Why do we care about a patient’s blood pressure?
Why do we get worried when the number on the monitor drops?
Blood pressure is really a proxy for the thing we actually care about; perfusion.
The tissues require perfusion to supply good stuff (such as oxygen, nutrients, immune factors, hormones, clotting factors etc) and get rid of waste (carbon dioxide, lactate, urea and creatinine, and hydrogen ions).
The microcirculation (capillaries, predominantly) is really the most important area of circulation, as this is where perfusion specifically occurs. But because it is not directly measurable, we use measurements of the macrocirculation (such as blood pressure), as well as clinical assessment, to get our closest approximation of perfusion.
“Capillaries are the business end of the circulatory system. In fact, maintaining an uninterrupted supply of blood to the capillaries is basically the purpose of the entire circulatory system.” – Alex Yartsev, Deranged Physiology
The ability to perfuse tissues and organs relies on an adequate pressure to drive blood toward them (perfusion pressure), as well as conditions that enable that blood to actually reach the target tissues (perfusion flow).
- Perfusion pressure = mean arterial pressure (MAP – driving pressure) – central venous pressure (CVP – opposing pressure)
Mean arterial pressure (MAP) is the mean, constant arterial pressure within the arteries during one full cardiac cycle. MAP provides the driving pressure for perfusion, whereas central venous pressure (CVP) is the downstream back-pressure within the thorax around the vena cava and right atrium. In health, CVP approaches zero mmHg, and so is essentially ignored. Pathological states that raise CVP such as pulmonary hypertension, right heart failure, massive PE and so on can alter this pressure gradient and negatively affect perfusion. Without a perfusion pressure, blood cannot flow to the end organs.
- Perfusion flow = mean arterial pressure (MAP) / regional vascular resistance (RVR)
Perfusion flow is the actual delivery of blood to the vascular beds of the target organs. It is determined by the central driving pressure (MAP) and the degree to which that pressure is transmitted through the target vascular beds, something which is tightly controlled by regional vascular resistance (RVR).
For the purposes of this illustration, imagine our body as like a tank of water.
The pressure in the tank (MAP) delivers water (blood) via a valve to individual flower beds (vascular beds). The degree to which the valves are open (RVR) determines how much of the water actually reaches each flower bed. Similarly, an adequate MAP may not guarantee perfusion if the RVR is high enough to impede blood flow.
Organ perfusion requires an adequate driving pressure (MAP-CVP) and the ability of that blood to be delivered to the target tissues (MAP/RVR).
Each organ has its own homeostatic set point of perfusion pressure, which is determined by the gradient between MAP and the local pressures around the target organs and tissues. When MAP falls below the lower limit of RVR autoregulation for any given tissue, blood flow, and thus perfusion, becomes linearly dependent on MAP. At this point, arterioles will be maximally dilated in an effort to maintain adequate blood flow.
The brain as an example:
A cerebral perfusion pressure below 50-60 mmHg risks suboptimal delivery of blood to brain tissue, which may lead to cerebral ischaemia. The intracranial pressure (ICP) normally sits around 5-15 mmHg, and so a MAP of 65 mmHg is required to ensure adequate cerebral perfusion. Increased intracranial pressure (such as a bleed, tumour, hydrocephalus and so on) will alter the pressure gradient by increasing the downstream intracranial pressure and may require a higher MAP to compensate.
Cerebral perfusion pressure = mean arterial pressure – intracranial pressure
Whilst there is no magic number that guarantees perfusion to all organs, a MAP of ≥65 mmHg is commonly targeted in clinical practice, with room up or down depending on the specific clinical context. A lower MAP is tolerated with a ruptured aortic aneurysm, in order to minimise haemorrhage, whereas a higher target may be set in a traumatic brain bleed, to prioritise cerebral perfusion, for example.
Why doesn’t blood pressure equal tissue perfusion?
We can imagine a situation where blood pressure is reassuringly normal, but CVP is similarly high due to right ventricular failure, resulting in central venous congestion, reduced venous return, poor cardiac output and compromised global tissue perfusion. This ‘normal’ blood pressure would be reliant on compensatory systemic vascular resistance, and would likely worsen as the RV failure affected preload and cardiac output. Another example is the patient whose reassuring MAP is sustained by vasopressors, but appears mottled and purple due to excessive pharmacological vasoconstriction, impeding actual blood flow to tissues.
On the other hand, someone may live with apparent hypotension without any ill-effect, as their tissue and organ perfusion is perfectly adequate, whereas a usually hypertensive patient may experience end-organ ischaemia with a ‘normal’ blood pressure due to acute changes in their usual raised baseline without adequate compensation.
One must start by remembering that the objective of increasing arterial pressure is to increase blood flow to tissues, and if that does not happen, the rise in pressure is of no value.
Blood pressure is required for, but not a guarantee of, perfusion.
Shock is defined by inadequate perfusion, regardless of the blood pressure.
There are many ways to assess perfusion outside of numerical blood pressure readings: volume status, urine output, clinical exam, biochemistry, echocardiography and so on. But without an adequate driving pressure, all these measures will deteriorate.
Given that MAP is the driving pressure that enables perfusion, we need to understand what determines it.
MAP = CO x SVR
This equation simply states that a heart (CO = cardiac output) and a circulatory system (SVR = systemic vascular resistance) will produce a pressure (MAP).
Unpacking these concepts will be the focus over the remainder of this series.