Preventing cerebral hypoperfusion is crucial for patient safety. Avoiding cerebral hypotension during the Beach Chair position can be difficult. Although a MAP of 50 is often considered the minimum for cerebral autoregulation. However, this level could lead to hypoperfusion or potentially stroke in anesthetized patients. It fails to account for individual patient history. Understanding how sitting affects hemodynamics is key to addressing these risks.
Case
My patient was an 80-year-old, 6 foot tall man scheduled for rotator cuff repair in a surgery center. He was healthy with no medical problems, no medications, and a resting blood pressure of 120/80. The surgeon planned on using the beach chair position. He preferred the angle of the captain’s table to be quite high, about 80 degrees. He requested a preoperative interscalene block for postoperative analgesia.
We now perform more complex outpatient surgeries on higher risk patients. Surgery centers have limited emergency support and lack invasive blood pressure monitoring. Careful patient selection is essential for these facilities. I assessed the risks.
Despite being healthy, the patient’s advanced age increased the risk of stroke and heart attack. The sitting position increased the risk of decreased cerebral perfusion. A successful regional block would eliminate surgical stimulus from the incision. When paired with a sitting position, that combination could cause significant systemic and cerebral hypotension. Being at a surgery center, my only monitor would be a blood pressure cuff.
The Anesthetic
We decided to continue with surgery since my elderly patient was healthy and active. To wanted to ensure that his mean arterial pressure (MAP) remained adequate in the sitting position. Therefore, I prepared a phenylephrine infusion using the syringe pump that we typically used for propofol sedation.
I performed an awake, sedated interscalene block with the patient on the captain’s table. To avoid hypotension on induction, I combined titrated propofol and forane with a non-depolarizing muscle relaxant to intubate. Post induction, we slowly raised the angle to 80 degrees. During the procedure I titrated anesthetic, fluids and phenylephrine to maintain his mean arterial pressure (MAP) higher than 80.
I used mean arterial pressure (MAP) because it’s a better representation of average organ blood flow. It is more accurate than using systemic arterial pressure (SAP) alone. MAP is the average arterial pressure throughout one cardiac cycle, systole and diastole. It’s a calculated value influenced by cardiac output and systemic vascular resistance. Most vital signs monitors will calculate the MAP automatically. The formula used for MAP is:
MAP= (2 x diastolic BP + systolic BP)/3
Surgery went well and the patient awoke comfortable and neurologically intact.
What Is the Sitting Position?
The sitting position is also called the Fowler’s or beach chair position. The patient’s head is elevated between 30 – 90 degrees above the horizontal plane. The head and face are typically padded, held securely in position using a headrest and various forms of facial restraint.
This position offers advantages in neurosurgery, orthopedics, and ENT procedures. It gives the surgeon better access to the operative site. It also lowers the chance of some complications in the lateral position like brachial plexus injury.
Hemodynamics and Gravity
Adequate blood flow to organs is essential. Mathematically flow is equal to the pressure gradient divided by the resistance (Ohm’s Law). To increase blood flow one must either increase the pressure difference (cardiac force), decrease the resistance, or both. The greater the resistance, the lower the flow.
Gravity creates a hydrostatic pressure gradient; this represents increased resistance to upward blood flow when we sit or stand. To quantitate the hydrostatic pressure gradient, there is an approximate 0.77 mmHg decrease for every 1 cm (1 mmHg for every 1.25 cm).
Cerebral perfusion pressure (CPP) = mean arterial pressure (MAP) – Intracranial pressure (ICP)
Anything that raises MAP increases CPP. In contrast, lowering MAP decreases CPP. Finally, raising ICP also lowers CPP.
When a person sits up or stands, their CPP will tend to drop. This occurs because gravity pulls blood to the legs, reducing blood flow return to the heart. However, the drop in non-anesthetized patients who are upright tends to be limited. CPP falls less than 15% due to a 50–80% increase in systemic vascular resistance caused by arterial constriction. Vasoconstriction maintains or even increases MAP and SAP. If compensatory vasoconstriction fails, such as with orthostatic hypotension, rapid standing can cause dizziness. It can also lead to fainting due to cerebral hypotension. Of note, anesthesia diminishes this reflex.
Autoregulation
Vasoconstriction is important. However, cerebral autoregulation is key to keeping cerebral perfusion just right. It ensures there is not too much and not too little perfusion. Avoidance of cerebral hypotension is the goal.
The brain is a closed system containing parenchyma, cerebrospinal fluid, and blood sealed inside the skull. An uncompensated increase in any component raises intracranial pressure, while reduced blood flow can cause ischemia. Autoregulation also ensures that cerebral perfusion pressure (CPP) is closely regulated according to metabolic needs.
Cerebral autoregulation keeps blood flow stable despite changes in systemic blood pressure, mainly by altering cerebral vessel diameter. This process is active between a MAP of 60–150 mmHg. It involves myogenic, neurogenic, endothelial, and metabolic mechanisms to prevent damage. This helps manage excessive or insufficient blood flow. Below 60, or above 150 autoregulation starts to fail.
