Systemic Vascular Resistance (SVR) quantifies the resistance the heart must overcome to pump blood into the body’s circulation. It is a fundamental measurement used in clinical settings to assess a person’s hemodynamic status, or the mechanics of blood flow. Understanding SVR allows medical professionals to gauge the tone of the blood vessels and determine if they are too constricted or too relaxed.
Understanding Systemic Vascular Resistance (SVR)
SVR represents the total resistance offered by all systemic blood vessels, excluding those in the lungs. It is often referred to as “afterload,” which is the pressure the left ventricle must push against to eject blood into the aorta. SVR is analogous to friction inside a hose; a narrower diameter creates more resistance, forcing the heart to work harder.
The physical diameter of the small arteries, or arterioles, is the primary factor determining SVR. When these vessels constrict, SVR increases, forcing the heart to exert more effort to push blood through the circulatory system. Conversely, when vessels dilate, SVR decreases, making it easier for the heart to pump blood. This dynamic measurement of vessel tone regulates blood pressure and the delivery of oxygen and nutrients to tissues.
Essential Variables for the SVR Formula
Calculating SVR requires three specific hemodynamic measurements: Mean Arterial Pressure (MAP), Central Venous Pressure (CVP), and Cardiac Output (CO). These variables define the pressure gradient and the flow rate within the circulatory system. They are typically obtained using specialized monitoring devices in a clinical setting.
Mean Arterial Pressure (MAP)
MAP represents the average pressure in the arteries during one complete cardiac cycle. It is the driving force that pushes blood through the systemic circulation. MAP is often estimated using the formula: Diastolic Blood Pressure plus one-third of the Pulse Pressure (Systolic minus Diastolic).
Central Venous Pressure (CVP)
CVP is the pressure of the blood in the large veins, specifically in the right atrium. It represents the pressure where blood returns to the heart at the end of the systemic circulation. Obtaining an accurate CVP reading usually requires the placement of an invasive monitoring catheter, often called a central line.
Cardiac Output (CO)
CO is the volume of blood the heart pumps per minute and serves as the flow component of the calculation. It is determined by multiplying the Stroke Volume (blood pumped per beat) by the Heart Rate (beats per minute).
Step-by-Step Calculation and Units
The calculation of SVR is based on a modification of Ohm’s Law, where resistance equals the difference in pressure divided by flow. The standard formula uses the three variables (MAP, CVP, and CO) to calculate the resistance to blood flow: SVR = [(MAP – CVP) / CO] x 80.
The first step is determining the pressure gradient by subtracting CVP from MAP. This difference represents the total pressure drop across the systemic circulation. For example, if a patient has a MAP of 95 mmHg and a CVP of 5 mmHg, the pressure gradient is 90 mmHg.
The second step is dividing this pressure gradient by the Cardiac Output (measured in L/min). If the pressure gradient is 90 mmHg and the Cardiac Output is 5 L/min, the initial result is 18 peripheral resistance units (PRU).
The final step involves multiplying this initial result by the conversion factor of 80. This factor converts the result from PRU into the standard clinical unit, dynes \(\cdot\) s \(\cdot\) cm\(^{-5}\). Using the prior example, 18 multiplied by 80 yields an SVR of 1,440 dynes \(\cdot\) s \(\cdot\) cm\(^{-5}\).
Interpreting Calculated SVR Values
The calculated SVR value provides objective data characterizing a person’s circulatory status. The normal range for Systemic Vascular Resistance is generally accepted to be between 800 and 1200 dynes \(\cdot\) s \(\cdot\) cm\(^{-5}\). Values within this range suggest appropriate vascular tone, allowing for efficient blood flow and adequate tissue perfusion.
A high SVR (above 1200 dynes \(\cdot\) s \(\cdot\) cm\(^{-5}\)) indicates severe vasoconstriction, or tightening of the blood vessels. This increased resistance forces the heart to work against a greater “load,” often seen in conditions like hypertension or cardiogenic shock. Managing high SVR typically requires medications that cause vasodilation to reduce strain on the heart.
A low SVR (below 800 dynes \(\cdot\) s \(\cdot\) cm\(^{-5}\)) suggests widespread vasodilation, where blood vessels are overly relaxed. This is common in distributive shock states, such as septic shock or anaphylaxis, where loss of vessel tone causes blood pressure to drop. Treatment often involves using vasopressor medications to increase vascular tone and restore resistance.