The human heart is one of the most studied and vital organs to life. There are many ways to describe the status of the heart’s function and health. One of the measures of the function used most by clinicians is the cardiac index. The cardiac index relies on another important parameter, the cardiac output, and turns it into a normalized form which accounts for the body size of the patient. For example, the cardiac output of a person who weighs 120 pounds would be expectedly different from a person who weighs 220 pounds. For this reason, a simple cardiac output alone cannot be a reliable indicator of cardiac performance. Calculating a cardiac index solves this problem.
Cardiac output (CO) can be further broken down as the product of stroke volume (SV), which is the blood volume ejected by one heartbeat, and heart rate (HR), which is the number of heartbeats per minute. Specifically, this is a measure of left ventricular output and a clinical indicator of left ventricular function. Conditions that increase heart rate or stroke volume then directly affect cardiac output. At the cellular level, increases of sympathetic tone or myocardial stretch increase cardiac output, albeit by slightly different mechanisms. Sympathetic fibers directly influence the adrenal medulla which then releases catecholamines, norepinephrine, and epinephrine. These circulating catecholamines then work their way to the receptors in the heart resulting increases in both contractility and rate.
Discrete increases in the stretch of the myocardium, or increases in pre-load, also increase cardiac output augmenting the myofibril - Ca2+ binding relationship. The term pre-load comes from the temporal relationship of being "pre" contraction of the myocardium. That is the load placed on the heart while in diastole, or its filling cycle. Specifically, stretching the muscle fibers increases troponin’s affinity for Ca2+ and decreases the space between thick and thin filaments of the cardiac muscle – ultimately leading to an increase in the number of cross-bridges which can form. 
Another variable that can have a profound effect on cardiac output is the afterload, which is aptly named, due to its temporal relationship with the heartbeat. Afterload then can be described as the load against which the heart must pump, or put another way, the pressure in the aorta that the heart must overcome to eject its left ventricular volume or preload.
The function of the cardiac index is to create a normalized value for the cardiac function, which effectively correctS for the patient’s body size. The units for the cardiac index are (Liters/minute)/(body surface area) measured in meters squared (m^2).
The goal of the heart is to keep blood circulating at an appropriate volume to meet the current metabolic demand of the body. In a healthy 70 kg, male patient the cardiac output should be 4.0 - 8.0 Liters/minute. This value accounts for both resting states with very low cardiac demand on the 4.0 end and high demand states such as strenuous exercise on the 8.0 side.
The normal value for the cardiac index should be between 2.5L/min/m^2 - 4.0L/min/m^2. With a value under 2.0 highly suspicious for cardiogenic shock.
The clinician has a few options for testing for the cardiac index at his or her disposal. Given the specific circumstances, necessity and acuity of the patient's medical picture the provider can choose from this battery of options to best suit the patient's needs. They range from non-invasive imaging techniques to highly invasive pressure readings. It bears mentioning that although non-invasive procedures are available and can provide a more accurate value, there is limited evidence that the benefits of value outweigh the risks and complications of the invasive procedures. Furthermore, given that there is no gold-standard for measuring cardiac output and, by proxy, cardiac index caution should be taken to pick the appropriate tests after weighing the motives for testing, goals of testing and patient condition.
Body Surface Area
While there are an array of methods for calculating the BSA, the most common method is the Mosteller formula where BSA = The square root of [bodyweight (kg) x height (cm) / (3600)]. The average BSA for an adult male is 1.9 and for the adult female 1.6. Today, a cellular phone application can be used to calculate the body surface area for a patient; however, caution must be taken to ensure the correct equation is selected for the situation and patient.
The pathophysiology behind cardiac index is rooted mainly in dysfunctions of the heart. These dysfunctions can be further broken down into systolic and diastolic dysfunctions.
The clinical significance of cardiac index comes from the fact that it is a measure of cardiac function that can be normalized for the patient's body habitus, which means that the clinician can gain critical insight into the patient's heart function given the variations in body type. Often, physicians need to make decisions on medications, treatment options and educate patients on prognosis given these objective parameters. The cardiac index's strength is that it is a number that includes a more detailed picture of how the heart is functioning relative to the body, and not independently.
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