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 goal of the heart is to keep blood circulating at an appropriate volume to meet the current metabolic demand of the body. The cardiac output varies by body size and activity level. In general, a normal range for cardiac output at rest is between 4.0 - 8.0 L/min, with the average being 5 L/min. During exercise, elite athletes can reach a cardiac output as high as ~40 L/min. The normal value for the cardiac index should be between 2.5 - 4.0 L/min/m^2. A value under 2.0 should raise suspicion for cardiogenic shock (characterized by <2.2 L/min/m2 with support or <1.8 L/min/m2 without support).[1][10][11]

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 an 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.[12][13]

Cardiac Output

Non-invasive 

• Doppler ultrasound 
  • Uses an ultrasound machine with a special probe that measures the Doppler shift in the returning ultrasound waves to decipher the blood flow rate and volume, both of which lead to the cardiac index
  • Benefits: relatively cheap, fast results, and non-invasive
  • Drawbacks: highly operator dependent
• Echocardiogram 
  • Uses two-dimensional ultrasound paired with Doppler shift measurements to elucidate blood flow rate and volume
  • Benefits: non-invasive, accurate (if properly trained)
  • Drawbacks: expensive, highly operator dependent
• Modified carbon dioxide Fick method
  • Utilizes the Fick principle and measures changes in CO2 elimination and end-tidal CO2 (which is a measure of atrial CO2)
  • Benefits: non-invasive, comparable accuracy to invasive methods
  • Drawbacks: can only be utilized in patients receiving mechanical ventilation and, like the other noninvasive techniques, does not provide a measurement of preload indexes
• Other non-invasive methods: cardiac magnetic resonance

Invasive

• The oxygen Fick method
  • This method uses the Fick equation (VO2)/(CaO2 - CvO2) to calculate the cardiac output numerically - the individual variables in the equation are measured through invasive testing, commonly pulmonary artery catheterization (PAC), and subsequently calculated
  • Benefits: highly accurate
  • Drawbacks: invasive (risking infection, arrhythmias, and pulmonary artery disruption), time-consuming [14] 
• Other invasive methods: lithium dilution

Body Surface Area (BSA)

While there are an array of methods for calculating the BSA, a commonly used 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.[15]

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.[16]

• Systolic dysfunctions (failure to pump)
  • High blood pressure (high afterload)
  • Cardiomyopathy 
  • Coronary artery disease 
  • Heart valve disease 
  • Other structural diseases, congenital or otherwise 
• Diastolic dysfunctions (inability to fill, usually secondary to other diseases)
  • Hypertrophy 
  • Sequelae of diabetes, hypertension, obesity, and inactivity
  • Other structural diseases, congenital or otherwise

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. For example, bedside echocardiogram monitoring in patients with septic shock can help guide the administration of vasopressors and/or dilators to certain patients.[17] 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.

Cardiac index is a hemodynamic measurement used to help evaluate the different forms of shock (circulatory disorders that lead to poor tissue perfusion). There are four main forms of shock:

• Cardiogenic – underlying heart dysfunction (e.g., as a result of myocardial infarction, arrhythmias, heart failure, cardiomyopathy, myocarditis, or severe mitral/aortic regurgitation)
• Obstructive – obstruction of the heart or its great vessels (e.g., as a result of cardiac tamponade, massive pulmonary embolism, or tension pneumothorax)
• Hypovolemic – loss of intravascular blood volume (e.g., as a result of hemorrhage or non-hemorrhagic fluid loss [diarrhea, vomiting, burns, third spacing, diuresis, adrenal insufficiency])
• Distributive (e.g., septic or anaphylactic shock) – redistribution of body fluid due to vasodilation with/without capillary leakage

One would expect to see a decrease in cardiac index in cardiogenic, obstructive, and hypovolemic shock. In contrast, there is usually a normal to increase in cardiac index in septic and anaphylactic shock.[18]