Abstract
Point-of-care ultrasound (PoCUS) is an essential diagnostic tool increasingly adopted by obstetric anaesthesiologists to complement the physical examination and rapidly assess and manage pregnant patients in a variety of clinical scenarios. This review explores the use of PoCUS focusing on the anatomical and physiological changes in pregnancy that impact its technique and interpretation. It examines the evidence supporting the use of lung, cardiac, lower limbs, abdominal and optic nerve sheath PoCUS in multiple clinical conditions, as well as its use in guiding preoperative aspiration risk via gastric assessment and in facilitation airway management in the obstetric patient. The review highlights both the strengths and challenges inherent to obstetric PoCUS and aims to equip the reader with a framework to combine different PoCUS modalities in order to achieve a diagnosis based on the combination of the pregnant patient’s symptomatology and ultrasonographic findings.
Ultrasound is a safe imaging modality in pregnancy and preferable, where possible, to most alternatives due to the lack of ionizing radiation and its potential impact on the unborn fetus. It has a long history of utility by the obstetrician for diagnosis and monitoring of the fetal condition as well as in emergency scenarios ,. It is therefore unsurprising that point-of-care ultrasound (PoCUS) has become a widely adopted tool to assist in making diagnostic and management decisions by the obstetric anaesthesiologist. The defining characteristics of PoCUS that distinguish it from imaging techniques such as echocardiography, are that it can be performed by the treating physician at the bedside and is goal-oriented, answering simple diagnostic questions in a binary (yes/no) fashion. Optimal utilisation requires immediate access to a portable ultrasound machine with a low-frequency curvilinear probe (2–7 MHz) and a phased array probe (2–8 MHz) in addition to the standard high-frequency linear probe (7–15 MHz) that is used for most point-of-care procedures such as vascular access and regional anaesthesia.
In pregnancy there are anatomical and physiological changes that must be taken into account when performing PoCUS, most of which are only relevant from the third trimester onwards. As the uterus expands, the diaphragm elevates up to 4 cm at term; this can impact the performance of lung PoCUS where evaluation of pleural effusions becomes more difficult due to the presence of the bowel which is pushed upwards. The heart is displaced to the left, cephalad and rotated in its long axis; this has implications on the probe positioning when performing cardiac PoCUS. Other abdominal organs like the stomach and bowel are displaced laterally and upwards by the uterus as women approach term, which can affect probe positioning for gastric PoCUS and may require some lateral translation to obtain a cross-sectional image of the antrum.
Physiological changes to the cardiovascular system begin during the first trimester and result in an increase of cardiac output (CO), heart rate and stroke volume. The ejection fraction remains constant due to concurrent expansion of the plasma volume and left ventricular end systolic and end diastolic dimensions. The increase in CO begins at 5 weeks gestation, peaks at 28–34 weeks gestation and rises again in the second stage of labour and in the immediate postpartum period. The maternal heart rate starts increasing in the first trimester and it reaches 15–20 beats. min −1 above baseline by 32 weeks gestation. Together, this results in a hyperdynamic heart on cardiac PoCUS in normal pregnancy, requiring some visual recalibration by the physician to identify abnormalities such as left ventricular (LV) dysfunction or hypovolemia, which result in a more hypodynamic or hyperdynamic LV, respectively.
1
Lung PoCUS
To perform lung PoCUS in the pregnant patient, curvilinear, phased array or linear probes have been used ,. Curvilinear probes are useful because they provide good penetration and the largest sector width so can image many intercostal spaces at once, while phased array probes have a small footprint so are able to fit between the ribspaces providing the best view of the lungs at a single interspace . However, some protocols also suggest using of a linear probe for the assessment of the pleura ,. The chosen probe is positioned in the longitudinal plane with the notch pointing to the patient’s head, perpendicular to the ribs . Different protocols have been used to perform lung PoCUS across the various studies that repeat the imaging in four, six, eight or 12 pre-specified regions of the chest or between all 28 rib interspaces ,,. Clinically, we recommend a 3-point examination of each lung including an upper anterior point, a lower anterior point and a posterolateral point, due to its speed, simplicity and high diagnostic accuracy .
