The left ventricular pressure-volume loops recorded in a human showed that as preload (left ventricular end-diastolic pressure) decreased, both end-diastolic volume and end-systolic volume decreased. However, the decrease in end-diastolic volume was greater, resulting in a decreased stroke volume. Cardiac function curves plotted as a function of left ventricular end-diastolic pressure for a normal heart, a heart with increased contractility, and a heart with reduced contractility demonstrated that at any given preload, stroke volume was higher in hearts with increased contractility and lower in hearts with decreased contractility compared to the normal basal state.

The approximate diastolic arterial blood pressure could be read when the aortic valve opens, and the approximate systolic arterial blood pressure is read during systole. Left ventricular pressure-volume loops recorded from a human were similar to the schematized version. The slope of the end-systolic pressure-volume relation defined contractility, with a steeper slope indicating increased contractility.

In a subject with partial occlusion of the inferior vena cava, the preload of the left ventricle on successive beats was reduced. This reduction in preload resulted in successively smaller stroke volumes of the left ventricle, demonstrating the operation of the Frank-Starling law of the heart in humans. The Frank-Starling law states that changes in preload (left ventricular volume) alter myocardial fiber length and, consequently, the strength of the subsequent contraction and stroke volume produced.

The cardiac function curve illustrates the relationship between preload and stroke volume. Increasing preload leads to an increase in stroke volume. If the contractility of the heart is increased, as seen with the action of norepinephrine, the slope of the end-systolic pressure-volume relation becomes steeper, and the entire cardiac function curve shifts upward. This indicates that the ventricle is now capable of producing a larger stroke volume at a given preload. The increased stroke volume is mainly a result of a decreased end-systolic volume. Hearts with increased contractility are able to contract more forcefully.

On the other hand, if the ventricle is damaged, such as after cardiac ischemia, or if its contractility is reduced from normal, as with calcium channel blockade, the cardiac function curve is shifted downward. This reflects a reduced stroke volume at any given preload. The cardiac function curve is also known as ventricular function curves or Starling curves.

To assess the integrated functioning of the cardiovascular system, cardiac output is usually measured rather than stroke volume. Preload of the heart is considered to be the filling pressure of the right ventricle, typically measured as mean right atrial pressure or central venous pressure.

Measurement of cardiac output can be done using the Fick principle, which was devised by the German physiologist Adolph Fick in 1870. The Fick principle is based on the law of conservation of mass and provides a method for measuring cardiac output in intact animals and people. The principle relies on the fact that the quantity of oxygen delivered to the pulmonary capillaries via.
Fig. 16.43 ). The •Fig. 16.44 A, Leftventricularpressure-volumeloopsrecordedinahuman.Theleftventriclewassubjectedtodifferentpreloadsbytransientocclusionofbloodflowintheinferiorvenacava.Aspreload(leftventricularend-diastolicpressure[LVEDP])wasdecreased,bothend-diastolicvolumeandend-systolicvolumedecreased,butend-diastolicvolumedecreasedmore,whichresultedindecreasedstrokevolume.B, CardiacfunctioncurveswhenstrokevolumeisplottedasafunctionofLVEDPforanormalheartinthebasalstate(blue line), aheartwithincreasedcontractility(red line), andaheartwithreducedcontractility(green line). Atanygivenpreload,strokevolumeishigherinheartswithincreasedcontractilityandlowerinheartswithdecreasedcontractilitythanstrokevolumeinthenormalbasalstate.(A, FromSenzakiH,etal.Single-beatestimationofend-systolicpressure-volumerelationinhumans.Anewmethodwiththepotentialfornoninvasiveapplication.Circulation. 1996;94[10]:2497-2506.) approximate “diastolic” arterial blood pressure may be read when the aortic valve opens (point D), and the approximate “systolic” arterial blood pressure is read during systole (point E). Left ventricular P-V loops recorded from a human (
Fig.
16.44A ) are similar to the schematized version (see
Fig.
16.43 ). The slope of the end-systolic P-V relation (line extending from the dot in
Fig. 16.44A ) defines contractility; a steeper slope indicates increased contractility. In this subject, partial occlusion of the inferior vena cava reduced the preload of the left ventricle on successive beats (as the inflow of blood to the left ventricle was reduced, as a consequence of the reduced return of blood to the right ventricle) and the effect was successively smaller stroke volume of the left ventricle.
This illustrates, in humans, the operation of the Frank-Starling law of the heart (
Chapter 13), whereby changes in preload (left ventricular volume) change myocardial fiber length, and thus the strength of the subsequent contraction, as well as stroke volume produced. This important phenomenon is characterized by the cardiac function curve (see
Fig. 16.44B).
As preload increases, stroke volume increases (solid blue line). If contractility of the heart is increased, as in the action of norepinephrine, the slope of the end-systolic P-V relation becomes steeper, and the entire cardiac function curve shifts upward (solid red line), which reflects the fact that the ventricle is now able to produce a larger stroke volume at a given preload. The increased stroke volume at any given preload is largely produced by a decreased end-systolic volume: Hearts with increased contractility are able to “squeeze down” to a greater extent. Conversely, if the ventricle is damaged, as after cardiac ischemia, or if its contractility is otherwise reduced from normal (as after calcium channel blockade), the cardiac function curve is shifted downward (green line), which reflects a reduced stroke volume at any given preload. The cardiac function curve is also known as ventricular function curves or Starling curves. To assess the integrated functioning of the cardiovascular system ( ), cardiac output, rather than stroke volume, is usually measured; preload of the heart is considered to be the filling pressure of the right ventricle, typically measured as mean right atrial pressure (Pra), or central venous pressure.
Measurement of Cardiac Output
The Fick Principle
In 1870 the German physiologist Adolph Fick contrived the first method for measuring cardiac output in intact animals and people. The basis for this method, called the Fick principle, is simply an application of the law of conservation of mass. The principle is derived from the fact that the quantity of O2 delivered to the pulmonary capillaries via