As a result of the immediate upregulation in force generation in response to acute increases in afterload, the steady state relationship between stroke work and end diastolic volume remains insensitive to wide alterations in afterload.85 This is true as long as there is a sufficient preload reserve; only then does the ventricular performance (generated stroke volume or stroke
Left atrial pressure +40 0
Right atrial pressure
Fig 2.12 Guyton's diagram showing right and left ventricular function curves. At steady state, the output of both ventricles is identical and corresponds to cardiac output (CO). The outputs of the right and left ventricles are determined by the intersections of ventricular output curves with the corresponding systematic and pulmonary venous (PV) return curves.68
work) not decrease with increasing afterload, because the ventricle can maintain its stroke volume by mobilising its preload reserve and making use of the Frank-Starling mechanism. In contrast, when preload reserve is exhausted - for example, by overtransfusion or inadequate venous return - stroke volume becomes dependent on systolic pressure and linearly declines with any farther increase in afterload. This can produce an apparent descending limb of the ventricular function curve.84 The condition has been termed "afterload mismatch" and can be described as the inability of the ventricle at a given level of myocardial contractility to maintain a normal stroke volume against the prevailing systolic load.86 A change in myocardial contractility will alter the afterload sensitivity of the ventricle, and the afterload mismatch will occur at different levels of systolic pressure and stroke volume. The concept of afterload mismatch is useful in the haemodynamic management of the perioperative phase even in patients with normal ventricular function. These patients may have a normal cardiac preload reserve, but peripheral factors may generate a venous return that is inadequate to maintain the end diastolic volume needed for adequate ventricular performance.84
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