From the Guidelines
Cardiac output is best calculated using the Fick equation: Q = V˙ o2 / [C(a–v)O2], where Q indicates cardiac output, V˙ o2 is oxygen consumption, and [C(a–v)O2] is the arteriovenous O2 content difference, as stated in the ats/accp statement on cardiopulmonary exercise testing 1. This equation is a linear function of V˙ o2 and does not vary as a function of either sex or state of training. The Fick equation provides a direct measurement of cardiac output, which is essential for assessing cardiovascular function during exercise. Key points to consider when calculating cardiac output include:
- Oxygen consumption (V˙ o2) is a critical component of the Fick equation
- Arteriovenous O2 content difference [C(a–v)O2] is related to O2 extraction, with maximal extraction thought to be about 75% of the arterial oxygen content (Ca O2) in healthy nonathletic individuals 1
- Increases in cardiac output are initially accomplished by increases in stroke volume and HR, and then at moderate- to high-intensity exercise almost exclusively by increases in HR. Understanding cardiac output is crucial for evaluating cardiovascular function, particularly during exercise, as it reflects how effectively the heart is supplying blood to meet the body's metabolic demands.
From the Research
Cardiac Output Equation
The cardiac output equation is not directly mentioned in the provided studies. However, the studies do discuss the factors that affect cardiac output, such as heart rate and stroke volume.
- Cardiac output (CO) is affected by dobutamine, which increases CO by augmenting stroke volume (SV) through enhanced left ventricular contractility and by decreasing systemic vascular resistance 2.
- The relationship between dobutamine plasma concentrations and CO is linear, with a clear increase in CO as dobutamine plasma concentrations increase 2.
- Heart rate (HR) also plays a role in determining CO, with dobutamine increasing HR in some patients, leading to an increase in CO 2.
- The choice of inotrope therapy, such as milrinone or dobutamine, can affect CO, with dobutamine trending towards a greater increase in cardiac index 3.
Factors Affecting Cardiac Output
Several factors can affect cardiac output, including:
- Heart rate (HR) 2
- Stroke volume (SV) 2
- Systemic vascular resistance 2
- Left ventricular contractility 2
- Inotrope therapy, such as milrinone or dobutamine 3
Limitations
The provided studies do not directly address the cardiac output equation, but rather discuss the factors that affect cardiac output. Therefore, the information provided is limited in its ability to fully answer the question. Additionally, study 4 is not relevant to the topic of cardiac output.