Factors like chronic hypertension, recent brain injury, and anesthesia can shift both the MAP curve and range of autoregulation. This means that this MAP autoregulatory range is not universal. It must be individualized to the patient and the circumstances, such as a sitting position. (see Intraoperative Hypotension: When Is a “Good” BP Bad)
Physiological Changes in the Sitting Position
Cerebral hypoperfusion can easily occur in the sitting position for a variety of reasons. These include reliance on a BP cuff located lower than the brain without taking the difference in values into account. Other reasons are venous pooling and the effects of mechanical ventilation.
Blood Pressure Cuff Location
Accurate monitoring is essential to detect hypotension. However, difficulties occur when the BP cuff, or arterial line transducer, and brain are positioned at different heights. Peripheral MAP varies significantly from cerebral MAP in the sitting position due to the hydrostatic gradients we discussed earlier. A normal peripheral SAP/mAP can coexist with cerebral hypotension.
When lying down, MAP in the arm matches that of the brain. Sitting causes cerebral MAP to drop below arm MAP. This occurs due to gravity, as hydrostatic pressure creates a gradient between the two.
When sitting at 90 degrees, the MAP at the brain base is 15–20 mmHg less than at the arm. It can be another 9 mmHg lower at the top of the cerebral cortex. If you don’t take this difference into account, your patient’s brain may experience poor perfusion. This can happen even if the cuff pressure seems normal.
An Example
For example, take our patient who had an arm BP (MAP) of 120/80 (MP of 93). The arm cuff was about 20 cm below the external auditory meatus. This is roughly the level of the base of the brain and the Circle of Willis. Using the hydrostatic gradient formula, 20 cm would equal 20 x 0.77 mmHg or a decrease of about 15 mmHg.
BP/MAP at the base of the brain would therefore be about 105/65 (MAP of 78).
The top of the cerebral cortex is about 10-12 cm higher than the base of the brain. This difference represents a further decrease of about 9 mmHg. So there, BP/MAP would be about 86/56 (MAP of 69).
Use of a leg cuff further exaggerates the difference between cuff pressure and brain pressure.
You can start to see the importance of taking this difference into account. This is crucial when deciding what an acceptable blood pressure/MAP should be during the case.
For another example, surgeons might ask me for controlled hypotension. I have been asked to drop SAPs into the 80-100 mmHg range to minimize blood loss. This would correspond to cuff MAPs of 50-80. However, at the brain’s cortex MAP would be in the 40s mmHg — way too low. This illustration shows the variation in BP/MAP with an SAP of 80 in the sitting position.
Drummond has recommended maintaining a cerebral MAP of 70-93. The mean should be 80 mmHg when the patient is in the sitting position.
Risks of Cerebral Hypoperfusion in the Sitting Position
Clearly the sitting position increases risks of hypoperfusion during anesthesia.
- Cerebral Hypoperfusion: The most serious risk of hypotension in the sitting position is decreased blood flow to the brain. As the head is elevated above the heart, cerebral perfusion pressure drops. If blood pressure falls too low, this can result in cerebral ischemia, leading to stroke, brain injury, or even death. Other causes of cerebral hypoperfusion include excessive flexion of the neck, which can impede arterial inflow or venous outflow.
- Cardiac Complications: Reduced blood pressure may compromise coronary perfusion. This increases the risk of myocardial ischemia or infarction. The risk is especially high in patients with preexisting heart conditions.
- Spinal Cord Ischemia: In spine surgeries performed in the sitting position, hypotension can reduce perfusion to the spinal cord. This increases the risk of neurological deficits.
- Organ Dysfunction: Other organs might suffer from inadequate blood flow. These include the kidneys and liver. This can result in postoperative complications.
Factors That Increase the Risk of Hypotension
Careful attention must be paid to tailor the anesthetic to your patient’s status and medical history.
- Type and Depth of Anesthesia: General anesthetics and certain regional blocks can cause vasodilation and decrease sympathetic tone.
- Patient Characteristics: Elderly patients, those with autonomic dysfunction, or those with preexisting cardiovascular disease are particularly vulnerable.
- Intraoperative Events: Blood loss, dehydration, and the use of vasodilatory drugs can further decrease blood pressure.
Strategies to Prevent Cerebral Hypotension
1. Preoperative Optimization: Assess and correct volume status, and review medications that may affect blood pressure.
2. Monitoring:
- Factor in the location of the blood pressure cuff, as well as the patient’s baseline MAP. Remember the autoregulation shifts with chronic hypertension
- Use invasive blood pressure monitoring (such as arterial lines) for early detection of hypotension in high-risk patients. Place the transducer at the level of the brain
- If your high-risk patient is scheduled at a surgery center, carefully consider whether this is the best strategy
3. Positioning: Gradually transition the patient to the sitting position to allow time for physiological adaptation.
4. Pharmacological Support: Administer vasopressors and intravenous fluids as needed to maintain adequate blood pressure and organ perfusion.
5. Communication: Ensure clear communication among the surgical and anesthesia teams to promptly recognize and manage hypotension.
Conclusion
Treating every patient as though an MAP of 50 is safe is suboptimal. This approach is especially concerning if the patient is in the sitting position. Avoidance of cerebral hypotension during anesthesia in the sitting position is of paramount importance. Vigilant monitoring, proactive management, and a thorough understanding of the physiological risks are essential to prevent serious complications. By prioritizing blood pressure stability, you can improve patient safety. You can also enhance surgical outcomes in procedures performed in the sitting position.
Further Reading
APSF Newsletter. Circulation 81,489 • Volume 22, No. 2 • Summer 2007
