The estimation of organ O2 consumption can be achieved using the Fick principle, which requires the determination of blood flow and the O2 content of arterial and venous blood. By rearranging the algebraic equation, we find that O2 consumption equals blood flow multiplied by the difference in arteriovenous O2 concentration. For instance, if blood flow through a kidney is 700 mL/minute, the arterial O2 content is 0.20 mL of O2 per milliliter of blood, and the renal venous O2 content is 0.18 mL of O2 per milliliter of blood, the rate of O2 consumption by that kidney would be 700 (0.20 - 0.18) = 14 mL of O2 per minute.

In noninvasive clinical settings, Doppler echocardiography is commonly used to measure cardiac output. This method measures the velocity of blood in the ascending aorta, and with the help of echocardiography, the cross-sectional area of the aorta can be obtained. This allows the determination of stroke volume, which is the volume of blood ejected in a single heartbeat, by multiplying stroke volume with the heart rate. The result gives us the cardiac output in liters per minute.

The consumption of O2 by the heart depends on the amount and type of activity it performs. Under basal conditions, myocardial O2 consumption (V˜O2) is approximately 8 to 10 mL/minute/100 g of heart. During exercise, it can increase severalfold, while conditions like hypotension and hypothermia can lead to a moderate decrease. The O2 content of cardiac venous blood is typically low (≈5 mL/dL), and the myocardium cannot extract much additional O2 from coronary blood. Therefore, the increased O2 demands of the heart are mainly met through an increase in coronary blood flow. Experiments with arrested heartbeats but maintained coronary perfusion show that O2 consumption falls to 2 mL/minute/100 g or less, which is still significantly higher than the O2 consumption of resting skeletal muscle.

Cardiac work involves external and internal components. Stroke work, which is the left ventricular work per beat, is roughly equal to the product of stroke volume and the mean aortic pressure against which the blood is ejected. External cardiac work (We) can be defined as the product of each small increment in volume (dV) pumped and the associated pressure (P), integrated over the time interval of interest (t2 - t1). Kinetic work, resulting from the velocity (v) of blood flow and the density (ρ) of blood, is added to this equation. At resting levels of cardiac output, the kinetic energy component is negligible. However, during high cardiac output activities such as strenuous exercise, the kinetic energy component can account for up to 50% of the total cardiac work.

The internal work (Wi) of the heart can be expressed as the product of a proportionality constant (α), wall tension (T), and time (dt). Measuring V˜O2 and left ventricular power clinically can be challenging, but both are closely related to the systolic pressure-time index, which is the integral of left ventricular pressure and time during systole. These measurements are crucial because internal work plays a significant role in determining the myocardial O2 requirement. An alternative approach to evaluate cardiac work and its relationship with O2 consumption involves the examination of P-V loops under conditions of varying preload and afterload while maintaining constant contractility.
The Fick principle is also used to estimate the O2 consumption of organs when blood flow and the O2 content of arterial and venous blood can be determined. Algebraic rearrangement reveals that O2 consumption equals blood flow multiplied by the difference in the arteriovenous O2 concentration. For example, if blood flow through one kidney is 700 mL/minute, the arterial O2 content is 0.20 mL of O2 per milliliter of blood, and the renal venous O2 content is 0.18 mL of O2 per milliliter of blood, the rate of O2 consumption by that kidney must be 700 (0.20 − 0.18) = 14 mL of O2 per minute.
In the clinic, cardiac output is most commonly measured noninvasively with Doppler echocardiography. By this method, the velocity of blood in the ascending aorta is measured. Obtaining the cross-sectional area of the aorta (also measured by echocardiography) allows the volume of blood ejected in a single beat (i.e., stroke volume) to be determined (see
Eq. 17.1 ). Multiplying stroke volume by the heart rate then yields a value for cardiac output in liters per minute.
Consumption of O2 by the heart depends on the amount and type of activity that the heart performs. Under basal conditions, myocardial O2 consumption (V˜O2) is approximately 8 to 10 mL/minute/100 g of heart. It can increase severalfold during exercise and decrease moderately in such conditions as hypotension and hypothermia. The O2 content of cardiac venous blood is normally low (≈5 mL/ dL), and the myocardium can receive little additional O2 by further extraction of O2 from coronary blood. Therefore, increased O2 demands of the heart must be met mainly by an increase in coronary blood flow (see ). In experiments in which the heartbeat is arrested but coronary perfusion is maintained, O2 consumption falls to 2 mL/ minute/100 g or less, which is still six to seven times greater than the O2 consumption of resting skeletal muscle.
Cardiac work has external and internal components. Left ventricular work per beat (stroke work) is approximately equal to the product of stroke volume and the mean aortic pressure against which the blood is ejected by the left ventricle. External cardiac work (We) may be defined as follows:
Equation 16.6
That is, each small increment in volume that is pumped, (dV) is multiplied by the associated pressure (P), and the products (PdV) are integrated over the time interval of interest (t2 − t1) to calculate total work. Added to this is kinetic work due to the velocity (v) of blood flow and the density (ρ) of blood. The mean pressure during expulsion is used to simplify this to
Equation 16.7
At resting levels of cardiac output, the kinetic energy component is negligible. However, with high cardiac output, as in strenuous exercise, the kinetic energy component can account for up to 50% of total cardiac work.
The internal work (Wi) of the heart can be written as
Equation 16.8 where α is a proportionality constant that converts T dt into units of work, T is wall tension, and dt is time. Clinically, V˜O2 and left ventricular power are difficult to measure; however, both are closely related to the systolic pressure–time index, the integral of left ventricular pressure, and time during systole. Such measurements are important because internal work is a large determinant of myocardial O2 need. An alternative approach to evaluate cardiac work and its relation to O2 consumption has been developed in which P-V loops (see
Fig. 16.44 ) are examined in conditions with varied preload and afterload; contractility is maintained constant.