What is cardiac output, its normal range, how is it measured, and how should low or high cardiac output states be managed?

Cardiac Output: Definition, Normal Range, Measurement, and Management

Definition and Normal Range

Cardiac output (CO) is the volume of blood pumped by the heart per minute, calculated as the product of heart rate and stroke volume, with normal values ranging from 4-8 L/min in adults, or a cardiac index (CI) of 1.9-4.3 L/min/m² when indexed to body surface area. 1, 2

• CO depends on four key determinants: heart rate, contractility, preload, and afterload 1
• Normal CO limits differ by gender: 3.3-7.3 L/min in females versus 3.5-8.2 L/min in males 2
• Cardiac index is more clinically useful than absolute CO because it remains stable regardless of age, gender, or body size 2
• CI normal limits are identical for both genders: 1.9-4.3 L/min/m² 2

Measurement Techniques

When to Measure Cardiac Output

CO monitoring should be initiated in patients who fail to respond to initial resuscitation therapy to determine shock type, guide treatment selection, and assess therapeutic response. 3

• Measure CO when clinical examination alone cannot clearly diagnose the type of shock 3
• Monitor CO in patients with very low cardiac output and poor tissue perfusion using arterial line with or without pulmonary artery catheterization 4
• Frequent reevaluation of CO is recommended during shock states 3

Measurement Methods

Echocardiography should be used as the first-line imaging modality to assess shock type and hemodynamic status, with the left ventricular outflow tract velocity-time integral (LVOT/VTI) method demonstrating the highest accuracy. 3, 5

Non-invasive Methods:

• Transthoracic echocardiography (TTE) using LVOT/VTI method shows superior accuracy (r=0.950) compared to other echo techniques 5
• The VTI method maintains reliability even in reduced CO states (r=0.606) and remains consistent regardless of heart rate variations 5
• Echocardiography provides rapid, non-invasive assessment suitable for stable patients and initial evaluation of unstable patients 6, 7

Invasive Methods:

• Transpulmonary thermodilution (TPTD) or pulmonary artery catheter (PAC) should be considered when accurate CO measurement is required in refractory shock 6, 3
• Use TPTD in patients without right ventricular failure, as it additionally measures extravascular lung water 3
• Use PAC in patients with right ventricular failure, as it measures pulmonary artery pressure 3
• PAC monitoring is indicated for progressive hypotension unresponsive to fluids or when fluid administration is contraindicated 8, 9
• Invasive arterial pressure monitoring is needed in cardiogenic shock 10

Management of Low Cardiac Output States

Initial Assessment and Stabilization

In low cardiac output states, immediately assess volume status with fluid challenge (>200 mL saline or Ringer's lactate over 15-30 minutes) if no overt fluid overload is present. 10

• Rapid volume loading should be administered to patients without clinical evidence of volume overload 8, 9
• Correct rhythm disturbances or conduction abnormalities causing hypotension 8, 9
• Echocardiography should evaluate mechanical complications unless assessed by invasive measures 8, 9

Pharmacologic Management

For low cardiac output with systolic blood pressure <90 mmHg, initiate dobutamine (2-5 μg/kg/min) or milrinone (0.275 μg/kg/min) as first-line inotropic therapy. 4

Inotropic Support:

• Dobutamine is the initial pharmacological intervention for low cardiac output 8, 9
• Levosimendan may be considered as an alternative, especially in patients on chronic beta-blocker therapy 10
• Once systolic blood pressure improves with inotropes, add vasodilator therapy to further lower filling pressures 4
• If blood pressure permits, add afterload-reducing agents to decrease cardiac work and pulmonary congestion 8, 9

Vasopressor Support:

• Norepinephrine is the recommended vasopressor (over dopamine) when mean arterial pressure requires pharmacologic support 10
• Vasopressors should only be used if there is strict need to maintain systolic BP in the presence of persistent hypoperfusion 10
• Vasopressor support should be given for hypotension that does not resolve after volume loading 8, 9
• Maintain mean arterial pressure >65 mmHg using the minimum necessary vasopressor dose 11

Medications to Avoid:

• Beta-blockers or calcium channel antagonists must not be administered to patients in low-output states due to pump failure 8, 9

Mechanical Circulatory Support

Intra-aortic balloon counterpulsation should be performed in patients who do not respond to pharmacologic interventions, unless further support is futile. 8, 9

• IABP is not routinely recommended in cardiogenic shock based on the IABP-SHOCK II trial 10
• Short-term mechanical circulatory support may be considered in refractory cardiogenic shock depending on patient age, comorbidities, and neurological function 10
• Consider device therapy rather than combining multiple inotropes when inadequate response occurs 10
• All cardiogenic shock patients should be rapidly transferred to tertiary care centers with 24/7 cardiac catheterization and ICU with mechanical circulatory support availability 10

Revascularization

Coronary artery revascularization with either PCI or CABG decreases mortality in patients with cardiogenic shock and is strongly recommended in suitable candidates. 8, 9

• Emergency surgical repair may benefit patients with ventricular septal rupture, papillary muscle rupture, or free wall rupture with pericardial tamponade 8, 9

Management of High Cardiac Output States

Assessment Considerations

When CO is monitored, its adequacy must be interpreted by evaluating organ function, tissue oxygenation, metabolism, and perfusion—not by absolute CO values alone. 3

• High cardiac output can occur in septic shock, especially after fluid resuscitation 3
• Vasodilatory shock may be characterized by unexpectedly low systemic vascular resistance despite elevated CO 12
• In vasoplegia-induced hypotension, use norepinephrine to maintain adequate perfusion pressure 12

Monitoring Adequacy

• Assess signs of hypoperfusion: oliguria (<0.5 mL/kg/h for ≥6 hours), altered mentation, cool extremities with livedo reticularis 10
• Lactate >2-4 mmol/L indicates organ hypoperfusion 10
• Mixed venous oxygen saturation <65% suggests inadequate tissue oxygenation 10

Critical Pitfalls

• Do not delay revascularization to perform right-sided heart catheterization in acute myocardial infarction with cardiogenic shock—defer until after PCI if it would cause undue delay 11
• Pulmonary artery catheter monitoring is not recommended in hemodynamically stable patients without respiratory compromise 8, 9
• Exclude hypovolemia in patients under vasopressors through repeated volume assessments 12
• In patients with right ventricular failure, initiation of positive pressure ventilation can abruptly lower systemic arterial pressure—use caution 11