2010A15 Explain how cardiac output is measured using thermodilution technique.

List:

· Principles: cardiac output, Stewart Hamilton Equation

· Pulmonary artery catheter description

· Thermodilution: methods, sources of error, pros, cons

Cardiac output

· Volume of blood ejected by the LV per unit time

· = HR x SV

· 70mL/kg/min in an adult

Stewart-Hamilton equation

· Q ∝ amount of coldness / effect of coldness

· Q = Volume injected x (T_Blood – T_Injectate).k1.k2 / AUC (∆ temp vs. time)

· Analogous to: Clearance = dose / AUC(concentration vs. time)

· k1: (SHC_Inj x SG_Inj) / (SHC_Blood x SG_Blood)

· k2: computational constant for heat gain in transit (injection time, dead space)

Components

· Sheath for insertion

· Flexible balloon-tipped catheter

· Thermistor near the tip

· Proximal ports in SVC and RA

· Distal port in pulmonary artery

Thermistor

· Semiconducting material e.g. metal oxide

· +/- incorporated into Wheatstone bridge

· ↓ temp -> ↑ resistance (non-linear overall, near linear in physiological range)

Method	· Cold saline injected into proximal port · Temperature known (e.g. 3°C) and volume known (e.g. 10mL) · Measure temperature vs time · Stewart-Hamilton equation used to calculate cardiac output · Some extrapolation required for the tail of the curve
Sources of error	Operator error · Injection too slow · Injection into wrong port · Measurement not at end-expiration Measurement error: accuracy decays with · Time · Heat sterilisation Potentially false assumptions: · That thermal equilibrium is established by the time mixture reaches thermistor · That injectate is warmed by blood only (not true in low flow states) · That flow is unidirectional (not true if tricuspid regurgitation) · That cold injectate does not depress cardiac output
Pros	· Can be performed frequently · No blood sampling required · No dye required · No recirculation error
Cons	· Requires invasive device: many risks of PAC insertion e.g. pulmonary artery rupture · Cold fluid may induce arrhythmias · Small extrapolation error