Obstructive hypotension indicates obstruction to flow within the core circulation and may be categorized as either intrinsic or extrinsic ( Table 2). Pulmonary embolism is the most important intrinsic cause and may be difficult to diagnose. The prevalence of potentially fatal acute pulmonary embolism among hospital in-patients is about 1 per cent. At autopsy, 14.6 per cent of patients had pulmonary embolism that may have contributed to their death. The most important cause of extrinsic obstruction is pericardial tamponade. This may follow blunt or penetrating chest trauma, cardiac surgery, and a variety of other chronic disease processes of which renal failure is probably the most common chronic cause seen in the intensive care unit.

Pulmonary embolism Atrial myxoma or thrombus Acute puifïwnaiy hypertension

Pericardial tamponade Cofislricliva poncartfitis Tension pneumothorax Obstructive inlramoraoc lumors

Table 2 Obstructive causes of hypotension

Tamponade can be difficult to distinguish from right ventricular infarction as both may present with signs and symptoms of right ventricular failure (the inability to provide sufficient preload to the left ventricle to maintain cardiac output). It should be noted that hypotension occurring in endstage hypertrophic or restrictive cardiomyopathy should not be classified as primarily obstructive, as circulatory failure related to these diseases is usually related to cardiogenic mechanisms (i.e. arrhythmia).

The clinical appearance of the patient with obstructive hypotension is variable and related to the underlying disease. For example, patients with pulmonary embolism may show signs similar to those with myocardial infarction, including severe chest pain and diaphoreses, while a patient with saddle embolism may show signs of acute arterial occlusion of the lower extremities in combination with severe left ventricular failure.

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