Cardiac output
Overview
Cardiac output (CO) is a term used in cardiology to describe the volume of blood being pumped by the heart, specifically by a ventricle in a minute. This measurement is crucial in assessing the heart's efficiency and overall cardiovascular health. It is typically measured in liters per minute (L/min).
Calculation
The calculation of cardiac output is a product of two factors: heart rate (HR) and stroke volume (SV). The formula is represented as CO = HR x SV. Heart rate is the number of heartbeats per minute, while stroke volume is the amount of blood pumped out of the heart with each beat.
Factors Affecting Cardiac Output
Several factors can affect cardiac output, including heart rate, stroke volume, and the body's demand for oxygen.
Heart Rate
An increase in heart rate can lead to an increase in cardiac output, assuming that stroke volume remains constant. However, if the heart rate becomes too high, it can decrease cardiac output because the ventricles don't have enough time to fill with blood during diastole.
Stroke Volume
Stroke volume can also affect cardiac output. It is determined by three factors: preload, contractility, and afterload. Preload refers to the volume of blood in the ventricles at the end of diastole. Contractility refers to the strength of the contraction of the heart muscle. Afterload refers to the pressure against which the heart must work to eject blood during systole.
Body's Demand for Oxygen
The body's demand for oxygen can also affect cardiac output. During exercise, for instance, the body's demand for oxygen increases, and the heart must pump more blood to meet this demand.
Pathophysiology
Changes in cardiac output can lead to various pathophysiological conditions.
Decreased Cardiac Output
Decreased cardiac output can result from a variety of conditions, including heart failure, myocardial infarction, and arrhythmias. These conditions can decrease the heart's ability to pump blood effectively, leading to decreased cardiac output.
Increased Cardiac Output
Conversely, increased cardiac output can occur in response to conditions such as exercise, pregnancy, and certain diseases like hyperthyroidism and anemia. In these conditions, the body's demand for oxygen is increased, and the heart must pump more blood to meet this demand.
Measurement
Cardiac output can be measured using several different techniques, including thermodilution, Doppler echocardiography, and Fick's principle.
Thermodilution
Thermodilution is a commonly used method for measuring cardiac output in critically ill patients. It involves the injection of a cold indicator into the bloodstream and measuring the change in blood temperature.
Doppler Echocardiography
Doppler echocardiography is a non-invasive method for measuring cardiac output. It uses ultrasound waves to measure the speed and direction of blood flow.
Fick's Principle
Fick's principle is a method that involves measuring the consumption of oxygen in the lungs and the difference in oxygen content in the arterial and venous blood.
Clinical Significance
Cardiac output is a crucial measurement in the management of many cardiovascular diseases. It provides valuable information about the functional status of the heart and can guide treatment strategies in conditions such as heart failure, shock, and sepsis.