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 amount of coldness / effect of coldness

  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. 3C) 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

 

 

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