How do we know if the heart is functioning efficiently? What tells us whether the heart is pumping enough blood to meet the body's needs? How do doctors measure heart failure severity?
The answers lie in Cardiac Output (CO) and Ejection Fraction (EF)—two key metrics that assess how well the heart pumps blood and delivers oxygen to tissues. Understanding these concepts is crucial for diagnosing heart failure, shock, and circulatory insufficiency.
In this article, we will explore:
✔ What cardiac output is and how it’s calculated
✔ Why ejection fraction is important and how it’s measured
✔ How these values change in different heart conditions
1. What is Cardiac Output (CO)?
🔹 Cardiac Output (CO) is the total volume of blood pumped by the heart per minute.
✔ Normal CO at rest = 4 to 8 liters per minute (L/min)
✔ In exercise, CO can increase up to 5-7× (up to 35 L/min in elite athletes!)
📌 Why is CO Important?
Cardiac output determines oxygen delivery (DO₂) to organs. If CO drops (heart failure, shock), organs don’t get enough blood, leading to organ dysfunction.
2. How is Stroke Volume (SV) Determined?
Stroke Volume (SV) is the amount of blood ejected by the left ventricle per beat.
✔ EDV (End-Diastolic Volume) = The maximum blood volume in the ventricle before contraction (~120 mL).
✔ ESV (End-Systolic Volume) = The remaining blood volume after contraction (~50 mL).
✔ Normal SV = 50 to 100 mL per beat.
✔ Higher SV = Better cardiac efficiency.
🔹 What Factors Affect Stroke Volume?
1️⃣ Preload (Frank-Starling Mechanism) → More venous return = Stronger contraction.
2️⃣ Afterload (Arterial Resistance) → Higher resistance = Lower SV.
3️⃣ Contractility (Strength of the Heart Muscle) → Stronger contraction = Higher SV.
📌 Example:
- Dehydration or hemorrhage → Lower preload → Lower SV → Lower CO.
- High blood pressure → Increased afterload → Lower SV → Lower CO.
- Exercise → Increased contractility → Higher SV → Higher CO.
3. What is Ejection Fraction (EF)?
🔹 Ejection Fraction (EF) is the percentage of blood ejected from the left ventricle per beat.
✔ Normal EF = 55% to 70%
✔ EF < 40% → Heart failure with reduced ejection fraction (HFrEF)
✔ EF > 75% → Possible hypertrophic heart disease
📌 Why Does EF Matter?
- Low EF means the heart is not pumping efficiently, leading to fatigue, breathlessness, and fluid retention (heart failure).
- EF is used to classify heart failure into HFrEF (<40%) and HFpEF (>50%).
🔹 How is EF Measured?
✔ Echocardiography (Most Common) – Uses ultrasound to assess chamber volumes.
✔ Cardiac MRI – Gold standard for precise EF calculation.
✔ Nuclear Scans (MUGA, PET Scan) – Used in certain cases.
📌 Example:
- EF 30% → Severe heart failure, requires medication & monitoring.
- EF 60% → Normal pumping function.
4. How Do CO and EF Change in Different Conditions?
A. Heart Failure (HF)
| Type of HF | CO | EF | Cause |
|---|---|---|---|
| HFrEF (Systolic HF) | ↓ | <40% | Weak contraction, dilated LV |
| HFpEF (Diastolic HF) | ↓ | >50% | Stiff ventricle, impaired filling |
📌 Why does HFrEF occur?
The ventricle is too weak to pump out enough blood, leaving more residual volume (high ESV), lowering EF.
📌 Why does HFpEF occur?
The ventricle does not relax properly, causing low stroke volume despite normal EF.
B. Shock States
| Type of Shock | CO | SVR | Key Feature |
|---|---|---|---|
| Hypovolemic Shock | ↓↓↓ | ↑ | Severe volume loss |
| Cardiogenic Shock | ↓↓↓ | ↑ | Heart pump failure |
| Septic Shock | ↑ | ↓↓↓ | Vasodilation, low BP |
| Neurogenic Shock | ↓ | ↓↓↓ | Loss of sympathetic tone |
📌 Why is CO low in cardiogenic shock?
The heart cannot pump effectively due to infarction, arrhythmias, or valve dysfunction.
📌 Why is CO high in septic shock?
Massive vasodilation decreases afterload, increasing cardiac output (compensatory response).
5. How Do We Improve Cardiac Output & EF?
A. Medications That Boost CO & EF
✔ Inotropes (Dobutamine, Milrinone) – Increase contractility for acute HF.
✔ Beta-Blockers (Metoprolol, Carvedilol) – Reduce heart strain & improve long-term EF.
✔ ACE Inhibitors (Lisinopril, Ramipril) – Lower afterload, making pumping easier.
B. Devices That Help Heart Function
✔ Implantable Cardioverter-Defibrillators (ICDs) – Prevent sudden cardiac death in low EF.
✔ Left Ventricular Assist Devices (LVADs) – Mechanical pumps for severe HF.
📌 Heart Transplant:
For end-stage heart failure, transplantation remains the final option.
6. Key Takeaways: What You Should Remember
💡 Cardiac Output (CO) = Stroke Volume × Heart Rate.
💡 Normal CO = 4-8 L/min, but increases massively in exercise.
💡 Ejection Fraction (EF) is a measure of heart efficiency, with normal values of 55-70%.
💡 Low EF (<40%) = Heart failure with reduced ejection fraction (HFrEF).
💡 Heart failure, shock, and hypertension all influence CO and EF differently.
💡 Management includes medications (ACE inhibitors, beta-blockers), devices (ICD, LVAD), and lifestyle modifications.
Conclusion
Measuring Cardiac Output (CO) and Ejection Fraction (EF) provides critical insight into heart function and circulation efficiency. These values help clinicians diagnose heart failure, shock, and cardiovascular diseases, guiding treatment strategies.
In the next article, we will explore "Vascular Physiology & Blood Pressure Regulation," diving into how vessels control resistance, perfusion, and long-term blood pressure stability.
References
- Braunwald E. Braunwald’s Heart Disease: A Textbook of Cardiovascular Medicine. 11th ed. Elsevier; 2018.
- Guyton AC, Hall JE. Textbook of Medical Physiology. 14th ed. Elsevier; 2020.
- Klabunde RE. Cardiovascular Physiology Concepts. 3rd ed. Lippincott Williams & Wilkins; 2021.
- American Heart Association. Ejection Fraction: What It Is and Why It’s Important. Available at: www.heart.org.
- Mayo Clinic. Heart Failure and Ejection Fraction. Available at: www.mayoclinic.org.
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