Preload and Afterload: Cardiac Output Overview

What is Cardiac Output?

Cardiac output (CO) is the amount of blood ejected from the left ventricle, into the aorta of your heart, then out to the rest of the body in one minute. Normally, an average person has about five liters of blood circulating throughout the body’s systems. 

Cardiac Output Equation

This CO equation helps you determine a patient’s cardiac output by calculating the amount of blood pumped by the heart in one minute. Here’s the equation (where CO = cardiac output (liters/min), HR = heart rate (beats/min), and SV = stroke volume (milliliters)):

 Heart rate (beats/min.) x Stroke volume (mL) = Cardiac output (l/min.)

What is Stroke Volume?

Stroke volume is the amount of blood in one clean pump – in other words, what your heart pumps with each beat. It’s a measurement of how much blood goes through your body every minute. Results depend on the size of the heart and how fast it contracts.

Normal stroke volume will depend on each individual patient, but it’s typically between 50 to 100 mL.

Backing up of traffic (too much blood) causes the left ventricle to swell or inflate because it’s trying its hardest to push blood out, going against the resistance of high blood pressure.

Decreased stroke volume = compromised cardiac output = left ventricular hypertrophy

Many of the same factors that regulate HR also impact cardiac function by altering SV. While a number of variables are involved, SV is ultimately dependent upon the difference between EDV and ESV. The three primary factors to consider are preload, or the stretch on the ventricles prior to contraction; the contractility, or the force or strength of the contraction itself; and afterload, the force the ventricles must generate to pump blood against the resistance in the vessels.

1.Preload:

Definition:

The initial stretching of the cardiac myocytes prior to contraction.

Determinants:

1. Ventricular volume (end-diastolic volume)
2. Venous return
3. Blood volume
4. Cardiac compliance

Factors affecting preload:

1. Volume status (hypovolemia/hypervolemia)
2. Venous tone
3. Cardiac function (heart failure)
4. Medications (diuretics, vasodilators)

2.Afterload:

Definition:

The resistance against which the heart must pump blood.

Determinants:

1. Systemic vascular resistance (SVR)
2. Arterial blood pressure
3. Cardiac output
4. Vascular compliance

Factors affecting afterload:

1. Blood pressure (hypertension/hypotension)
2. Vascular tone
3. Cardiac function (heart failure)
4. Medications (vasodilators, vasoconstrictors)

Relationship between Preload and Afterload:

1. Increased preload → Increased stroke volume (Frank-Starling mechanism)
2. Increased afterload → Decreased stroke volume
3. Decreased preload → Decreased stroke volume
4. Decreased afterload → Increased stroke volume

Clinical Implications:

1. Heart failure: Decreased preload, increased afterload
2. Hypovolemia: Decreased preload
3. Hypertension: Increased afterload
4. Septic shock: Decreased afterload, increased preload

3.Cardiac contractility

Cardiac contractility or myocardial contractility refers to the ability of the ventricles to contract during the systolic phase.

In other words, the force with which ventricular ejection occurs is called contractility of the heart.

Factors affecting cardiac contractility

Several factors affect the contractility of the heart.

Factors that increase cardiac contractility are:

• Hormonal changes. For example, adrenaline, glucagon, and thyroxine increase cardiac contractility.
• Activation of the parasympathetic nervous system also increases the heart’s contractility due to noradrenaline release.

Factors that decrease cardiac contractility are:

• Increased hydrogen ion concentration in the blood (known as acidemia).
• Increased potassium level in the blood (known as hyperkalemia)

Measurement:

1. Preload: Central venous pressure (CVP), pulmonary capillary wedge pressure (PCWP)
2. Afterload: Systemic vascular resistance (SVR), arterial blood pressure

Manipulation:

1. Preload: Fluid administration, diuretics
2. Afterload: Vasodilators, vasoconstrictors

Other Factors Influencing Heart Rate and Force of Contraction

Many factors affect HR and SV, and together, they contribute to cardiac function. HR is largely determined and regulated by autonomic stimulation and hormones. There are several feedback loops that contribute to maintaining homeostasis dependent upon activity levels, such as the atrial reflex, which is determined by venous return.

SV is regulated by autonomic innervation and hormones, but also by filling time and venous return. Venous return is determined by activity of the skeletal muscles, blood volume, and changes in peripheral circulation. Venous return determines preload and the atrial reflex. Filling time directly related to HR also determines preload. Preload then impacts both EDV and ESV. Autonomic innervation and hormones largely regulate contractility. Contractility impacts EDV as does afterload. CO is the product of HR multiplied by SV. SV is the difference between EDV and ESV.

REFERENCES

1. Kobe J, Mishra N, Arya VK, Al-Moustadi W, Nates W, Kumar B. Cardiac output monitoring: Technology and choice. Ann Card Anaesth. 2019 Jan-Mar;22(1):6-17. 
2. Huang SJ. Measuring cardiac output at the bedside. Curr Opin Crit Care. 2019 Jun;25(3):266-272. 
3. Kaufmann T, Clement RP, Hiemstra B, Vos JJ, Scheeren TWL, Keus F, van der Horst ICC., SICS Study Group. Disagreement in cardiac output measurements between fourth-generation FloTrac and critical care ultrasonography in patients with circulatory shock: a prospective observational study. J Intensive Care. 2019;7:21. 
4. Patel N, Durland J, Awosika AO, Makaryus AN. StatPearls [Internet]. StatPearls Publishing; Treasure Island (FL): Jun 8, 2024. Physiology, Cardiac Index.
5. Huber W, Zanner R, Schneider G, Schmid R, Lahmer T. Assessment of Regional Perfusion and Organ Function: Less and Non-invasive Techniques. Front Med (Lausanne). 2019;6:50.
6. Argueta EE, Paniagua D. Thermodilution Cardiac Output: A Concept Over 250 Years in the Making. Cardiol Rev. 2019 May/Jun;27(3):138-144.
7. Norton JM. Toward consistent definitions for preload and afterload. Adv Physiol Educ. 2001 Dec;25(1-4):53-61. 
8. Ernstmeyer K, Christman E, editors. Nursing Fundamentals [Internet]. 2nd edition. Eau Claire (WI): Chippewa Valley Technical College; 2024. PART IV, NURSING PROCESS. Available from: https://www.ncbi.nlm.nih.gov/books/NBK610818/

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