for pain relief but not considered reliable for prolonged analgesia.[39] Epidural analgesia is an alternative but not preferable in emergency cesarean delivery. TAP block using local anesthetic mixtures can be performed after surgery in a supine position. It was introduced by Rafi in 2001[40] using landmark technique by identification of iliac crest and lumbar triangle of Petit. Hebbard and colleagues introduced the ultrasound-guided TAP block technique in 2007.[41] They described the approach using a linear array ultrasound probe, which is to be placed in the mid-axillary line. A 150 mm, 20G short bevel needle is to be introduced anteromedially and inserted in the plane between the internal oblique and transverse abdominis muscles. The local anesthetic solution can be injected on both sides in this plane after negative aspiration for intravascular placement. The successful injection creates an echolucent lens-shaped space between the muscles.

Mc Donnell et al.[42] conducted a study on landmark-guided TAP block procedure using two-pop technique after abdominal surgeries including cesarean delivery. However, visceral injury and intraperitoneal injection were reported as complications. Fusco et al. reviewed 11 studies based on the use of TAP block and reported safety, efficacy, and outcomes of TAP block as postcesarean analgesia.[43] Ultrasound guidance enables clear needle visualization along with the local anesthetic spread and, thus, improves the accurate placement of local anesthetic and minimizes accidental injuries.

A randomized, double-blind, placebo-controlled trial was done by Belavy et al. in a maternity hospital, in 50 women undergoing cesarean delivery.[44] Bilateral ultrasound-guided TAP block with either 40 mL of 0.5% ropivacaine or 40 mL saline (placebo) was given to the parturients after surgery. Hydrodissection was not done to separate the fascial layers. Patients were given oral acetaminophen and ibuprofen as advised after surgery. Morphine infusion was continued for 24 h after the surgery and total morphine dose was noted. The median morphine dose (18 mg) was 43% which was lower than the placebo group (31.5 mg) and as compared to 80% in Mc Donnell's trial. This result suggested that refining of the ultrasound technique by placing the needle below fascial layers using hydrodissection[45] can lead to better analgesia and concluded that ultrasound-guided TAP block has morphine sparing effect after cesarean section.

It was observed in the audit of ultrasound-guided TAP blocks that there was no sensory block in the L1 distribution[46], signifying that local anesthetic may have missed the iliohypogastric and ilioinguinal nerves which enter the muscle plane along the anterior axillary line.[47] A Pfannenstiel incision, innervated by L1 branches, by blocking these branches can provide more effective analgesia after cesarean section.[48] Mankikar et al. evaluated analgesic quality of ultrasound-guided TAP block with ropivacaine for 24 h after cesarean delivery via Pfannenstiel incision[49] in a controlled study. A posterior approach was used for USG-guided TAP block after skin closure and tramadol was used a rescue analgesia. It was concluded that TAP block delayed the time for the demand of rescue analgesia and reduced the need of opioids and also decreased VAS scores.

TAP block is a safe and efficacious technique for postoperative pain relief. Intravenous injection, local anesthetic toxicity, and lacerations to liver/spleen have been reported but rarely.[50] A rare complication which was noted in a study done by Salaria et al. was transient femoral nerve palsy.[51] It could have been due to incorrect injection between transverse abdominis muscle and fascia transversalis, where local anesthetic accumulates around femoral nerve leading to weakness of lower extremities and affects ambulation or could be due to use of high volume and concentration of local anesthetic and pressure by the use of abdominal binders. A newer and novel approach of surgical TAP block is nowadays becoming popular, which can reduce the technical difficulties faced in obese patients using ultrasound or landmark technique, also reducing the need of special equipment or skills of ultrasound techniques.[52]

  Vascular Access Top

Vascular access can be a difficult task in obstetric population and predominant causes of intravascular access failure include failure to identify vein or failure to advance the needle/catheter into lumen properly.[53] The usefulness of ultrasound-guided peripheral vascular access has been shown in two recent studies and its use is supported in guidelines from U.K.[54]

