The student recognizes several techniques of assessing cardiac output and cardiac contractility:
- Given data, calculates cardiac output using the Fick principle.
- Defines ejection fraction and identifies visualization methods used to determine it.
- Describes the end-systolic pressure–volume relationship.
The student understands the physiological basis of the electrocardiogram:
- States the relationship between electrical events of cardiac excitation and the P, QRS, and T waves, the PR and QT intervals, and the ST segment of the electrocardiogram.
- States Einthoven's basic electrocardiographic conventions and, given data, determines the mean electrical axis of the heart.
- Describes the standard 12-lead electrocardiogram.
There are a variety of methods available to assess cardiac function. Some of these are noninvasive (eg, auscultation of the chest to evaluate valve function, electrocardiography to evaluate electrical characteristics, and echocardiography to visualize mechanical pumping action) and others require various types of invasive instrumentation. This chapter provides a brief overview of some of these commonly used clinical tools.
Fick principle is one of the most accurate methods of measuring cardiac output, which is discussed in detail in Chapter 6. Briefly, this principle states that the amount of a substance consumed by the tissues, Xtc, is equal to what goes in minus what goes out, which is the arterial–venous concentration difference in the substance ([X]a − [X]v) times the blood flow rate, Q̇. This relationship can be algebraically arranged to solve for blood flow:
A common method of determining cardiac output is to use the Fick principle to calculate the collective flow through the systemic organs from (1) the whole body oxygen consumption rate (tc), (2) the oxygen concentration in arterial blood ([X]a), and (3) the concentration of oxygen in mixed venous blood ([X]v). Of the values required for this calculation, the oxygen content of mixed venous blood is the most difficult to obtain. Generally, the sample for venous blood oxygen measurement must be taken from venous catheters positioned in the right ventricle or pulmonary artery to ensure that it is a mixed sample of venous blood from all systemic organs.
The calculation of cardiac output from the Fick principle is best illustrated by an example. Suppose that a patient is consuming 250 mL of O2 per minute when his or her systemic arterial blood contains 200 mL of O2 per liter and the right ventricular blood contains 150 mL of O2 per liter. This means that, on the average, each liter of blood loses 50 mL of O2 as it passes through the systemic organs. In order for 250 mL of O2 to be consumed per minute, 5 L of blood must pass through the systemic circulation each minute:
Dye dilution and thermal dilution...