Systemic Vascular Resistance (SVR) and the Heart

Introduction

Systemic Vascular Resistance (SVR) is a critical parameter in cardiovascular physiology. It represents the resistance that the systemic circulation offers to blood flow, primarily influenced by the diameter of blood vessels and the tone of the vascular smooth muscle.

What is SVR?

SVR is a measure of the resistance blood faces when flowing through the body's peripheral circulation. It is determined by: - Blood vessel diameter – Narrower vessels increase resistance, while wider vessels reduce it. - Vascular tone – The contraction state of the smooth muscles in artery walls. - Blood viscosity – Thicker blood leads to higher resistance.

Formula for SVR: [ SVR = \frac{(Mean Arterial Pressure - Right Atrial Pressure)}{Cardiac Output} ] Measured in dynes·sec·cm⁻⁵.

SVR and Heart Function

SVR plays a crucial role in cardiovascular health. Here’s how it affects the heart:

1. High SVR (Increased Afterload):

   • Increases the workload of the heart, particularly the left ventricle.
   • Can contribute to conditions like hypertension, left ventricular hypertrophy, and heart failure.

2. Low SVR (Decreased Afterload):

   • Reduces resistance, which can happen in sepsis, vasodilatory shock, or anaphylaxis.
   • May lead to hypotension and inadequate organ perfusion.

Common Conditions Affecting SVR

1. Hypertension (High SVR): Chronic high vascular resistance increases heart strain.
2. Sepsis (Low SVR): Widespread vasodilation leads to reduced SVR and low blood pressure.
3. Heart Failure: The heart struggles to overcome high SVR, leading to inadequate cardiac output.
4. Aortic Stenosis: A narrowed aortic valve increases afterload, mimicking high SVR effects on the heart.

Treatment Considerations

• High SVR: Treated with vasodilators (e.g., ACE inhibitors, calcium channel blockers) to reduce resistance.
• Low SVR: Managed with vasopressors (e.g., norepinephrine, vasopressin) to restore vascular tone.

Conclusion

SVR is a key determinant of cardiovascular function, affecting the heart’s workload and systemic blood flow. Its proper regulation is essential in treating various cardiac and circulatory disorders.

Source recommendations

1. American Heart Association Guidelines on Hypertension

1. https://www.heart.org/en/health-topics/high-blood-pressure
2. https://www.ahajournals.org/doi/10.1161/hyp.0000000000000065
3. https://www.heart.org/-/media/Files/Professional/Quality-Improvement/Pain-Management/AHA-GUIDELINEDRIVEN-MANAGEMENT-OF-HYPERTENSION--AN-EVIDENCEBASED-UPDATE.pdf
4. https://professional.heart.org/en/science-news/2017-hypertension-clinical-guidelines
5. https://www.aafp.org/pubs/afp/issues/2018/0315/p413.html

2. European Society of Cardiology Guidelines on Heart Failure

1. https://www.escardio.org/Guidelines/Clinical-Practice-Guidelines/Acute-and-Chronic-Heart-Failure
2. https://academic.oup.com/eurheartj/article/42/36/3599/6358045
3. https://www.escardio.org/Guidelines/Clinical-Practice-Guidelines/Focused-Update-on-Heart-Failure-Guidelines
4. https://academic.oup.com/eurheartj/article/44/37/3627/7246292
5. https://www.escardio.org/Guidelines/Clinical-Practice-Guidelines

3. Surviving Sepsis Campaign Guidelines

1. https://www.sccm.org/clinical-resources/guidelines/guidelines/surviving-sepsis-guidelines-2021
2. https://journals.lww.com/ccmjournal/fulltext/2021/11000/surviving_sepsis_campaign__international.21.aspx
3. https://www.sccm.org/survivingsepsiscampaign
4. https://pubmed.ncbi.nlm.nih.gov/34599691/
5. https://link.springer.com/article/10.1007/s00134-021-06506-y