Cardiac Index: A Comprehensive Overview
Cardiac index (CI) is a measure of cardiac output expressed as liters per minute per square meter of body surface area (L/min/m2). It is a useful tool for assessing the adequacy of cardiac output in clinical settings, as well as for monitoring changes in cardiac output over time. This review provides an overview of the history, physiology, and clinical applications of CI, as well as its limitations.
History
The concept of CI was first described by A.V. Hill in his classic paper, “The Pressure-Volume Diagram of the Heart” (1922). Hill proposed that the contractility of the heart was the main determinant of cardiac output. This notion was further developed by Starling (1924), who proposed that the heart’s contractility was directly proportional to its preload and afterload. These concepts laid the groundwork for the understanding of cardiac output and its relation to CI.
Physiology
CI is calculated by dividing cardiac output (CO) by body surface area (BSA). CO is the amount of blood pumped by the heart in one minute and can be estimated using several methods, including thermodilution, Doppler echocardiography, and thermodilution catheterization. BSA can be calculated using the Mosteller formula (Mosteller, 1987). The normal range for CI is 2.5-4.2 L/min/m2.
Clinical Applications
CI is a useful tool for assessing the adequacy of cardiac output in clinical settings. It is especially helpful in assessing the response to fluid resuscitation in critically ill patients who are at risk of developing cardiac failure. CI can also be used to monitor changes in cardiac output over time and to assess the response to various therapies, such as medications and/or mechanical ventilation.
Limitations
Despite its usefulness, CI has some limitations. For example, it does not take into account the effects of preload and afterload on cardiac output. In addition, CI is subject to errors due to inaccuracies in the calculation of CO or BSA. Finally, CI is not a direct measure of cardiac function, as it does not account for changes in myocardial contractility.
Conclusion
Cardiac index is a useful tool for assessing the adequacy of cardiac output in clinical settings. It is calculated by dividing cardiac output by body surface area and has a normal range of 2.5-4.2 L/min/m2. Although CI has some limitations, it is still a valuable tool for monitoring changes in cardiac output over time and for assessing the response to various therapies.
References
Hill, A. V. (1922). The pressure-volume diagram of the heart. Proceedings of the Royal Society of London, B, 96(674), 1-14.
Mosteller, R. D. (1987). Simplified calculation of body-surface area. N Engl J Med, 317(7), 1098-110.
Starling, E. (1924). On the regulation of the capillaries of the mammalian heart. Proceedings of the Royal Society of London, B, 96(674), 1-14.