Shock States, MODS and ECMO

Hemodynamic Classifications and Bedside Assessment of Shock

Shock represents a critical failure of tissue perfusion requiring rapid bedside differentiation. In hypovolemic shock, loss of intravascular volume leads to decreased preload, presenting hemodynamically with a low central venous pressure (CVP), low pulmonary artery occlusion pressure (PAOP), and decreased cardiac output (CO). In response, the body mounts a compensatory increase in systemic vascular resistance (SVR) to maintain perfusion to vital beds. Distributive shock is characterized by decreased SVR; cardiac output and filling pressures vary by cause and stage, with normal or elevated cardiac output possible in early hyperdynamic sepsis but decreased cardiac output possible in neurogenic shock or later septic shock. Sepsis-induced distributive shock features profound systemic vasodilation combined with endothelial barrier breakdown, which precipitates capillary leak, widespread tissue edema, and impaired microcirculatory oxygen delivery. In contrast, neurogenic shock typically follows a high spinal cord injury, usually at or above T6; the sudden loss of sympathetic tone causes hypotension, warm and dry skin, and possible bradycardia.

Obstructive shock occurs when physical barriers impair cardiac filling or outflow, generating a low cardiac output state. In this category, right-sided filling pressure is often elevated, whereas left-sided filling pressure varies based on the specific anatomical cause. For instance, cardiac tamponade restricts diastolic expansion, producing hypotension, pulsus paradoxus, and near-equalization of diastolic filling pressures, including CVP, right ventricular end-diastolic pressure, and PAOP. Massive pulmonary embolism presents with acute right ventricular failure, an elevated CVP, elevated pulmonary artery systolic pressure, and a normal or low PAOP. Tension pneumothorax creates an acute obstructive emergency characterized by sudden hypotension, elevated CVP, and markedly reduced or absent breath sounds on the affected side; nurses must remember that tracheal deviation away from the affected side is a late and often absent sign.

Evaluating shock severity and volume status demands a shift from static numbers to dynamic trends. Serial lactate and base-deficit trends help assess the depth of shock and the response to ongoing resuscitation; however, neither measurement is entirely specific for tissue hypoperfusion, requiring continuous integration with physical assessment. Central venous oxygen saturation (ScvO2) trends reflect global oxygen delivery relative to consumption; an ScvO2 below approximately 70% may indicate inadequate oxygen delivery or increased cellular demand, though a normal or elevated value does not exclude persistent tissue hypoperfusion. When evaluating fluid responsiveness, passive leg raising functions as an endogenous challenge by transiently mobilizing approximately 300 mL of venous blood, predicting fluid responsiveness when paired with real-time measurement of stroke volume or cardiac output. Respiratory variations like stroke volume variation (SVV) and pulse pressure variation (PPV) are most reliable during controlled positive-pressure ventilation without spontaneous breathing efforts, with a regular cardiac rhythm and adequate tidal volume. Low tidal volumes and right ventricular dysfunction reduce the accuracy of these dynamic indicators.