Applied cardiovascular physiology

The cardiovascular responses to exercise, haemorrhage, a Valsalva manoeuvre, postural changes and exposure to microgravity are complex patterns made up of a number of common physiological ‘building blocks’. The main cardiovascular response to isotonic exercise is an increase in cardiac output to elevate oxygen delivery to exercising muscle. This is largely mediated via an increase in heart rate and facilitated by an increase in venous return. By contrast, in haemorrhage a reduced venous return leads to a reduction in cardiac output, a fall in arterial pulse pressure and unloading of the arterial baroreceptors. The baroreceptor reflex response is designed to maintain arterial blood pressure by increasing peripheral vascular resistance and limiting the fall in cardiac output via a positive inotropic effect and tachycardia. But, because venous return falls in haemorrhage, the tachycardia is relatively ineffective in sustaining cardiac output (in contrast to the situation in exercise). As the haemorrhage becomes severe, a second reflex, causing a profound vagally mediated bradycardia and hypotension, becomes apparent. This ‘depressor’ reflex is blocked by the response to musculoskeletal injury, hence the biphasic response to ‘simple’ haemorrhage is often not seen in trauma patients. Changing posture and performing the Valsalva manoeuvre also reduce venous return and lead to a reflex tachycardia, an increase in peripheral resistance, and maintenance of arterial blood pressure by mechanisms similar to those seen in the early response to blood loss. By recording the responses to a Valsalva manoeuvre or postural changes, aspects of the reflex control of the cardiovascular system can be assessed and quantified. Finally, movement from supine to standing and exposure to microgravity lead to a movement of fluid from the vascular space to the interstitium because of a disturbance in Starling’s forces, leading to hypovolaemia. By contrast, after haemorrhage an alteration in Starling’s forces results in the movement of fluid from the interstitium to the vasculature to aid an early restoration of intravascular volume.