Agency for Healthcare Quality and Research USA and National Institute for Health and Care Excellence U.K recommends the use of ultrasound for central venous catheterization in pregnant females.[55],[56] Ultrasound-guided central venous catheterization has better safety margin and consumes lesser time as compared to landmark palpation method.[57] A study by Siddiqui et al. concluded that needle puncture for internal jugular vein catheterization guided by anatomical landmarks would have resulted in carotid artery puncture in 19% of pregnant women compared with 10% of nonpregnant women.[58]

Ultrasound guidance decreases the number of punctures and is particularly beneficial in parturients with coagulopathy or platelet function defects. It is also useful in problematic patient positions like prone/sitting or in parturients having deformity/kyphosis.[59] Dietrich et al. reviewed the use of ultrasound for vascular access and its associated complications of inadvertent arterial puncture and number of attempts.[60] In addition, misplaced guidewire and cannula can be identified using ultrasound guidance.

  Airway Assessment Top

Airway assessment and management using ultrasound is a challenging area of interest. Pregnancy is associated with airway edema and tissue edema due to hormonal effect. The incidence of failed intubation in pregnant women has been reported as 2.3 per 1000 general anesthetic for cesarean section and 2.6 per 1000 spinal anesthetic for obstetric procedures.[61] A strong antenatal plan is recommended for predicted difficult airway under Obstetric Anesthesia Airway guidelines.[62]

Identification of cricothyroid membrane position is of utmost importance for difficult airway management and digital palpation of hyoid bone is very difficult in obese pregnant women. Ultrasound scan was used to predict difficult airway by Hui and Tsui in nonpregnant population. Inadequate hyoid bone visualization was taken as a sign for difficult intubation and this technique was applied to pregnant women too.[5] The transverse technique identifies cricothyroid membrane as a hyperechoic marking between cricoid and thyroid cartilage. The longitudinal technique shows the tracheal rings, when the probe is first placed transversely then rotated to 90 degree, then cephalad till the cricoid and thyroid cartilage are pin-pointed with cricothyroid membrane in between.[63] You-Ten et al. studied ultrasound-guided and digital palpation of cricothyroid membrane in 56 women and found the improved location of cricothyroid membrane with the use of ultrasound and it was of particular use in obese women.[64]

Ultrasound examination of trachea during intubation allows visualization of tube as a hyperechoic area in the trachea in real time.[65] Adhikari et al. studied the measurements of soft tissue thickness of neck anteriorly at the level of hyoid bone using ultrasound for the prediction of difficult laryngoscopy.[66]

It was revealed in a study done by Kinsella et al. that one in 60 of women who could not be intubated may need surgical airway access which can be lifesaving.[61] Other applications of airway ultrasound are evaluation of pathologies of airway, size of endotracheal tubes, airway-related nerve blocks, and successful extubation prediction by prediction of airway edema and vocal cord movements.

