Anesthetic considerations of surgery during sitting position

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Anesthetic Considerations for Surgery in the Sitting Position

The sitting (or semi-recumbent/"beach chair") position is used primarily in posterior fossa neurosurgery (access to the fourth ventricle, cerebellopontine angle, quadrigeminal plate, vermis, pineal region) and shoulder/elbow surgery. While it provides excellent surgical access and reduces intraoperative bleeding, it carries a constellation of unique anesthetic hazards that demand meticulous preparation and monitoring.

1. Indications and Alternatives

The sitting position is chosen when access to midline posterior fossa structures is needed. Surgeons who are concerned about its risks may opt for:

Prone position
Semilateral or lateral decubitus

Contraindications to the sitting position include:

Patent foramen ovale (PFO) - risk of paradoxical air embolism
Vascular insufficiency when awake
Poorly controlled hypertension
Severe cardiomyopathy
Significant cervical spine degenerative disease with stenosis

2. Achieving the Position Safely

The patient is typically semirecumbent rather than truly sitting:

Legs elevated (with pillows under the knees) to promote venous return
Hip flexion kept less than 90 degrees to minimize sciatic nerve stretch
Arms supported with shoulders slightly elevated to avoid brachial plexus traction
Feet supported and padded
Head fixed with rigid pins (Mayfield) or a special headrest

Critical: The head holder support should be attached to the back portion of the table - not to the thigh section. This allows emergency head-down positioning and closed chest compressions without first detaching the head holder.

Neck flexion: There must be a minimum of 2-3 fingerbreadths between the chin/mandible and the sternum/clavicle to prevent jugular vein compression, macroglossia, and cord ischemia. The preoperative cervical range of motion should always be assessed.

3. Cardiovascular Effects and Management

This is the first major concern after patient positioning.

Hemodynamic changes: MAP is relatively preserved (due to reflex increase in SVR), but wedge pressure, stroke volume, and cardiac index decrease - the latter by ~15%.
The combination of unchanged MAP and reduced cardiac index means SVR increases significantly. Patients who tolerate sudden SVR rises poorly (e.g., severe cardiomyopathy) are at particular risk.
Blood pressure transduction: The arterial line transducer must be referenced to the level of the external auditory meatus (EAM), not the arm, to reflect true cerebral perfusion pressure (CPP). If using a manual arm cuff, a correction for the hydrostatic difference must be applied (~0.77 mmHg per cm).
CPP target: Maintain CPP (MAP - estimated ICP) at a minimum of 60 mmHg in healthy patients. This threshold should be raised in the elderly, hypertensives, those with cerebrovascular disease, and those with cervical stenosis.
Prevention: Preoperative hydration, graduated compression stockings, and slow incremental table tilting. IV vasopressors may be required.

4. Venous Air Embolism (VAE)

VAE is the most feared and frequent complication of surgery in the sitting position. It occurs because the surgical field lies above the level of the heart, creating a subatmospheric pressure gradient in open venous sinuses/diploe that draws in air.

Incidence and Severity

Reported in 25-40% of neurosurgical sitting cases by sensitive monitors
Clinical severity depends on the volume and speed of air entrainment
Animal data: ~3-5 mL/kg is lethal in adults, but clinically significant events occur with far less

Effects of VAE

Arrhythmias, O2 desaturation
Acute pulmonary hypertension and right heart strain
"Mill-wheel" murmur (late sign)
Cardiovascular collapse and cardiac arrest

Monitoring for VAE (in order of sensitivity)

Monitor	Sensitivity	Notes
TEE	Most sensitive - detects 0.02 mL/kg	Invasive; use pediatric probes to reduce macroglossia risk
Precordial Doppler (TTD)	0.05 mL/kg - most sensitive noninvasive	Place on 2nd-4th ICS at left or right sternal border
Transcranial Doppler (TCD)	Detects paradoxical emboli	Monitors middle cerebral artery
End-tidal CO2 (ETCO2)	Decreases with VAE	Standard monitor; less sensitive than Doppler
End-tidal N2O	Rises as entrapped gas exits via lungs	Useful when N2O is being used
PA catheter	Pulmonary arterial pressure rise	Detects moderate-large embolism

ETCO2 monitoring is standard; precordial Doppler is the recommended non-invasive continuous monitor.

Treatment of VAE (Sequential Steps)

1. Stop further air entrainment:

Notify surgeon to flood the field with saline and apply bone wax
Compress the jugular veins
Lower the head (Trendelenburg if possible)

2. Treat the intravascular air:

Aspirate the right heart catheter (CVP/multiorifice catheter pre-placed in right atrium)
Discontinue N2O immediately (switch to FiO2 = 1.0)
Administer pressors and/or inotropes for hemodynamic compromise
Consider placing patient left side down + Trendelenburg to dislodge right ventricular outflow air lock
Chest compressions if cardiac arrest occurs

Right Heart Catheter Placement

A multiorificed catheter is optimal, positioned with its tip 2 cm below the SVC-atrial junction. Placement is confirmed by:

Intravascular ECG (biphasic P wave in right atrium is characteristic)
TEE
Radiography

Right internal jugular venous access is preferred. If neck veins are unavailable, alternate central access is used.