Lung PoCUS can rapidly and accurately diagnose pulmonary pathology at the bedside and allows for an immediate correlation of the clinical status with the image findings. When compared to chest x-ray, it demonstrates better diagnostic accuracy and is also dynamic so can be repeated frequently without the need for ionizing radiation ,,. Prospective studies of 32 patients with acute respiratory distress syndrome and 42 ventilated patients in intensive care identified that lung PoCUS outperformed physical examination skills and chest x-ray in the diagnosis pleural effusions and pulmonary oedema ,,,. If combined with cardiac PoCUS, it has greater diagnostic utility and can accurately diagnose the cause of acute dyspnoea ( Table 1 ), a clinical presentation with substantial relevance in the obstetric setting .
Table 1
The PoCUS findings following a presentation of acute dyspnoea in pregnancy.
| Lung PoCUS | Cardiac PoCUS | Other Findings | Likely diagnosis |
|---|---|---|---|
| Normal pattern | RV dilated | IVC distension Lower limb: thrombus visualized or lack of venous compression | Pulmonary embolism |
| RV severely impaired | |||
| McConnel’s sign: hyperkinesis of the RV apex relative to a hypokinetic free wall with RV dilatation | |||
| RV dilated | IVC distension | Amniotic fluid embolism | |
| RV severely impaired | |||
| In the acute phase a mobile threadlike mass may be seen in the right heart | |||
| Large pericardial effusion | IVC distension | Pericardial tamponade | |
| Normal pattern or bilateral interstitial syndrome | LV dilated | IVC normal | Peripartum cardiomyopathy |
| LV severely impaired (global) | |||
| LV severely impaired (regional) | IVC normal | Acute myocardial infarction | |
| History of chest pain | |||
| Bilateral interstitial syndrome | LV and RV normal | IVC distension | Iatrogenic fluid overload |
| Hypertension | Severe preeclampsia | ||
| Peripheral oedema | |||
| Pleural effusion may be present | |||
| Optic nerve sheath diameter: increased | |||
| LV and RV severely impaired | IVC distension | Congestive cardiac failure | |
| Peripheral oedema | |||
| Absence of lung sliding | LV and RV usually normal | IVC usually normal | Pneumothorax |
| Hepatization or consolidation with air/fluid bronchogram | LV and RV usually normal | IVC usually normal | Pneumonia |
PoCUS, point-of-care ultrasound; RV, right ventricle; LV left ventricle; IVC, inferior vena cava.
When performing lung PoCUS, the only visible structure in normally aeriated lungs is the pleura which appears as a shimmering hyperechoic horizontal line between rib shadows . A-lines are seen as horizontal reverberation artifacts parallel to the pleura and indicate homogenous lung tissue without interstitial pathology . B-lines are present when the air content decreases due to transudate or exudate in the interstitium and ultrasound waves can then be conducted through the lung; they are seen as vertical hyperechoic reverberation artifacts arising from the pleura and move in synchrony with the lung sliding where they extend to the end of the image without fading . Up to two B lines are a normal finding in healthy lung tissue, however, three or more in a single view are considered abnormal and indicate interstitial syndrome . Interstitial syndrome is the clinically silent accumulation of extravascular lung water that precedes alveolar oedema ,. The presence of three or more B lines in at least two bilateral lung zones is usually indicative of pulmonary oedema ,. The sensitivity and specificity of lung ultrasound in identifying pulmonary oedema in non-pregnant patient is up to 93 % and 98 %, respectively .
The physiological changes of pregnancy, the presence of preeclampsia and peripartum cardiomyopathy predispose parturients to the accumulation of interstitial lung water and development of pulmonary oedema . One study identified that 75 % of 150 healthy pregnant women in the third trimester had a lung PoCUS pattern that was similar to non-pregnant individuals with A lines present in all regions. Of the remainder, no patient fulfilled the criteria for interstitial syndrome . In contrast, an observational study of 24 healthy parturients found that 21 % developed interstitial syndrome during labour, in the early postpartum period, or both. However, all patients remained asymptomatic questioning the clinical relevance in this setting . Lung PoCUS can’t, however, differentiate between cardiogenic and non-cardiogenic pulmonary oedema so it should be combined with cardiac PoCUS to improve diagnostic utility . We would recommend that a finding of bilateral interstitial syndrome suggestive of pulmonary oedema should prompt the physician to assess the LV function , and it may be a useful finding in cases of right ventricular (RV) failure, to assess for concomitant LV failure .