  Gastric Ultrasound Top

During pregnancy, the gastrointestinal tract undergoes significant anatomical and physiological changes along with delayed gastric emptying which increase the propensity of aspiration of the gastric contents during general anesthesia. Ultrasound is particularly useful for assessing the gastric volume and, thus, estimates the risk of perioperative aspiration.[67] To protect against the acid aspiration, rapid sequence intubation (RSI) is advised in nonfasting pregnant women presenting for emergency procedures and reduced tone of lower esophageal sphincter in pregnancy. If stomach can be found empty by gastric ultrasound, RSI with associated risks at intubation can be avoided. Gastric ultrasound can evaluate the volume and type of stomach contents, thereby providing detection of anatomical hazards and help in reaching a proper decision. Pulmonary aspiration was first described by Mendelson in 1946[68] and since then, parturients requiring general anesthesia during labor or cesarean delivery are considered as higher risk for pulmonary aspiration of gastric contents. The hypothesis of pregnancy impairing the gastric motility and delayed gastric emptying[69] has been recently challenged and now guidelines encourage fluid intake during labor. Wong et al. reported normal gastric emptying during pregnancy by measuring the clear fluid content in both obese and nonobese pregnant women.[70],[71] In their cross-over study in 2002,[70] they assessed the emptying of clear fluids when compared with emptying after an overnight fasting in term pregnant women. There was no delay in gastric emptying in term healthy pregnant women after ingestion of 300 mL water. In another cross-over study by Wong et al., gastric emptying was studied in obese and term pregnant women using ultrasound and acetaminophen absorption and it was suggested that half time in gastric emptying in pregnant obese women did not show any difference after oral intake of 300 mL water as compared to 50 mL water.[71] Thus, it was concluded that pregnant women can take clear fluids until 2 h before surgery without any added risk of aspiration. Techniques to assess gastric content such as polyethylene glycol dilution, acetaminophen absorption, CT, and MRI are expensive and have lower clinical feasibility.[72] Real-time ultrasound assessment of gastric content is an easy and noninvasive reliable method for qualitative analysis of the gastric antrum and measurement of gastric antral cross-sectional area (ACSA). Various studies have documented good correlation between predicted gastric ACSA and predicted gastric volume, even in the obstetric population.[4] Bedside confirmation of empty stomach by ultrasound during labor would, thus, be useful in view of such uncertainties, which could help plan for airway management accordingly.

The proposed lowest cut off value of antral cross-sectional area[73] is taken as 320 mm2. Arzola C et al. conducted a study in fasting term pregnant women[74] in which ultrasound was performed 1 h before scheduled cesarean, in supine and right lateral decubitus (RLD) positions, at 45 degree head elevation. Ultrasonic assessment of antrum was done both qualitatively and quantitatively. Patients were classified into three grades after ruling out the presence of solid content. Grade 0 was considered as absence of fluid content in both supine and RLD positions with estimated gastric volume as 0.4 mL/kg, i.e., entirely empty state. Grade 1 antrum, if fluid was seen only in RLD position with gastric volume as 1 mL/kg and Grade 3, if fluid was seen in both positions with gastric volume as 2.7 mL/kg. Quantitative assessment of volume was done by measuring the antral cross-sectional area. It was concluded that 95% of fasting pregnant women with gestational age >36 weeks for cesarean section under neuraxial blockade showed gastric ACSA >9.6 cm2 in RLD position which corresponded to gastric volume of <1.5 mL/kg. It was suggested to use these values as upper limit of normal value in full-term pregnant women.

Barboni et al. in a prospective case-control study using ultrasound evaluated the rate of gastric emptying after a full meal in third-trimester pregnant women.[75] The point at which pregnant women are considered full stomach was determined. Their method was different from the study done by Arzola et al., which included pregnant women with empty stomach or who had taken clear fluids before elective cesarean delivery or during labor. In addition, no ultrasound was done beyond 6 h in this study. Bataille et al. also determined cut off value for gastric content by ultrasound antral cross-sectional measurement in laboring women before and after the ingestion of nonclear fluids.[4] This study suggested the cut-off value of 3.2 cm2 for labelling an at-risk pregnant woman. Perlas et al. also described a 3-point grading scale to estimate gastric antral volume using ultrasound guidance.[76] Grade 0 was taken as empty gastric antrum in both RLD and supine positions, Grade 1 was defined as having fluid content in RLD position indicating <100 mL volume in 77% patients, Grade 2 was labelled if fluid content was observed in both the positions indicating >100 mL volume in 75% and >250 mL volume in >50% patients, and Grade 3 was taken likely to be at high risk for aspiration.

Bouvet et al. proposed for measuring antral cross-sectional area in the semirecumbent position to differentiate between the gastric contents above/below the threshold risk, which is regarded as gastric volume >0.8 mL/kg or presence of solid food.[73] However, RLD position is more dependable for the assessment of gastric fluid volume, when a formula taking age of the patient in consideration is being used.

