The heart is fundamentally a muscular hydraulic pump that can create pressure as well as flow. It is comprised of a pump (heart) and pipes (arteries) that generate energy. Cardiac output is the flow through this closed circuit that is influenced by cardiac contractility and a multifaceted interaction between vascular resistance, compliance to flow, filling pressures, and overall intravascular volume. The cardiac pumping capability is defined as cardiac power output (CPO) (Yildiz, et al., 2017). CPO is also a direct correlate of end-organ perfusion (Mishra, 2016). This novel hemodynamic measurement uses the physical rule of fluids: power = pressure x flow and is the product of simultaneously measured cardiac output (or cardiac index) and mean arterial pressure (Fincke, et al., 2004).
Formulas (Yildiz, et al., 2017)
- Resting CPO is measured in watts using the following formula: cardiac output (L/min) x mean arterial pressure divided by 451. CPO < 0.6 W is indicative with hemodynamic compromise and is associated with increased risk of mortality.
- Resting cardiac power index (CPI) is measured in watts per square meter (W/m2) using the following formula: cardiac index (L/min/m2) x mean arterial pressure divided by 451. Normal CPI is 0.5-0.7 W/m2.
Under the CPO umbrella reside three sub-categories which include maximal CPO, reserve CPO, and resting CPO. Maximum CPO and reserve CPO can be measured non-invasively or invasively during cardiopulmonary stress testing. When cardiac (or pump) dysfunction occurs, whether due to myocardial infarction, valvular heart disease, myocarditis, etc., maximum and reserve CPO decrease to a level that causes impairment of resting CPO. Deficits in maximum and reserve CPO manifest in exercise intolerance symptoms, for example dyspnea on exertion. Severe decline in resting CPO can be seen in patients in advanced stages of heart failure and/or cardiogenic shock (Yuldiz, et al., 2017).
You might be wondering how these pieces of data are helpful or relevant to caring for a cardiac patient. Well, here’s why! Invasive hemodynamic monitoring, namely right heart catheterization, has been utilized for decades for risk stratification and guiding medical therapy for patients in heart failure or cardiogenic shock. The measurements of pulmonary capillary wedge pressure (PCWP) and cardiac output were most heavily focused on, and while tailoring treatment to improve these parameters did improve patient symptoms, it did not consistently improve overall outcomes. Notably, neither of these hemodynamic measurements gave a reflection of the essence of cardiac contractility, meaning PCWP is a measurement of intracardiac pressure
and cardiac output or index are measurements of cardiac flow
. It was recognized these measurements did not authentically represent the synergistic nature of our cardiac system that is composed of an energy source (heart) and its pipes (arteries) that conduct this energy throughout the body. Therefore, the actual pumping power of the heart is more accurately represented by the measurements of cardiac power output and index (Fincke, et al., 2004).
Utility of the utility of CPO and CPI has been the focus of many of research studies. Below is a short list of a few of these studies including the highlights:
- Fincke, et al. (2004) studied 541 patients with cardiogenic shock and found that CPO was the strongest independent indicator of in-hospital mortality. Patients with a CPO ≤ 0.53 W had a 58% probability of in-hospital mortality.
- George, et al. (2012) found that CPO was a reliable discriminator compared to other hemodynamic measurements in predicting successful left ventricular assist device (LVAD) weaning and explantation. They reported all patients with CPO > 1.1 W experienced myocardial recovery and tolerated LVAD explantation.
- Grodin, et al. (2015) studied 495 ambulatory advance heart failure patients and found that lower resting CPI was associated with higher left and right sided filling pressures and abnormal cardio-renal biomarkers. Their research found a CPI < 0.44 W/m2 was associated with increased adverse outcomes (mortality, cardiac transplantation, and left ventricular assist device placement). They concluded that CPI is a reliable prognostic measurement and support its use for risk stratification in advanced heart failure patients.
- Yildiz, et al. (2017) studied 172 advanced heart failure patients who were referred for cardiac transplantation. They hypothesized that resting CPO would correlate with survival. They found that indeed a resting CPO < 0.54 W was an independent prediction of adverse outcomes in this patient population.
Cardiac power output and index are hemodynamic parameters that provide insight into the current clinical status of your cardiogenic shock or advanced heart failure patients, and are a powerful predictor of patient prognosis. While these terms may not yet be part of daily clinical dialogue at your institution, their utility and validity are quite evident in the literature surrounding these high-risk patients.
Fincke, R., Hochman, J.S., Lowe, A.M., Menon, V., Slater, J.N., Webb, J.G., ...Cotter, G. (2004). Cardiac power is the strongest hemodynamic correlate of mortality in cardiogenic shock: A report from the SHOCK trial registry. Journal of American College of Cardiology, 44(2), 340-348. doi: 10.1016/jacc.2004.03.060
George, J.K., Shabari, F.R., Abdul-JAbbar, A.H., Gazzala, J., Rossell, G., Odegaard, P.,...Delgado, R.M. (2012). Cardiac power output predicts myocardial recovery and successful explantation of left ventricular assist device. The Journal of Heart and Lung Transplantation, 31(4S), S74. doi: 10.1002/ejhf.268
Grodin, J.L., Mullens, W., Dupont, M., Wu, Y., Taylor, D.O., Starling, R.C., Wilson, W.H. (2015). Prognostic role of cardiac power index in ambulatory patients with advanced heart failure. European Journal of Heart Faliure, 17(7), 689-696. doi: 10.1002/ejhf.268
Mishra, S. (2016). Upscaling cardiac assist devices in decompensated heart failure: Choice of device and its timing. Indian Heart Journal, 68, S1-S4. doi: 10.1016/j.ihj.2015.12.012
Yildiz, O., Gamze, A., Zumrut, D.T., Yenigun, C.D., Yazicioglu, N. (2017). Evaluation of resting cardiac power output as a prognostic factor in patients with advanced heart failure. The American Journal of Cardiology, 120(6), 973-979. doi: 10.1016/j.amjcard.2017.06.028