The student recognizes several techniques of assessing cardiac mechanical activity:
- Describes echocardiography and other cardiac visualization techniques for estimating cardiac ejection fraction.
- Describes the end-systolic pressure–volume relationship and how it reflects cardiac contractility.
- Given data, calculates cardiac output using the Fick principle.
- Defines cardiac index.
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 various imaging techniques to assess mechanical pumping action) and others require some invasive instrumentation. This chapter provides a brief overview of some of these commonly used clinical tools.
Advances in several noninvasive imaging techniques have made it possible to obtain 2- and 3-dimensional images of the heart throughout the cardiac cycle. Visual or computer-aided analysis of such images provides information useful in clinically evaluating cardiac function. These techniques are especially suited for detecting abnormal operation of cardiac valves or contractile function in portions of the heart walls. They can also provide estimates of heart chamber volumes at different times in the cardiac cycle that are used to assess cardiac function. Cardiac contractility, as assessed by the cardiac ejection fraction (ie, stroke volume divided by end-diastolic volume, SV/EDV), can be estimated by most of these imaging techniques.
Echocardiography is the most widely used of the cardiac imaging techniques currently available. This noninvasive technique is based on the fact that sound waves reflect back toward the source when encountering abrupt changes in the density of the medium through which they travel. A transducer, placed at specified locations on the chest, generates pulses of ultrasonic waves and detects reflected waves that bounce off the cardiac tissue interfaces. The longer the time between the transmission of the wave and the arrival of the reflection, the deeper the structure is in the thorax. Such information can be reconstructed by computer in various ways to produce a continuous image of the heart and its chambers throughout the cardiac cycle. Doppler echocardiography can provide additional information about blood flow velocity and direction across the cardiac valves. It is particularly useful in detecting valve stenosis or insufficiency.
Other imaging techniques are available for assessing cardiac function. Cardiac angiography involves the placement of catheters into the right or left ventricle and injection of radiopaque contrast medium during high-speed x-ray filming (cineradiography). Radionuclide ventriculography (also known as multigated acquisition scan or MUGA scan) involves the intravenous injection of a radioactive isotope that stays in the ...