JUSTIN WEINER, JOSE CARDENAS-GARCIA, RUBIN I. COHEN and SETH KOENIG

Overview

Shock is an emergency medical condition caused by inadequate tissue oxygenation. Treatment of shock depends on the underlying cause of the circulatory failure. Ultrasound facilitates rapid evaluation of hemodynamically unstable patients by providing valuable information about not only myocardial function but also the peripheral vasculature.¹ When performed in real time, intensivist-guided bedside ultrasonography can be used to differentiate between shock states, facilitate efficient early goal-directed therapy, and monitor the response to therapy.^2,3 Proficiency in basic critical care ultrasonography allows the intensivist to distinguish among shock secondary to obstruction (pulmonary embolism or cardiac tamponade), hypovolemia, and distributive causes (septic shock). Ultrasound is a bedside, reproducible, and noninvasive imaging modality. Furthermore, it obviates the need to transport unstable patients to the radiology department. This chapter discusses the role of ultrasound in the diagnosis and management of circulatory failure in the intensive care unit (ICU).

Ultrasound-based evaluation of circulatory failure in the intensive care unit

We start by first noting that ultrasound should not in any way substitute for a thorough history and physical examination. The presence of heart murmurs, pericardial rubs, elevated neck veins, previous history of gastrointestinal bleeding, known malignancy, or unilateral absence of air entry on examination will suggest the cause of the shock. Ultrasound can be used to confirm clinical suspicion. Specific emergency department protocols exist for patients in shock with and without a history of trauma4–7; however, these protocols are not reviewed in this chapter. Moreover, ultrasound techniques have been described in detail elsewhere in this book and are not restated here. Implicit in the discussion of the use of ultrasound in shock patients is the dynamic nature of the symptoms and signs. The state of patients in circulatory failure is constantly changing secondary to both the initial insult and its subsequent treatment. Accordingly, ultrasound can be used to aid in both diagnosis of the cause of the shock and the response to interventions.⁸

Ultrasound examination of patients usually starts by assessing the heart with a low-frequency transducer. The following views are obtained¹: parasternal long-axis (PSL), parasternal short-axis (PSS), apical four-chamber (4CV), subcostal (SC), and inferior vena cava (IVC) views (Table 39-1). The purpose is to evaluate myocardial function. The basic algorithm is shown in Figure 39-1. We assess left ventricular (LV) size and function, right ventricular (RV) size and function, gross valvular abnormalities, the pericardial space, and the size and variation of the IVC with respiration. Occasionally, the cause of the shock can be unambiguously diagnosed with ultrasonography. For example, visualization of a thrombus in one of the main pulmonary arteries necessitates rapid intervention. However, the cause of the shock is more likely to be found by ultrasound examination of several organs in a coordinated fashion, once again underlining the benefits of the holistic approach (HOLA) critical care ultrasound concept (see Chapters 1 and 57). A normal finding on ultrasound examination is very useful because it rules out components of the differential diagnosis. For example, a hypotensive patient with a known underlying malignancy and a normally functioning right ventricle on ultrasound assessment virtually rules out the presence of a massive pulmonary embolus.

The suggested algorithm begins with evaluation of the pericardial space. Here it is important to clearly differentiate pleural from pericardial fluid. On the PSL view, it is easy to differentiate a pericardial from a pleural effusion by noting whether the fluid travels anteriorly in front of the aorta. A significant pericardial effusion should prompt the intensivist to assess for RV diastolic or right atrial systolic collapse. If either is present, tamponade physiology is probably present and drainage of the pericardial fluid is recommended. Ultrasound-guided pericardiocentesis can be then performed.^1,9

Next, LV function is assessed with the PSS and PSL views. Moderate to severe LV dysfunction suggests cardiogenic shock, especially if it is a new finding. By using the PSS view at the level of the papillary muscles, regional wall motion abnormalities can be assessed. Although the PSS view is primarily used, the 4CV and PSL views can alternatively be used for the same purpose. A “normal” left ventricle (or hyperdynamic circulation) without RV dysfunction is suggestive of either a hypovolemic state or distributive shock. Reappraisal of LV function is crucial since contractility of the heart may change following changes in preload and afterload (after volume resuscitation or the use of diuretics or vasopressors).

Assessment of the right ventricle follows. RV dilatation (as seen on the PSS and 4CV views) with bowing of the interventricular septum into the left ventricle suggests obstructive shock (Figure 39-2). Furthermore, akinesia of the midportion of the RV free wall with sparing of the RV apex (McConnell sign) is also suggestive of pulmonary embolism causing obstructive shock.¹⁰