What is one way the heart compensates when cardiac output fails?

When cardiac output fails, one of the heart's compensatory mechanisms is ventricular hypertrophy. This occurs as a response to the increased workload placed on the heart due to conditions such as hypertension or valve regurgitation. To maintain adequate blood flow despite a decreased cardiac output, the myocardium (heart muscle) responds by thickening, or hypertrophy, which allows the heart to generate more forceful contractions. This adaptation helps to improve the efficiency of the heart, enabling it to pump blood more effectively.

The other options, while they may relate to cardiac function or blood flow, do not represent primary compensatory mechanisms of the heart in response to failing cardiac output. For instance, increased blood viscosity can occur due to dehydration or certain medical conditions, but it does not actively compensate for cardiac output failure. Decreased heart rate is generally a maladaptive response in this context because a slower heart rate can contribute to further decreased cardiac output. Vascular constriction can support blood pressure but isn't a direct compensatory mechanism of the heart itself in failing cardiac output scenarios, as it does not address the heart's innate capacity to pump blood. Thus, ventricular hypertrophy stands out as a direct and established compensatory response.