58 Fick Equation And Pa Catheter Principles
58.1 What this covers
- Direct Fick
- Indirect Fick
- Thermodilution
58.2 Learning objectives
- Direct Fick
- Indirect Fick
- Thermodilution
- DO2
- Pressures
- Central line
- PA Cathether
58.3 Bottom line / summary
- VO2 (CO Ca) - (CO Cv)
- Inspired O2 vs expired O2 measured (e.g.
- in a stress test) to calculate VO2.
- Then, you calculate the cardiac output
- Estimate CO by assuming a value for 1 of: VO2 (using a nomogram), CaO2 (using SpO2), or CvO2 (using EtCO2)
58.4 Approach
- 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.
- CVP ( R heart filling pressure )
- MAP ( pressure generated by the left heart)
58.5 Red flags / when to escalate
- DO2 (Deliver of oxygen) and the DO2:VO2 ratio is particularly useful in differentiating types of shock.
58.6 Common pitfalls
- TODO: Capture common errors or missed steps.
58.7 References
- https://link.springer.com/article/10.1186/cc9348
- https://photos.collectednotes.com/photos/5187/faf12185-ce07-40b9-94d8-f2ebe92c15e1
- https://pubmed.ncbi.nlm.nih.gov/33564995/
- https://pubmed.ncbi.nlm.nih.gov/33646499/
58.8 Source notes
58.8.1 Fick Equation And Pa Catheter Principles
59 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)
59.1 Direct Fick
Inspired O2 vs expired O2 measured (e.g. in a stress test) to calculate VO2. Then, you calculate the cardiac output
59.2 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
59.3 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.
59.4 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.
60 DO2
DO2 (Deliver of oxygen) and the DO2:VO2 ratio is particularly useful in differentiating types of shock.
see separate article
61 Pressures
61.1 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
61.2 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
61.3 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
62 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:
- Part 1: https://pubmed.ncbi.nlm.nih.gov/33564995/
- Part 2: https://pubmed.ncbi.nlm.nih.gov/33646499/
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