Nicole JohnsonWhat determines cardiac output? It’s an important concept to understand because there can be many different contributing factors that can impact your patient’s cardiac output! The equation to calculate cardiac output is heart rate multiplied by stroke volume OR C.O. = HR x SV.
Did you know there are three components of stroke volume? I’m sure you’ve heard this before but let’s refresh! Stroke volume is composed of preload, afterload and contractility. Let’s do a deeper dive.
What is stroke volume (SV)?
- The amount of blood ejected out of the heart.
- Normal SV is 50 – 100 mL/beat.
- Normal SV index is 35 – 60 mL/beat/m2
What is preload?
- It’s the myocardial fiber length at the end of diastole prior to contraction. Think of it as the “stretch” of the heart.
- It can be equated to volume status (with caution!)
- High preload = fluid overload (But not always clinically true!)
- Low preload = fluid deficit
| Estimated volume on the right side of the heart | Estimated volume on the left side of the heart: |
| Right Atrial Pressure (RAP)Central Venous Pressure (CVP)Normal 2 – 6 mm Hg | Pulmonary Artery Occlusive Pressure (PAOP)Normal 8 – 12 mm Hg |
What is afterload?
- What your heart is pushing against.
- It’s the resistance or pressure the ventricles must overcome to eject during systole.
| Systemic Vascular resistance (SVR) | Pulmonary Vascular Resistance (PVR) |
| Defined as left ventricular afterload Normal = 900 – 1400 dynes/sec/cm-5 | Defined as right ventricular afterload Normal = 90 – 250 dynes/sec/cm-5 |
| ↓ afterload may be caused by: | ↑ afterload may be caused by: |
| Vasodilation↓ pressure needed to open the aortic valve↑ stroke volume | Vasoconstriction Chamber size Stenosis of an A-V valve (aortic stenosis) Wall tension (hypertrophy) More pressure needed to open the aortic valve |
What is contractility?
- It is the FORCE of ventricular ejection.
- It is difficult to measure
- It is influenced by changes in preload & afterload.
- “Inotrope” +/-
Factors affecting contractility:
- Myocardial injury (MI, contusion)
- Sepsis
- Inadequate stretch
- ↑ resistance
- Metabolic disturbances/acidosis
If you found this interesting and want to learn more about the heart, check out Nicole’s Cardiac Medicine Certification Course “Ace the CMC” which is currently on sale through June 18th at 30% off!
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