Other pathological findings in lung PoCUS are the presence of pneumothorax, pleural effusions and lung hepatization seen in lobar pneumonia. In the presence of a pneumothorax, normal lung sliding seen between the visceral and the parietal pleura is absent anteriorly and lateral translation of the probe can identify the lung point, where the two pleural layers rejoin and lung sliding returns. Pleural effusions can be found at the base of the lung in an upright patient and are visualized as anechoic areas between the parietal and the visceral pleura. Hepatization of lung tissue, where the lung has a similar echogenicity to the liver, represents a lung that is highly fluid filled or consolidated . Consolidated lung may also present as subpleural hypoechoic areas with comet-tail reverberation artifacts at the far-field margin, air bronchograms, fluid bronchograms or with a vascular pattern within the consolidation .
1.2
Lung PoCUS in severe preeclampsia
Women with severe preeclampsia are 10–30 fold more likely to develop complications including pulmonary oedema, cerebrovascular accidents, coagulopathy, bleeding and multiorgan failure . Pulmonary oedema is associated with significant maternal morbidity , and the contributory factors in preeclampsia include decreased plasma oncotic pressure, endothelial dysfunction, arterial hypertension, elevated pulmonary vascular hydrostatic pressure, diastolic LV dysfunction and iatrogenic causes like fluid overload and medications like magnesium sulphate ,,,. An observational study including 38 patients with preeclampsia and 48 patients with severe preeclampsia concluded that the presence of B lines was more common on lung PoCUS in severe preeclampsia than in non-severe preeclampsia (p < 0.01), suggesting an increased propensity towards pulmonary oedema as the severity of preeclampsia worsens . Further studies of patients with severe preeclampsia identified that 25–65 % fulfilled the criteria for interstitial syndrome prior to delivery ,. Lung PoCUS may also detect findings consistent with pulmonary oedema before clinical symptoms appear ,,, therefore providing a window for the treating physician to tailor fluid management and prevent further deterioration ,. Excessive intravenous fluid administration has previously been identified as a contributory factor in maternal deaths related to preeclampsia . Once again, combining lung PoCUS with cardiac PoCUS or inferior vena cava (IVC) assessment can be used to help the physician evaluate fluid responsiveness in patients with severe preeclampsia and avoid excessive fluid administration .
2
Cardiac PoCUS
A phased array probe is dedicated towards cardiac imaging . Echocardiography is a widely used imaging modality that provides a detailed assessment of the heart anatomy and function; it requires extensive training and is practiced by a limited number of practitioners and is therefore not always readily available . Cardiac PoCUS on the contrary, provides an abbreviated assessment of ventricular abnormalities, pericardial conditions and volume status; it requires less extensive training through accredited schemes such as the Focused Ultrasound in Intensive Care (FUSIC) Heart qualification, and is performed at the bedside by physicians across various medical specialties. As it provides immediate information on major ventricular abnormalities and fluid status, it can be used to complement clinical assessment and monitor the effects of interventions; it does not, however, replace echocardiography .
Combined with lung and other PoCUS modalities, cardiac PoCUS can direct the initial steps to stabilize an acutely unwell patient, such as one presenting with shock, and together with clinical symptoms and laboratory findings, guide further treatment ( Table 2 ) . It consists of four standard views: parasternal long axis, parasternal short axis, apical four chamber and the subcostal view. The parasternal long axis view allows identification of gross LV dysfunction and enlargement as well as the structure and opening of the mitral and aortic valves. The parasternal short axis view is obtained at the midpapillary level to allow for detection of global and regional left ventricular motion, as well as right ventricular strain and reduced intravascular volume status. The apical four chamber view allows for assessment of both ventricles and atria sizes as well as right ventricular function. Finally, the subcostal view allows a four chamber view of the heart as well as the assessment of the IVC.
Table 2
The PoCUS findings following a presentation of shock in pregnancy.
| Lung PoCUS | Cardiac PoCUS | Other Findings | Type of shock | Likely diagnosis |
|---|---|---|---|---|
| Normal pattern | LV hyperdynamic Kissing papillary muscles evident in PSAX view | IVC flattening Abdominal: intraperitoneal fluid | Hypovolemic shock | Haemorrhage |
| Normal pattern | RV dilated RV severely impaired | Lower limb: thrombus visualized or lack of venous compression | Obstructive shock | Pulmonary embolism |
| Normal pattern | Large pericardial effusion | IVC distension | Tamponade | |
| Absence of lung sliding | LV and RV usually normal | IVC distension | Tension pneumothorax | |
| Bilateral interstitial syndrome | LV severely impaired | IVC normal or distended | Cardiogenic shock | Peripartum cardiomyopathy |
| Normal pattern | LV hyperdynamic Kissing papillary muscles evident in PSAX view | IVC flattening Rash or wheeze with precipitating allergen | Distributive shock | Anaphylaxis |
| Normal pattern OR consolidation | LV hyperdynamic | IVC flattening Fever, lactataemia and infectious source | Sepsis |
PoCUS, point-of-care ultrasound; RV, right ventricle; LV left ventricle; IVC, inferior vena cava.
