Measurement of cardiac output

* Thermodilution: The measuring system comprises a disposable insulated syringe, cooling coils and injectate temperature probes with the microprocessor incorporated into a modular physiological monitor. A multilumen balloon catheter with a thermistor near the distal end is inserted via a peripheral vein. The tip is floated into a branch of the pulmonary artery. Ten millilitres crystalloid (ice-cold saline), drawn from a reservoir kept cool by ice, is injected through a proximal opening within the right ventricle. The saline mixes with the blood in the right ventricle. The temperature change is measured by the distal thermistor, which is downstream in the pulmonary artery. The fall in temperature is plotted against time (temperature-dilution curve). The area under the curve correlates with cardiac output.

* Indicator dye dilution: Evans blue, indocyanine green

A dose of indicator dye injected into the superior vena cava or the right atrium will flow as a bolus in the pulmonary circulation and through the left U

heart and then into the arterial system before it recirculates and completely pu mixes with the blood.

If an amount A of suitable indicator is injected into an unknown volume of distribution (V), the V can be estimated from the resultant indicator concentration (C) with the equation: V = A/C.

The flow of a fluid (Q) can be measured if the mean concentration of the indicator is determined for the time (t) required for that indicator to pass a given site. This is measured with the equation: Q=A / Ct. The dye concentration in arterial blood is plotted on a logarithmic scale against time during the first circulation of the dye bolus on a linear scale. The log-linear plot makes the descending limb of the curve a straight line -i.e. the indicator concentration follows an exponential time course, which can be extrapolated to zero concentration. The point of intersection gives the theoretical time taken by the dye bolus to circulate through the lungs and to leave the heart (the duration of indicator passage). The average concentration of the dye in the blood is calculated by integration of the area under the curve. Dividing the amount of dye (Q) by the average arterial concentration (C) gives the volume of blood (V) required to carry the dye, i.e. Q / C = V

This volume is the cardiac output during the time given by the curve for one circulation of the dye.

* Direct Fick principle: The amount of substance taken up by an organ in a given time is equal to the difference in concentration of that substance between arterial and venous blood, multiplied by the volume of blood flowing through the organ during the same period of time.

Cardiac output =-°xygen consumPtion (ml/min)-x 100

arterial oxygen content-mixed venous oxygen content (ml/100 ml)

The technique for cardiac output measurement requires sampling of systemic arterial blood and of mixed venous blood from a pulmonary artery catheter.

* Duplex ultrasound, using an oesophageal Doppler monitor

Doppler cardiac output (ml/min) = mean aortic blood flow velocity (cm/s) x aortic cross-sectional area (cm2) x 60

* Thoracic bioimpedance: this measures stroke volume on a beat-by-beat basis.

Table 6.1. Pressures in the central circulation

Pressures

mm Hg

Central venous pressure

0-5

Right atrium (mean)

0-5

Right ventricle systolic

20-30

diastolic

0-5

Pulmonary artery systolic

20-30

diastolic

8-12

Left atrium (mean)

8-12

Left ventricle systolic

100-150

diastolic

8-12

Aortic systolic

100-150

diastolic

70-90

Derived variables

Cardiac output = stroke volume x heart rate Cardiac index = cardiac output/body surface area in m2

= 3.0-4.5 l/(min/m2) Left ventricular stroke work = the integral of the LV pressure as a function of volume over the cardiac cycle Systemic vascular resistance = mean arterial pressure/cardiac output Left ventricular ejection fraction = stroke volume/end diastolic volume x 100% = 55%-70%

Right ventricular end-diastolic volume = 70-100 ml Left ventricular end-diastolic volume = 70-100 ml

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