It is not suitable for bedside clinical use. However, the method is cumbersome and time consuming, and usually performed in the laboratory. It is based on a very simple principle that blood flow through an organ is related to the uptake of a marker (oxygen) and the difference in concentration of that marker between blood entering (arterial) and blood leaving (venous) that organ, in the case of the Fick method, the heart and lungs. It involves measuring oxygen uptake by the body and comparing oxygen content in arterial and venous blood samples. The Fick method is considered the most accurate method and gold standard. Knowing these variables becomes important when treating critically ill patients with low blood pressures who may be either hypovolaemic or septic, as it helps one to differentiate between the two conditions.Ĭardiac output has proved very difficult to measure reliably in the clinical setting. Usually, the physician is only able to measure the pulse rate, and thus does not know how much blood the heart pumps each minute, nor the degree of the peripheral vasoconstriction. However, cardiac output and peripheral resistance are much less easy to obtain. Voltage = Current x Resistance).ĭuring clinical assessment pulse rate and blood pressure are very easy to measure. A very simple formula exists that describes the model of Blood Pressure = Cardiac Output x Peripheral Resistance, which is often compared to Ohm’s law for electricity (i.e. Blood pressure is generated in the arteries by the heart pumping against this resistance. The haemodynamics of the model has flow, the cardiac output, leaving the heart, and passing through a resistance, the peripheral capillaries. When evaluating the circulation, and thus haemodynamics, a very simply model can be drawn of the heart pumping blood through the arteries to peripheral capillaries and then returning to the heart via the veins. In parallel with these clinical developments the technology also became available to make more sophisticated cardiac output monitors and in particular monitors that can be used continuously at the bedside. The need to measure cardiac output in a clinical setting arose in the 1970s because of the development of intensive care units and the increasing need to manage unstable patients during high risk surgery. It is equal to the volume pumped out by the heart in one contraction, known as stroke volume, multiplied by heart rate. Today pulse rate and blood pressure measurement is performed in almost every patient.Ĭardiac output is the volume of blood that is pumped by the heart around the systemic circulation in a given time period, usually one minute. However, it was not until the 1940s that the clinical sphygmomanometer was invented, and blood pressure measurement became routinely available. The Egyptian physicians used simple palpation of the pulse and the use of the pulse in Chinese medicine dates back over two thousand years. Physicians have been assessing the circulation long before the birth of Christ (BC). “Cardiac output the “Holy Grail” of haemodynamic monitoring”
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