Some adjustments need to be considered when performing cardiac PoCUS in the pregnant patient, both from the patient and the probe positioning point of view. They may have to assume a left lateral semirecumbent position during the exam, which optimizes the echocardiographic views by bringing the heart closer to the chest wall, due to the risk of aortocaval compression in the supine position . In addition, the presence of uterine enlargement pushes the heart upwards and to the left so probe positioning will need to be adjusted accordingly; while usually the probe is positioned on the third or fourth intercostal space to obtain a parasternal view, in advanced pregnancy the probe might need to be placed at a higher intercostal space . Further consideration needs to be given to the cardiovascular changes that happen in pregnancy which lead to reversible structural changes, like atrial dilatation and left ventricular hypertrophy , as well as mild mitral and tricuspid regurgitation and the presence of small pericardial effusions ,.
2.1
Cardiac PoCUS in right ventricular failure
The RV size, which is best assessed in the apical four chamber or subcostal views, should be two thirds the size of the LV . Due to the relatively low pressure in the pulmonary arterial system, the RV has a decreased ventricular wall thickness compared to the LV and is prone to dilatation when either right-sided volume or pressure rises .
Acute RV failure in pregnancy can be caused by pulmonary embolism, peripartum cardiomyopathy, congenital heart disease, pulmonary hypertension, amniotic fluid embolism and myocardial infarction . In cases of increased right ventricular pressure, a classic finding in the parasternal short axis is a D shaped left ventricle, due to interventricular septum flattening . A dilated right ventricle and a reduction in longitudinal and free-wall function can also be identified in the apical four chamber and subcostal views . Whilst measurements are not within the scope of cardiac PoCUS, the commonest methods to rapidly quantify right ventricular size and function are the right to left ventricular ratio and tricuspid annular plane systolic excursion (TAPSE). The right to left ventricular ratio is identified by dividing the size of the right ventricle with the size of left ventricle at the valve level; a ratio of RV/LV higher than one suggests severe RV dilation . However, a visual estimation of RV to LV size can also be useful and apicalisation of the RV is a consistent sign of RV dilatation. The TAPSE is a measure of longitudinal excursion of the tricuspid valve during systole and a good surrogate for RV function but is beyond the scope of this review.
A further sign strongly associated with pulmonary embolism can be seen on cardiac PoCUS in the apical four chambers view known as McConnell’s sign; this describes the observation of normal or increased motion at the apex of the RV associated with severe hypokinesis of the RV free wall . Hyperacute amniotic fluid embolism is also associated with sonographic features of RV dilation and dysfunction, and occasionally a highly mobile threadlike mass can be seen in the right heart . A dilated IVC, considered when the diameter is above 2 cm at the end of expiration in spontaneous breathing and with no variation in diameter during the respiratory cycle, further confirms the presence of acute RV failure .
2.2
Cardiac PoCUS in left ventricular failure
Parasternal and apical views are the most useful to assess the LV . LV dysfunction is associated with peripartum cardiomyopathy, amniotic fluid embolism and myocardial infarction, all of which are critical conditions encountered in pregnant patients.
The classical sonographic findings in peripartum cardiomyopathy are a dilated LV with globally impaired function either at the end of pregnancy or in the first month postpartum ,. Acute LV dysfunction may also present following a myocardial infarction (MI), of which the most common aetiology in pregnancy and the postpartum period is a spontaneous coronary artery dissection . A MI is distinguishable by its history, ECG and cardiac enzyme changes as well as regional wall motion abnormality on cardiac PoCUS . Acute RV failure, such as following an amniotic fluid embolism, may progress to LV failure in the context of ongoing shock. In a scoping review of 82 case reports of amniotic fluid embolism, 67 % had RV dysfunction while only 29 % had LV dysfunction .
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