Physiology Note 1: Perfusion Pressures

 

PERFUSION PRESSURES

Contributed by Srinivas Samavedam

What is it?

Perfusion pressure is a term commonly used in the description of cardiovascular physiology, referring to the force that drives blood flow through the circulatory system to reach tissues and organs.

This pressure is essential for ensuring that tissues receive an adequate supply of oxygen and metabolic requirements while also facilitating the elimination of the products and by-products of tissue energy utilisation.

 Proper regulation of perfusion pressure is vital for maintaining homeostasis and overall functioning of organs

What is its importance?

Perfusion pressure is defined as the difference in pressure across a vascular bed.

This difference in pressure creates the driving force that enables blood to flow from arteries, through capillaries, and into veins.

Adequacy of perfusion of tissues and tissue oxygen delivery is determined by an effective perfusion pressure.

How is it calculated?

In clinical settings, perfusion pressure is often relative to specific organs, such as the brain or kidneys, and is calculated differently based on context. Defending and optimizing such pressures tailored for each organ remains a challenge among critically ill patients with simultaneous dysfunction of several organs

 

 1. Cerebral Perfusion Pressure (CPP): This is the most well known of all the perfusion pressures. It is crucial for brain perfusion and oxygen delivery and is calculated as the difference between Mean Arterial Pressure (MAP) and Intracranial Pressure (ICP). The formula is: 

   {CPP} = {MAP} - {ICP}

   Most interventions in neurocritical care are tailored to ensure an optimal CPP. Its normal range is 60 to 80 mm Hg. A decrease in the CPP suggests that the gradient required to push blood towards the brain is not being maintained.  This can cause brain ischemia from reduced cerebral blood flow. The fall in CPP can be caused by a fall in Mean Arterial Pressure on an increase in Intra Cranial Pressure or both.

 

 2. Renal Perfusion Pressure: This is vital for kidney function and often assessed by the difference between mean arterial pressure and central venous pressure or intra-abdominal pressure. Adequate renal perfusion pressure ensures a good glomerular filtration and tubular exchange. Among patients with volume overload both the CVP as well as the IAP will have an influence on the Renal Perfusion Pressure

In normal physiology, intact autoregulation ensures that the renal blood flow remains constant despite variations in renal perfusion pressure (RPP).

However, in critical illness, autoregulation may be impaired or lost, resulting in a direct pressure-flow dependency. Consequently, RBF may fall significantly despite a theoretically normal RPP.

 

3. Coronary Perfusion Pressure:

It is the pressure that drives the blood flow through the coronary arteries into the myocardium

It's calculated as the

Aortic diastolic pressure - left ventricular end-diastolic pressure

4. Abdominal Perfusion Pressure

APP is the difference between Mean Arterial Pressure (MAP) and Intra-Abdominal Pressure (IAP), and it reflects the pressure available for perfusion of abdominal organs

 

 

* Factors Influencing Perfusion Pressure

Several physiological and pathological factors can influence perfusion pressure:

Blood Pressure: It is the primary determinant of perfusion pressure. Both systemic hypertension and hypotension affect perfusion pressure by altering the force driving blood flow through the circulatory system.

Vascular Resistance: The resistance within blood vessels can influence perfusion pressure. Vasoconstriction (narrowing of blood vessels) increases resistance and can decrease perfusion, while vasodilation (widening) decreases resistance and increases flow.

Cardiac Output: This is the amount of blood the heart pumps per minute. A decrease in cardiac output, such as in heart failure, can lead to reduced perfusion pressure, compromising blood delivery to organs.

*Clinical Significance

Understanding and monitoring perfusion pressure is essential in critical care.                   

In critical care settings, maintaining adequate perfusion pressures is crucial for patients with conditions such as shock, sepsis, or traumatic injury. Treatments may involve medications to support blood pressure, fluids to expand blood volume, or interventions to relieve increased intracranial or intra-abdominal pressures. Perfusion pressures may be one of the ways to achieve personalized BP targets

 Conclusion

Perfusion pressure is a fundamental concept in physiology, pivotal for the delivery of blood to tissues and organs. Its maintenance is critical for overall health, and disturbances in perfusion pressure can lead to significant clinical issues. Understanding the mechanisms and influences on perfusion pressure allows for better management and treatment of conditions that affect blood flow and tissue oxygenation.