Cardiac output monitoring

By the Fick method

Oxygen uptake equals the arterio-venous oxygen difference times flow rate

$$CO \cdot (CaO_2 - CvO_2) = \dot{V_{O_2}}$$

VO2 can be directly measured by knowing the minute volume, the partial pressure of O2 in mixed expired gas, and the FiO2: $$\dot{V_{O_2}} = MV \cdot (P_aO_2 - P_{\bar{E}}O_2)$$

The CaO2 and CvO2 can be computed with knowledge of the ceHb, and the arterial and mixed venous PO2 and saturations, because $$C_{O_2} = 1.34 \cdot ceHb \cdot sO_2 + 0.03 \cdot P_{O_2}$$ with \(C_{O_2}\) measured in ml/L

Limitations

• Pulmonary O2 consumption (increase VO2 \(\to\) overestimate CO)
• Intracardiac shunt (reduces the a-v difference \(\to\) overestimate)
• Assumes steady-state

By the indirect Fick method

As the Fick method, but some terms are estimated from a nomogram \(\to\) more error

By indicator dilution (including thermodilution)

The Stewart Hamilton equation reports that, for a concentration measured distal to an infusion of indicator, $$CO = \frac{\text{indicator dose}}{\int_0^{\inf} C(t) dt}$$

If a thermal indicator i.e. cold saline is used, then this becomes $$CO = k\frac{(T_{blood} - T_{saline}) \cdot V_{\text{saline}}}{\int_0^{\infty} \Delta T(t) dt}$$ where k depends on the injectate.

Limitations of dye dilution

• Dye accumulates and recirculates, which limits how often CO measurements can be taken
• Assumes uniform mixing of blood and unidirectional flow
• Spot measurement \(\to\) inaccurate with erratic respiration

Limitations of thermodilution

• Heat sinks (e.g. pleural effusions) may interfere
• Intracardiac shunt \(\to\) overestimates cardiac output (in essence a left-to-right shunt effectively reduces the thermal dose)
• Rapid infusion of IV fluids will interfere (effectively increases the thermal dose)

By pulsed wave doppler

Pulsed wave doppler is placed over the LV outflow tract in A5C or A3C. The velocity signal is integrated over one systole to give a velocity-time integral (VTI), the displacement that a line blood has accumulated during that systole. The LVOT radius is measured in PLAX. Then,

$$CO = HR * VTI * \pi r^2$$

Limitations

• Operator dependent; doppler beam must be aligned with LVOT; small errors in LVOTd are magnified when it is squared
• Assumes each stroke volume is equal \(\to\) inaccurate in AF or erratic respiration
• Not continuous

By pulse contour analysis

The area under an arterial pressure waveform is integrated. Since pressure is proportional to flow, this integral is proportional to the stroke volume.

$$SV \propto \int_0^{\text{end of systole}}P_{\text{arterial}}(t) dt$$

The coefficient of proportionality can be determined by using an intermittant method (typically thermodilution), then pulse contour analysis can be used for continuous monitoring.