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

 

Principles:

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 = Volume injected x (TBlood – TInjectate).k1.k2 / AUC (∆ temp vs. time)

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

·  k1: (SHCInj x SGInj) / (SHCBlood x SGBlood)

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

 

Pulmonary artery catheter:

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)

 

Thermodilution technique:

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

·   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

 

 

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