In critical scenarios, such as acute fetal or maternal compromise, RLD position might be difficult for the parturient. In this context, Jay et al. in 2017, in a prospective observational cohort study,[77] described a qualitative ultrasound technique in pregnant women in a supine position for calculation of ceiling value of the antral CSA. Grade 0 was considered as empty stomach and scores ranged from grade 0 to 3 with cut off value of 3.81 cm2. The success rate of antral CSA measurement reported in this study was found to be 90% in laboring women as compared to 96% in the study by Bataille et al.[4] Roukhomovsky et al., in their prospective cohort study in pregnant women, developed a mathematical model for prediction of the gastric content/volume and assessed the adequacy of qualitative grading in identifying fluid volume.[78] Gastric content volume was measured using MRI and it was concluded that antral CSA measurement correlated well with MRI-measured gastric content volume.

But, the gastric antrum examination using ultrasound [Figure 2] is challenging in pregnant women, owing to the epigastric probe location, which may be hampered by the sharp angle between the xiphoid process and abdomen. Upward migration of stomach due to the gravid uterus can also obscure the ultrasound view of the antrum via left hepatic lobe and aorta in the sagittal plane.[79] It has been observed that the gastric antral depth and the antral cross-sectional area significantly increase during pregnancy.[80] In addition, prolonged fasting can lead to hindrance in the gastric antrum identification.[81] More studies are still needed for validation of its use.
Figure 2: Gastric ultrasound showing full view of gastric antrum (A). Image reproduced with permission

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  Lung Ultrasound Top

Pulmonary edema is the major cause of acute breathlessness during pregnancy and preeclampsia or HELP syndrome can lead to pulmonary edema. Lung ultrasound is a standard modality for the diagnosis of lung disorders and pulmonary edema.[82] Endobronchial intubation can be ruled out on ultrasound when auscultation is difficult in obese pregnant women.[83] Ramsing et al. concluded a sensitivity of 93% for identification of tracheal/bronchial intubations compared to 66% for auscultation.[84] Picano and Pellikka used ultrasound to diagnose pulmonary congestion and pleural effusion.[85]

A prospective observational study was done by Ambrozic et al. in pregnant women with severe preeclampsia and healthy controls.[86] The lung ultrasound echo comet score (ECS) denoting the amount of extravascular lung water was calculated. Response to fluid was measured by observing changes in stroke volume (SV) with passive leg raising (PLR) and changes of >12% signified response to fluid. ECS was greater in women with preeclampsia than in controls before and after delivery. An increase in SV >12% was seen in 14% preeclamptic women and 33% controls before delivery and in 19% preeclamptic women and 17% controls postdelivery. It was concluded that excess lung water can be assessed by lung ultrasound in severe preeclamptic women before the clinical symptoms/signs. Zieleskiewicz et al. presented two case reports based on lung ultrasound done in 2 parturients with acute respiratory failure.[87] In case I, lung ultrasound showed vertical lines (B-Pattern) radiating into the lung tissue, consistent with severe pulmonary edema in undiagnosed preeclampsia which lead to timely management of acute dyspnea in the parturient. In case II, preeclamptic woman was assessed with TTE and lung ultrasound, which helped in the identification of interstitial edema at a preclinical stage, followed by adequate resuscitation.

Reduced blood volume can result in reduced tissue oxygen delivery and excess fluid can result in fluid overload and edema. The risk of fluid overload is much more in preeclamptic women and poor fluid management is associated with maternal deaths. Lung ultrasound examination is a technique with an easy learning curve,[88] i.e., <10 examinations. Parturient is placed in supine position and probe is positioned perpendicular to the ribs. B-lines are seen as vertical, long, hyperechoic bands that move along with the lung sliding. Identification of three or more B-lines on scanning the chest anteriorly is called B-pattern, used for rapid diagnosis of pulmonary edema. Number of B-lines are counted at parasternal, mid-clavicular and anterior and mid-axillary line from 2nd-4th intercostal spaces of left hemithorax and 2nd-5th space in the right hemithorax. It gives a semiquantitative idea for the severity of interstitial edema. A 28 rib space method is used for more detailed assessment. Lung ultrasound can distinguish between bronchial disorders (normal pattern) and pulmonary edema (B-pattern).[89] It can be performed during high-risk stages of peripartum period, i.e., before and after fluid expansion and during delivery. Ultrasound is better than chest X-ray and is of similar accuracy to CT scan[90] and lack of irradiation provides an advantage in the obstetrics.

