Cardiac power output
Metric originally designed to represent “Stroke work per unit time” as measured in watts and correaltes with the area within a pressure-volume loop (see below for example pressure-volume loops) CPO was originally described in 1986 but didn’t gain use in mainstream settings until it’s inclusion in a subgroup analysis looking at the SHOCK dataset.
![Pressure-volume loops in various disease states describing some of the drawbacks of using CPO as a measure of ventricular efficiency [3]](https://prod-files-secure.s3.us-west-2.amazonaws.com/85319cf9-504f-42ae-8dc3-9aa0766251b7/18b67668-74d6-4318-ab72-d86e77b9557a/Screenshot_2024-11-04_at_16.45.03.png?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Content-Sha256=UNSIGNED-PAYLOAD&X-Amz-Credential=AKIAT73L2G45FSPPWI6X%2F20241128%2Fus-west-2%2Fs3%2Faws4_request&X-Amz-Date=20241128T121013Z&X-Amz-Expires=86400&X-Amz-Signature=e7ff2b226a828ab6badac83be3e326d1334df3a029a418cc3c34f42ae219a2f5&X-Amz-SignedHeaders=host&x-id=GetObject)
“Equation 1” has been the most widely used measure for CPO since its inclusion in a subgroup analysis of the seminal SHOCK paper [1] In this study CPO was found to be the strongest independent metric for mortality in patients with cardiogenic shock.
451 Constant
The 451 constant is derived from an earlier study which recognised that if CVP is 3mmHg, BP is 120/80 (and MAP is 90.3mmHg) and cardiac output is 5.0l/min then 90.3x5 = 451
This equation using the 451 constant creates a “normal” CPO of 1, values of < 0.6 are thought to be abnormal representative of cardiogenic shock.
Using “Equation 1” omits CVP which in normality is not a huge contributor. Note that in the original SHOCK paper CVP values ranged from 13 +-6mmHg i.e. were elevated. Omission of RAP from the equation in these cases leads to an increased calculated CPO. CPO8 can be used in cases with elevated CVP and gives a more accurate reflection of “true” CPO. [2]
CPO appears to correlate well with outcomes in CS-AMI but may be less useful in patients who present with decompensation of known heart failure where the press-volume loop has had time to shift to the right due to neuro-hormonal adaptations resulting in near-normal stroke work (CPO) but other parameters being abnormal (e.g. PCWP, PASP etc…)
References