Brian Locke

Fick Equation and PA Catheter Principles

  • VO2 = amount of oxygen consumed.
  • CO = cardiac output
  • Ca, Cv = content of O2 in the arterial circulation and venous circulation = (Hb * BO2 * % saturation) + (0.0032 * PaO2), where BO2 = the maximum amount of oxygen binding in a unit of blood, usually 1.39.

VO2 = (CO * Ca) - (CO * Cv)

rearranged to

CO = VO2 / (Ca - Cv)

Direct Fick

Inspired O2 vs expired O2 measured (e.g. in a stress test) to calculate VO2. Then, you calculate the cardiac output

Indirect Fick

Estimate CO by assuming a value for 1 of: VO2 (using a nomogram), CaO2 (using SpO2), or CvO2 (using EtCO2)

Practical usage: VO2 estimated with 125 ml / O2 * BSA

Thermodilution

Principle: cardiac output = indicator dose * area under the time-concentration curve.

relies on uniform mixing and unidirectional flow. Margin of area ~25% with Fick.

##Can you do this with a central venous catheter? Yes - with the Fick principle (to the extent that scvO2 approximates svO2... which is somewhat variable and contested - https://link.springer.com/article/10.1186/cc9348). You need the mixing for thermodilution to be effective.

DO2

DO2 (Deliver of oxygen) and the DO2:VO2 ratio is particularly useful in differentiating types of shock.

see separate article

Pressures

Central line

You can measure:

  • CVP ( R heart filling pressure )
  • MAP ( pressure generated by the left heart)

With cardiac output*, you can then calculate SVR

PA Cathether

You can measure:

  • CVP (R heart filling pressure )
  • MPAP ( pressure generated by the right heart )
  • LVEDP (L heart filling pressure )
  • MAP (pressure generated by the left heart)

With cardiac output, you can calculate SVR and PVR

alt

Comparison

Thus, with a central line, the heart & pulmonary vasculature is treated as a single unit (in a system with the systemic vasculature). With a PA catheter, you can resolve information about each of the

Vascular Resistance

Hydraulic version of Ohm's law

Voltage drop = I R is rearranged to R = V / I

SVR: (MAP-CVP)/CO

PVR: (PA_m - PCWP)/CO

Contemporary review:

In sum: times when PA catheter derived indices are particularly helpful:

  • R heart failure or pulmonary hypertension (thus limits traditional metrics of fluid responsiveness and assessment). Think: how do you differentiate R heart failure (low CO and high CVP) caused by elevated LVEDP, increased PA pressure, or decreased RV contractility?
  • times when TTE / Pulse contour analysis can't be used: AFib, IABP, or VAD