Total Peripheral Resistance (TPR) is a measurement reflecting the overall opposition to blood flow within the systemic circulatory system. Understanding TPR provides insight into how efficiently blood circulates and how blood pressure is maintained throughout the body.
Understanding Total Peripheral Resistance
Total Peripheral Resistance represents the sum of all resistances offered by blood vessels in the systemic circulation. This resistance is primarily determined by the diameter of the arterioles, which are small arteries. Changes in their diameter, through either constriction or dilation, significantly influence the overall resistance to blood flow. When these vessels narrow, resistance increases, and when they widen, resistance decreases, both of which affect blood pressure and circulation.
The Core Calculation Formula
The core formula for Total Peripheral Resistance relates mean arterial pressure and cardiac output: TPR = MAP / CO. TPR stands for Total Peripheral Resistance, MAP for Mean Arterial Pressure, and CO for Cardiac Output. The standard unit of measurement for TPR is millimeters of mercury per minute per liter (mmHg·min/L), sometimes referred to as Peripheral Resistance Units (PRU) or Wood units.
Calculating Key Components
Before calculating Total Peripheral Resistance, two components, Mean Arterial Pressure (MAP) and Cardiac Output (CO), must be determined. Mean Arterial Pressure is the average arterial pressure during a single cardiac cycle, providing a comprehensive view of the pressure driving blood through the organs. It is calculated using the formula: MAP = Diastolic Pressure + 1/3 (Systolic Pressure – Diastolic Pressure). For example, if a blood pressure reading is 120/80 mmHg, the systolic pressure is 120 mmHg and the diastolic pressure is 80 mmHg.
Cardiac Output is the volume of blood pumped by the heart per minute. It is derived from two measurements: Heart Rate (HR) and Stroke Volume (SV). The formula for cardiac output is: CO = Heart Rate × Stroke Volume. Heart Rate is the number of times the heart beats per minute. Stroke Volume is the amount of blood ejected from the left ventricle with each heartbeat.
Putting It All Together: A Practical Example
To illustrate TPR calculation, consider a hypothetical individual with a systolic blood pressure of 120 mmHg, a diastolic blood pressure of 80 mmHg, a heart rate of 70 beats per minute, and a stroke volume of 70 milliliters per beat. First, calculate Mean Arterial Pressure (MAP): MAP = 80 mmHg + 1/3 (120 mmHg – 80 mmHg) = 93.33 mmHg.
Next, determine Cardiac Output (CO). Convert the stroke volume from milliliters to liters (70 mL = 0.07 L). CO = 70 beats/min × 0.07 L/beat = 4.9 L/min. Finally, calculate TPR using the core formula: TPR = MAP / CO. TPR = 93.33 mmHg / 4.9 L/min = 19.05 mmHg·min/L. This value represents the overall resistance to blood flow in this individual’s systemic circulation.