07 February, 2012
How to Calculate Cardiac Efficiency From a Resting BPM to a Post-Test Heart Rate
Cardiac efficiency is the ratio of work done by the heart to the energy used to perform the work. This measurement is crucial for the diagnosis and management of several cardiac diseases including cardiomyopathy, hypertension and cardiac failure. Invasive techniques such as cardiac catheterization, as well as non-invasive methods such as positron emission tomography, can be used to measure cardiac efficiency. It is often important to calculate cardiac efficiency at resting-phase heart rate and at post-activity heart rate to evaluate the functioning of the organ at all times.
The heart requires energy for pumping blood; this energy is acquired from oxidation of glucose and fatty acids obtained from the diet. However, it is difficult to measure the amount of energy obtained from these sources, hence cardiac efficiency measurements use oxygen consumption of the heart cells as a measure of its energy use. The work or output involves the functioning of the ventricles, and is generally expressed in terms of the ventricular stroke work -- the work done by the heart to pump a certain volume of blood. The efficiency of the normal heart is about 20 to 25 percent.
The stroke volume or the amount of blood pumped by your heart per minute is calculated by multiplying the amount of blood pumped by the heart per minute to the BPM, or beats per minute, number. The ratio of the stroke volume and mean arterial blood pressure is the cardiac efficiency of your heart.
Cardiac efficiency can be calculated using the formula, mV02 ¼ H + EW, where mVO2 is the oxygen consumption, H is the heat and EW is the external work. To determine oxygen consumption, blood is drawn from the coronary sinus and systemic artery of the patient; oxygen content is calculated as a product of the hemoglobin concentration, oxygen saturation and oxygen binding capacity of the blood sample. Heat or the temperature of the blood can be measured using catheter-mounted thermistors. The external work or output of the heart can be measured using conductance catheters.
Oxygen consumption is also calculated by injecting a chemical known as C-Acetate, and measuring its clearance rate using positron emission tomography, or PET. This is, in turn, used to determine the work metabolic index, an index of cardiac efficiency. The blood pressure and heart rate are measured before injecting the acetate. The ratio of the C-Acetate and the time from resting BPM to post-test heart rate is also calculated using data obtained from PET scans. This is the stroke volume, or the cardiac output, of your heart.
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