5. Paradoxical Air Embolism (PAE)

PFO is present in approximately 25% of adults
Air can pass from the right to left heart, causing stroke or myocardial infarction
Preoperative contrast echocardiography (bubble study) is recommended to screen for PFO
PFO is classically a contraindication to the sitting position
However, a recent review found VAE detected in 40% of patients with a known PFO but zero paradoxical emboli - so the decision can be individualized with informed consent and shared surgical/anesthesia decision-making

6. Pneumocephalus

Air enters the supratentorial space as CSF drains and brain "shrinks" during surgery
Almost universally present on postoperative imaging after sitting posterior fossa surgery

N2O and pneumocephalus:

N2O expands trapped gas spaces and can convert a simple pneumocephalus into tension pneumocephalus
N2O is not absolutely contraindicated before dural closure (and may actually promote faster gas absorption by facilitating rapid N2O diffusion vs. slow N2 diffusion)
N2O must be discontinued before complete dural closure to prevent tension pneumocephalus postoperatively
Tension pneumocephalus: causes delayed awakening, severe headache; requires prompt surgical evacuation

7. Macroglossia and Airway Complications

Prolonged neck flexion with an oral airway or esophageal stethoscope in situ can cause compression ischemia of the tongue base, soft palate, and posterior pharyngeal wall. Upon reperfusion, edema accumulates, causing rapid-onset airway obstruction after extubation.

Prevention:

Maintain at least 2-3 fingerbreadths between chin and sternum
Avoid unnecessary oropharyngeal foreign bodies
Use a rolled gauze bite block (small bulk) rather than a large oral airway
Use pediatric TEE probes if TEE is employed
This risk rises sharply with procedure duration

8. Quadriplegia and Spinal Cord Injury

Sitting position with neck flexion can stretch or compress the cervical spinal cord, especially in the presence of osteophytes or pre-existing stenosis
Mechanisms: hyperflexion, hyperextension, rotation, or vascular compromise (arterial hypoperfusion or venous congestion)
ASA Closed Claims data (1970-2007) identified sitting position as an independent risk factor for cervical spinal cord injury
Mitigation: Preoperative assessment of cervical range of motion; strict CPP targets; consider intraoperative somatosensory and motor evoked potential (SSEP/MEP) monitoring during positioning in high-risk patients

9. Preoperative Assessment Checklist

Assessment	Rationale
Echocardiography (bubble study)	Screen for PFO - PAE risk
Cardiac function / ejection fraction	Assess tolerance of reduced cardiac index
Cervical spine imaging/MRI	Identify stenosis, osteophytes - cord injury risk
Cervical range of motion	Establish safe neck flexion limits
Peripheral vascular exam	Sitting position tolerated awake?
Blood pressure awake upright	Baseline for CPP management

10. Intraoperative Monitoring

Minimum recommended monitoring for sitting position surgery:

Arterial line (transduced at EAM)
Precordial (transthoracic) Doppler - VAE detection
Central venous catheter (right heart) - VAE aspiration
ETCO2 + EtN2O (if N2O used)
TEE (ideal, especially in high-risk patients)
SSEP/MEP (for patients with cervical spine risk)
Urinary catheter (procedures >1-2 hours)
Temperature monitoring (heat loss is greater in this position)

11. Peripheral Nerve Injury Risks

Nerve	Mechanism	Prevention
Brachial plexus	Arm/shoulder traction, dependent arm	Support arms; slightly elevated shoulders
Sciatic nerve	Hip flexion >90°, knee extension	Hip flexion <90°; slight knee flex; padding
Common peroneal	Knee pressure	Padding at fibular head
Ulnar nerve	Elbow pressure	Avoid pressure on medial elbow

Summary

Complication	Key Consideration
Hemodynamic instability	Pre-hydrate; transduce BP at EAM; SVR rises
Venous air embolism	Precordial Doppler + CVP catheter mandatory
Paradoxical air embolism	Screen for PFO; relative contraindication
Pneumocephalus / tension	Stop N2O before dural closure
Macroglossia / airway	2-3 fingerbreadth chin-sternum clearance
Quadriplegia	CPP management; SSEP/MEP monitoring
Peripheral nerve injury	Careful padding and positioning

Sources: Miller's Anesthesia 10e (Chapters 30, 53); Morgan & Mikhail's Clinical Anesthesiology 7e (Chapter 30)

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