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27.5
11%
29/259
37.5
45%
117/259
47.5
20%
52/259
57.5
3%
9/259
67.5
7/259
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The cardiac output is obtained through the Fick principle [cardiac output (CO) = volume of oxygen consumption (VO2) / tissue extraction of oxygen] and the cardiac output equation [CO = stroke volume (SV) x heart rate (HR)]. After calculating a CO of 4 L/min, the total peripheral resistance (TPR) can be calculated from the equation [TPR = (mean arterial pressure (MAP)-right atrial pressure (RAP)) / CO], yielding a TPR of 37.5 mmHg x min/L. The Fick principle essentially states that blood flow can be calculated if oxygen extraction and consumption by the body are known. Oxygen consumption (VO2) is defined as the amount of oxygen taken up per unit time. Extraction of oxygen is indirectly defined by subtracting venous oxygen content (e.g. from pulmonary artery) from arterial oxygen content (e.g. from pulmonary vein). In practice, due to difficulties with acquiring and analyzing gas concentrations for VO2, an assumed oxygen consumption of 125 mL O2/min/m^2 body surface is used. Also known as systemic vascular resistance, TPR represents the body’s resistance that must be overcome for blood flow. Its formula is basically Ohm’s law, such that R = V/I is equivalent to TPR = (MAP-RAP)/CO. Note that, just as in Ohm's law where V refers to the voltage drop across the resistor, similarly, the pressure drop across the vascular bed (MAP-RAP), not the absolute pressure at the start of the vascular bed (which would just be MAP). For simplicity, the unit mmHg x min/L is referred to as a Wood unit. Incorrect Answers: Answers 1 & 3-5: Filling in the patient values, the Fick principle equation states [CO = 800 mL O2/min oxygen consumption / (0.3 mL O2/mL of pulmonary vein blood - 0.1 mL O2/mL pulmonary artery blood)]. Resolving the equation yields CO = 4,000 mL / min = 4 L / min. Using this value for the TPR equation will reveal TPR = (155 mmHg-5 mmHg) / (4 L / min) = 37.5 mmHg x min/L. Bullet Summary: The Fick principle equation is CO = VO2 / (Ca – Cv), where cardiac output (CO) is equal to volume of O2 consumption (VO2) / [O2 in arterial blood (Ca) – O2 in venous blood (Cv)].
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