The novel coronavirus infection (COVID-19) has emerged as a global pandemic. The typical symptoms include fever, cough, sore throat, headache, diarrhea and in severe cases, develops into acute respiratory syndrome. The incidence of COVID-19 in pregnant women is on the rise globally due to associated immunocompromised state which places them in the high-risk category for infection and its potential effects on the pregnant women and the growing fetus demand attention. In addition, the diagnosis of associated pneumonia is challenging along with a need for rapid assessment of pneumonia in pregnant suspects as chest X-ray and CT scan cannot be done due to damaging radiation exposure to mother and fetus. Thus, lung ultrasound has proven to be useful as an accurate imaging method in pregnant suspects for the detection of pneumonia and other pulmonary pathologies.[91] The findings of COVID-19 pneumonia[92] on a 12-point lung ultrasound examination (6 points on each side) are as follows:

  1. Patchy distribution of interstitial artefactual signs
  2. Small subpleural consolidations (focal or lobar) with associated areas of white lung and occasional air bronchograms
  3. Thickening of pleural line with pleural irregularity
  4. B-lines—vertical, discrete, laser-like hyperechoic reverberation artifacts, focal or confluent, arising from the pleural line, occur due to extraalveolar fluid collection and A-lines—horizontal lines in recovery phase
  5. Ground-glass appearance of lung; these patterns extend to multiple areas of lung surface leading to peripheral lung disease and mild pleural effusion and pneumothorax in rare cases.

Point-of-care bedside lung ultrasound minimizes the need of transferring the patient, further reducing the risk of infection in healthcare personnel and plays a crucial role for diagnosis as it avoids chest X-ray and CT scan and helpful for earlier localization of peripheral lung disease and consolidation.

  Transthoracic Echocardiography Top

Its advocated use in pregnant women has been reviewed by Dennis.[93] TTE helps to diagnose postpartum cardiomyopathy associated with genetic conditions or severe preeclampsia. Emergency diagnosis of maternal critical illness can be made by the use of TTE[94] along with the assessment of myocardial contractility, preload and end-diastolic volumes of ventricles. Hemorrhage and sepsis can lead to pathological changes which might overlap with normal features of pregnancy leading to delayed diagnosis and difficult distinction between the causes of tachycardia and hypotension. Use of TTE can help in differentiating causes of hypotension.[95] Dennis described a rapid obstetric screening echocardiographic scan (ROSE scan) in pregnant women for bedside diagnosis and monitoring response to treatment.[93]

Lung ultrasound and TTE when combined together can be used for the assessment of hemodynamic profile. TTE can assess the intravascular volume and fluid responsiveness[96] and contractility of heart, while lung ultrasound provides information on pulmonary edema and quantifies extravascular lung water.[97] Recent studies have shown the combined use of TTE and lung ultrasound for management of the fluid resuscitation in severe preeclampsia.[98] Echocardiography and PLR are used to identify women with preeclampsia who can respond to fluid therapy by an increase in cardiac output. In a study done by Ambrozic et al., it was observed that 14% women with severe preeclampsia were fluid responsive.[86] A study done by Vartun et al. showed 4%–15% of pregnant women responsive to fluids, based on echocardiography results.[99] Melchiorre et al. observed the parameters of diastolic dysfunction in preeclamptic women through echocardiographic findings.[100]

Lung ultrasound and TTE are combined for assessing women with severe preeclampsia, in whom excess fluid is to be avoided for avoiding pulmonary edema or women who are responding to fluids.

