What is systemic vascular resistance (SVR)?

From the Guidelines

Systemic vascular resistance (SVR) is the total resistance to blood flow in the systemic circulation, representing the force that the left ventricle must overcome to pump blood through the arterial system, and is crucial in maintaining a balance between pulmonary vascular resistance (PVR) and systemic blood pressure, particularly in critically ill patients with pulmonary arterial hypertension (PAH) 1.

Definition and Calculation of SVR

SVR is primarily determined by the diameter of small arteries and arterioles, which can constrict or dilate in response to various stimuli. The calculation of SVR is based on the formula: SVR = (Mean Arterial Pressure - Central Venous Pressure) / Cardiac Output, as mentioned in the context of managing PAH patients in the ICU 1.

Clinical Significance of SVR

In the management of PAH, maintaining SVR greater than PVR is a major guideline to prevent right ventricular ischemia, which can occur if the gradient shifts during systole to a state in which PVR exceeds SVR 1. This highlights the importance of SVR in clinical practice, particularly in critical care settings where the balance between SVR and PVR can significantly impact patient outcomes.

Factors Influencing SVR

Factors that influence SVR include sympathetic nervous system activity, hormones (e.g., angiotensin II, vasopressin), and local metabolic factors, all of which can be relevant in the context of PAH and other cardiovascular conditions 1. Understanding these factors is essential for the effective management of SVR and the prevention of complications associated with its imbalance.

Management of SVR in Clinical Practice

In clinical practice, particularly in the ICU setting, the management of SVR involves the careful selection of inotropes and vasopressors, as well as the use of therapies like inhaled nitric oxide (iNO) to decrease PVR and improve cardiac output without detrimental effects on SVR 1. The goal is to maintain optimal SVR levels to ensure adequate blood pressure and organ perfusion, which is critical for morbidity, mortality, and quality of life outcomes.

From the Research

Definition of Systemic Vascular Resistance (SVR)

• Systemic vascular resistance (SVR) refers to the resistance offered by the systemic circulation to the flow of blood 2, 3, 4, 5, 6
• It is an important factor in determining cardiac output and blood pressure

Factors Affecting SVR

• SVR is affected by various factors, including:
  • Vasoconstrictor and vasodilator mechanisms 3
  • Exercise-induced vasodilatation 3
  • Pharmacological manipulation of afterload 2, 6
  • Use of vasoactive drugs 5

Measurement and Clinical Significance of SVR

• SVR can be measured using techniques such as transpulmonary thermodilution 4
• Changes in SVR can have significant clinical implications, including:
  • Effects on cardiac output and blood pressure 2, 4, 5
  • Requirements for vasoactive drugs 5
  • Outcomes in patients with cardiopulmonary resuscitation 3

Clinical Situations Involving SVR

• SVR is an important consideration in various clinical situations, including:
  • Pediatric cardiac surgery 2, 6
  • Hypothermic cardiopulmonary bypass 5
  • Septic shock 4
  • Exercise-induced reduction in SVR 3