Systemic vascular resistance (SVR) refers to the resistance offered by the systemic circulation against the flow of blood through blood vessels. It is a key determinant of blood pressure and plays a significant role in cardiovascular homeostasis. Measuring SVR provides valuable insights into a patient’s hemodynamic status and helps in diagnosing and managing various cardiovascular disorders. This article will guide you on how to calculate systemic vascular resistance and its clinical applications.

Formula for calculating Systemic Vascular Resistance

The formula for calculating SVR is as follows:

SVR = (MAP – CVP) / CO

Where:

– SVR: Systemic Vascular Resistance (measured in dynes·sec/cm^5)

– MAP: Mean Arterial Pressure (measured in mmHg)

– CVP: Central Venous Pressure (measured in mmHg)

– CO: Cardiac Output (measured in L/min)

How to measure the variables?

1. Mean Arterial Pressure (MAP): MAP is an indicator of overall perfusion pressure experienced by organs. It is calculated using the following formula:

MAP = [(2 x DBP) + SBP] / 3

Where:

– DBP: Diastolic Blood Pressure

– SBP: Systolic Blood Pressure

2. Central Venous Pressure (CVP): CVP represents the pressure within the thoracic vena cava near the proximity of the right atrium. It can be measured invasively, using a central venous catheter inserted into a large vein, or estimated non-invasively by assessing jugular venous distension.

3. Cardiac Output (CO): CO is defined as the volume of blood ejected by the heart per minute and serves as a critical assessment of cardiac function. It can be measured using various approaches such as thermodilution, echocardiography (Doppler-based), or non-invasive techniques like bioimpedance.

Working through an example

Consider the following values:

– SBP: 120 mmHg

– DBP: 80 mmHg

– CVP: 5 mmHg

– CO: 5 L/min

Firstly, calculate the MAP:

MAP = [(2 x 80) + 120] / 3 = [160 + 120] / 3 = 280 / 3 ≈ 93.3 mmHg

Now, substitute the values into the SVR formula:

SVR = (93.3 – 5) / 5 = 88.3 / 5 ≈ 17.6 dynes·sec/cm^5

In this example, the calculated SVR is approximately 17.6 dynes·sec/cm^5.

Clinical Applications

Calculating systemic vascular resistance helps in diagnosing and managing various medical conditions, including:

1. Hypertension: High SVR values contribute to a higher blood pressure, which is a primary risk factor for cardiovascular diseases.

2. Heart failure: Low or elevated SVR values can suggest cardiac dysfunction and indicate therapeutic adjustments.

3. Shock: Low SVR indicates distributive shock (e.g., septic shock), while high SVR is seen in some forms of cardiogenic shock.

4. Pulmonary hypertension: Normalizing SVR helps in assessing the pulmonary vascular resistance to diagnose pulmonary hypertension.

Conclusion

Understanding how to calculate systemic vascular resistance aids healthcare professionals in evaluating a patient’s hemodynamic status and making informed clinical decisions. Accurate measurement of variables such as mean arterial pressure, central venous pressure, and cardiac output is essential for correct calculations.