Cardiac Output (CO):  
 
Definition: 
 
Cardiac Output is the volume of blood pumped by the heart per minute (measured in litres per minute) 
 
CO=Stroke Volume (SV)×Heart Rate (HR) 
Normal Range: 4–8 L/min (in adults) 
 
Stroke Volume (SV) = the amount of blood ejected with each heartbeat (normal – 60–80 mL/beat). 
 
Thus, both heart rate and stroke volume directly affect CO. 
 
Determinants of Cardiac Output 
 
Preload – Volume of blood returning to the heart (end-diastolic volume). 
↑ preload → ↑ stroke volume → ↑ CO (until the heart is overstretched). 
Managed with fluids (if low) or diuretics (if high). 
 
Afterload – Resistance the heart must overcome to eject blood. 
↑ afterload → ↓ CO. 
Managed with vasodilators (to reduce resistance) such as GTN, milrinone, enoximone, sodium nitroprusside. 
Managed with vasoconstrictors (to increase resistance – systemic vascular resistance (SVR)) such as Noradrenaline, vasopressin, phenylephrine, high dose dopamine. 
 
Contractility – The strength of heart muscle contraction. 
↑ contractility → ↑ CO. 
Affected by inotropes (e.g., adrenaline, dobutamine, dopamine, milrinone, levosimendan). 
 
Heart Rate – Beats per minute. 
Too slow (bradycardia) → ↓ CO (due to low heart rate) 
Too fast (tachycardia) → ↓ filling time → ↓ CO (due to low stroke volume) 

 
Cardiac Output Studies / Measurement Methods 
 
1. Invasive Methods 
 
a. Pulmonary Artery Catheter (PAC) / Swan-Ganz Catheter 
Measures CO via thermodilution technique. 
Also gives pulmonary artery pressure, central venous pressure (CVP), and mixed venous oxygen saturation (SvO₂). 
 
Nursing applications: 
Maintain sterility. 
Monitor waveform and calibration. 
Observe for complications: infection, arrhythmia, thrombosis, or pulmonary artery rupture. 
 
b. Fick Principle (Indirect) 
Uses oxygen consumption and arteriovenous oxygen difference. 
Accurate but complex; usually done in cardiac catheterization labs. 
 
2. Less-Invasive / Non-Invasive Methods 
 
a. Doppler Ultrasound (Echocardiography) 
Measures stroke volume and CO using blood flow velocity across valves. 
Non-invasive and repeatable. 
 
b. Bioimpedance / Bioreactance (e.g., NICOM, Cheetah system) 
Measures changes in electrical conductivity with each heartbeat. 
Common in ICU and operating theatre settings. 
 
c. Pulse Contour Analysis (e.g., PiCCO, LiDCO) 
Uses arterial line waveform data to estimate CO (area under the curve). 
Requires arterial line and sometimes a central line. 
 
Nursing Applications 
 
1. Hemodynamic Monitoring 
 
Interpret CO, SV, SVR (Systemic Vascular Resistance), and CI (Cardiac Index). 
Identify if the patient is: 
 
Hypovolemic: ↓ preload, ↓ CO → give fluids. 
Cardiogenic: ↓ contractility → give inotropes. 
Distributive (e.g., sepsis): ↓ afterload → vasopressors. 
 
2. Early Detection of Shock 
Monitor trends in CO and tissue perfusion indicators (MAP, urine output, lactate). 
Guide timely interventions before irreversible organ damage occurs. 
3. Evaluating Therapy Effectiveness 
Assess response to fluid resuscitation, inotropes, vasopressors, or mechanical devices (e.g., IABP, ECMO). 
 
4. Education and Coordination 
Teach patients/families about cardiac performance and medications. 
Collaborate with physicians to adjust therapy based on CO trends. 
 
Key Nursing Considerations 
 

 
 
Clinical Example 
– see Cardiac Output Studies interpretation Cardiac Output Studies – reference range and examples