Beach chair position: phenylephrine holds the pressure and loses the saturation

Design

2x2 factorial randomised controlled trial with time-resolved near-infrared spectroscopy and diffuse correlation spectroscopy

Population

40 adults (39 analysed) undergoing shoulder surgery in the beach chair position

Primary outcome

Relative cerebral tissue oxygen saturation and relative cerebral metabolic rate of oxygen over 30 minutes

Effect

No main effect of anaesthetic or vasopressor; phenylephrine gave a 9% greater 30-minute decline in saturation (interaction P=0.004)

Forty adults having shoulder surgery in the beach chair position were randomised in a 2x2 design to propofol or sevoflurane maintenance and to a phenylephrine infusion or ephedrine boluses, with cerebral tissue oxygen saturation and cerebral metabolic rate of oxygen measured directly by a hybrid optical system across the first 30 minutes after positioning.

Neither anaesthetic nor vasopressor changed the 30-minute averages. The time course did change. Saturation fell and metabolic rate rose over the half hour in everyone, and phenylephrine accelerated both: a 9 per cent greater fall in relative saturation, and a rising relative metabolic rate.

The practical reading is not to abandon phenylephrine. It is that in the position where cerebral perfusion is most vulnerable, a pressure restored by peripheral vasoconstriction may not be a brain that is better perfused — and 30 minutes of drift would be invisible without a saturation monitor. Thirty-nine patients analysed, one centre, one position, half an hour of data.

• Consider cerebral oximetry for beach chair cases, especially in older patients and long lists.
• Where phenylephrine is used in this position, watch the trend rather than a single reading.
• Measure blood pressure at the level of the brain, not the arm, in the beach chair.
• Do not read this as evidence that ephedrine improves outcomes; no clinical outcome was measured.
• Thirty minutes of data cannot describe a two-hour shoulder reconstruction.

Why it matters

It separates the number the monitor shows from the perfusion the drug was given to achieve.

Don't overread it

Surrogate physiological endpoints in 39 patients over 30 minutes; no neurological or clinical outcome was assessed.

The statistics, in plain English

Coprimary outcomes with no significant main effect and a significant time-by-treatment interaction is a pattern that needs care: the averages were the same, and what differed was the direction of travel. With 39 patients across four cells — around ten each — an interaction term is fragile, and the analysis of temporal change was one of several that could have been run. This is physiology worth following up, not a comparison ready to act on.

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