Cardiac output measurement

Critchley, L.A.H., Critchley, J.AJ.H., 1999. A meta-analysis of studies using bias and precision statistics to compare cardiac output measurement techniques. J. Clin. Monit. Comput. 15, 85—91. 

The heart, a four-chambered muscular pump has as its primary purpose the propelling of blood throughout the cardiovascular system. The left ventricle is the principal pumping chamber and is therefore the largest of the four chambers in terms of muscle mass. The efficiency of the heart as a pump can be assessed by measuring cardiac output, left ventricular pressure, and the amount of work requHed to accomplish any requHed amount of pumping. 

Monitoring the patient in shock requires vigilance on the part of the nurse The patient s heart rate, blood pressure, and ECG are monitored continuously. The urinary output is measured often (usually hourly), and an accurate intake and output is taken. Monitoring of central venous pressure via a central venous catheter will provide an estimation of the patient s fluid status. Sometimes additional hemodynamic monitoring is necessary with a pulmonary artery catheter. The use of a pulmonary artery catheter allows the nurse to monitor a number of parameters, such as cardiac output and peripheral vascular resistance The nurse adjusts therapy according to the primary health care provider s instructions. 

Pulmonary artery catheter An invasive device used to measure hemodynamic parameters directly, including cardiac output and pulmonary artery occlusion pressure calculated parameters include stroke volume and systemic vascular resistance. 

Radionuclide angiocardiography is used to measure ejection fraction (EF), regional ventricular performance, cardiac output, ventricular volumes, valvular regurgitation, asynchrony or wall motion abnormalities, and intracardiac shunts. 

Anaesthetized studies conducted using data capture systems to record six lead ECG (I, II, III, aV, and aVf), left ventricular pressure variables, arterial blood pressure and respiratory measurement of arterial blood flow in selected vascular beds, cardiac output and arterial blood gas measurement. ECG intervals are measured from the lead II ECG and Q-T interval can be corrected for heart rate using Bazett s, Friderecia s or Van De Water s formulas. 

Anaesthetized studies using MI2 data capture system with additional measurement of blood flow in selected vascular beds, cardiac output, respiratory and left ventricular function. 

Endpoint. An indicator measured in a patient or biological sample to assess safety, efficacy, or another trial objective. Some endpoints are derived from primary endpoints (e.g., cardiac output is derived from stroke volume and heart rate). Synonyms include outcome, variable, parameter, marker, and measure. See surrogate endpoint in the text. Also defined as the final trial objective by some authors. 

A marker substance is injected into a central vein. A peripheral arterial line is used to measure the amount of the substance in the arterial system. A graph of concentration versus time is produced and patented algorithms based on the Stewart-Hamilton equation (below) are used to calculate the cardiac output. 

Sound waves are emitted from the probe (P) at a frequency F0. They are reflected off moving red blood cells and back towards the probe at a new frequency, FR. The phase shift can now be determined by FR-F0. The angle of incidence (9) is shown on the diagram. If a measurement or estimate of the cross-sectional area of the blood vessel is known, flow can be derived as area multiplied by velocity (m2.m.s 1 = m3.s 1). This is the principle behind oesophageal Doppler cardiac output monitoring. 

Normal The LVEDP may be used as a measure of preload or initial fibre length . Cardiac output increases as LVEDP increases until a maximum is reached. This is because there is an optimal degree of overlap of the muscle filaments and increasing the fibre length increases the effective overlap and, therefore, contraction. 

Cournand AF, Riley RL, Breed ES, et al. Measurement of cardiac output in man using the technique of catheterization of the right auricle. J Clin Invest 1945 24 106. 

Measurements of arterial pressure, cardiac output, stroke work index, and pulmonary capillary wedge pressure are particularly useful in patients with acute myocardial infarction and acute heart failure. Such patients can be usefully characterized on the basis of three hemodynamic measurements arterial pressure, left ventricular filling pressure, and cardiac index. One such classification and therapies that have proved most effective are set forth in Table 13-4. When filling pressure is greater than 15 mm Hg and stroke work index is less than 20 g-m/m2, the mortality rate is high. Intermediate levels of these two variables imply a much better prognosis. 

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