  Intracranial Tension Top

In obstetrics, increased ICP can be observed as a complication of preeclampsia and associated with progression to eclampsia. Optic nerve sheath diameter (ONSD) is an accurate marker for increased intracranial tension in severe pregnancy induced hypertension (PIH) patients. In obstetrics, one can encounter an at-risk patient for increased intracranial tension but for whom neuraxial anesthetic is indicated for any procedure. Subarachnoid injection and accidental dural puncture during epidural injection can lead to acute herniation of brain stem in patients with raised intracranial pressure.[101]

Dubost et al. introduced a noninvasive ocular ultrasound technique for detection and monitoring of raised intracranial pressure (ICP) in preeclampsia.[102] It was concluded that ocular ultrasound can help to assess the alteration of optic nerve sheath as seen with preeclampsia. ONSD was measured 3 mm behind the globe with a linear probe. Five of the 26 preeclamptic parturients had increased ONSD with mean measurement of >5.8 mm, i.e., threshold diameter, which showed 95% sensitivity for raised intracranial tension. Normal control cases showed a value of 4.5 mm. In several clinical trials, ONSD has been proven predictive of ICP in the settings of trauma and hemorrhage. It was postulated by Rollins and Flood that optic nerve sheath allows assessment of cerebrospinal fluid leak.[103] In another prospective controlled observational study[104] done by Singh andBhatia for the evaluation of incidence of raised ICP in severe preeclampsia patients using ocular ultrasound with ONSD measurement, 25 patients of severe preeclampsia (group II) and 24 patients of eclampsia (group III) were compared with 25 normal term women (group I). It was observed that 44% of patients in group II and 66% of patients in group III had ONSD value >5.7 mm. These 44% patients were at greater risk for deteriorating into eclampsia and need emergency management. The gold standard methods for ICP measurement are invasive but these result in infection and hemorrhage and are contraindicated in states of coagulopathy and thrombocytopenia.[105] ONSD can be used as a guide for assessing clinical severity and help in making decision of right anesthesia modality in severe PIH cases.

Kimberly and Noble used MRI for measuring ONSD for raised ICP detection.[106] MRI use is limited in emergency situations. Rajajee et al.[107] and Amiri et al.[108] used invasive techniques and identified cut-off value of range 4.8–5.9 mm for ONSD, values >5.8 mm are associated with ICP levels >20 mmHg. Ocular ultrasound is a noninvasive technique and much more easier to perform by anesthesiologists than the retinal examination done using an ophthalmoscope.

  Shock in Pregnancy Top

Maternal shock during pregnancy is usually due to uncontrolled bleeding or secondary to pathological conditions of sepsis, amniotic fluid or pulmonary embolism, myocardial infarction, or cardiomyopathy. Bedside ultrasonography can help in the diagnosis and treatment of shock in intensive care settings or in emergency circumstances.[88] Pulmonary embolism when associated with shock during pregnancy or in postpartum period shows dilatation of right ventricle along with paradoxical septal wall motion on ultrasound. Ultrasound assessment of lower limb venous system can be done as well for the location of thrombus.

  Conclusion Top

It has been observed that for the administration of neuraxial anaesthesia by anesthesiologist experienced in both landmark and ultrasound guided techniques, the use of ultrasound did not offer much benefit in the success rate of anaesthesia administration or number of attempts or complications in obstetric population with easily palpable spines. Another point of criticism in the use of ultrasound is the subjectivity of the observer in drawing conclusions. Despite the improvement in healthcare facility, most of the centers do not have dedicated ultrasound machine in obstetric units. In addition, there are lack of finances, experiences, proper curvilinear probes, difficulty in real-time visualization, and time constraints due to the urgency of cesarean section in some cases. Ultrasound has no place in emergency sections. If landmarks are easily appreciable, ultrasound is of no use in saving time or in technique. It's potential to improve the safety of the patient, satisfaction, and success rate anticipated in difficult cases is beyond any doubt in skilled hands.

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Conflicts of interest

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