Overview and Relevance
Positioning the pregnant patient during anaesthesia requires integration of two overlapping domains: the general principles that apply to all surgical patients, and the unique physiological adaptations of pregnancy that amplify those risks. Anaesthesia abolishes the patient's normal protective responses, the awake patient shifts position when uncomfortable, providing continuous feedback that prevents sustained pressure, stretch, or ischaemia. Under general or neuraxial anaesthesia, this protective mechanism is lost entirely. The pregnant patient is additionally vulnerable because of cardiovascular, respiratory, and musculoskeletal changes that alter the consequences of any given position.
Positioning injuries represent approximately 22% of cases in the ASA Closed Claims Project. The mechanisms of injury are fundamentally three: stretching, compression, and ischaemia. All three are relevant to the pregnant surgical patient.
Physiological Considerations Unique to Pregnancy
Cardiovascular
The gravid uterus from approximately 20 weeks causes aortocaval compression in the supine position. The inferior vena cava is compressed against the lumbar vertebrae, reducing venous return and cardiac output by up to 30%. Simultaneously, aortic compression reduces uteroplacental perfusion.
$$\text{Uteroplacental perfusion} \propto \frac{MAP_{uterine} - CVP_{uterine}}{Uterovascular\ resistance}$$
Any position that fails to displace the uterus laterally risks haemodynamic compromise to both mother and fetus.
Respiratory
- Functional residual capacity (FRC) is reduced by approximately 20% at term due to diaphragmatic splinting by the uterus
- Oxygen consumption is increased by 20%
- These changes narrow the safe apnoeic window and worsen any further positional reduction in FRC
- Any position that further restricts diaphragmatic or chest wall movement, such as steep Trendelenburg, lithotomy, or lateral decubitus, compounds these pre-existing deficits
Musculoskeletal
- Ligamentous laxity from relaxin increases joint mobility and susceptibility to traction injuries
- The lumbar lordosis is exaggerated, altering the neutral spine position and making torsional injuries more likely
- These changes are relevant particularly to lithotomy positioning, where failure to raise both legs simultaneously can cause torsional lumbar injury
Other
- Oedema, particularly in the third trimester, may reduce the threshold for compartment syndrome and nerve compression injury
- The enlarged uterus limits abdominal exposure and may alter the optimal table position
General Principles of Safe Positioning
Core Tenets
The key principle is that anesthetised patients should be positioned in a way that could be comfortably tolerated if awake. When more extreme positions are unavoidable, their duration should be minimised. Preoperative assessment should document:
- Pre-existing neurological symptoms or neuropathies
- Comorbidities that increase vulnerability: diabetes mellitus, peripheral vascular disease, obesity
- Positions the patient can comfortably tolerate while awake
Positioning injuries arise from stretch, pressure, and compression. Stretch injuries occur when anatomical structures are placed at the limit of their excursion. Compression injuries occur at bony prominences, table hinges, monitoring equipment, and IV lines. Padding is essential but note: padding that is too tight may itself cause injury.
The Role of the Whole Team
Safe positioning is a multidisciplinary responsibility requiring active coordination between anaesthetist, surgeon, scrub and scout nurses. All team members must understand the risks associated with changes in position intraoperatively. In the pregnant patient this is especially important when repositioning is required for obstetric or surgical access reasons.
Position-Specific Considerations in the Pregnant Patient
Supine Position
The supine position is the most common intraoperative position but carries the highest risk of aortocaval compression in pregnancy beyond 20 weeks.
| Risk | Mechanism | Mitigation |
|---|---|---|
| Aortocaval compression | Gravid uterus compresses IVC and aorta | Left lateral uterine displacement (15-30°); wedge under right hip |
| Reduced FRC | Diaphragmatic splinting worsened in supine | PEEP, head-up position if feasible, minimise apnoea time |
| Peripheral nerve compression | Prolonged pressure on bony prominences | Padded surfaces; regular position checks |
| Pressure injury | Immobility under anaesthesia | Padded mattress; attention to heels, sacrum, occiput |
Left lateral uterine displacement is the cornerstone measure. A firm wedge (typically 15 cm) placed under the right hip, or manual displacement of the uterus, relieves caval compression. Full left lateral tilt is avoided during surgery as it impairs surgical access.
Lithotomy Position
The lithotomy position is used for gynaecological, perineal, and urological procedures that may be required during pregnancy. The hips are flexed 80-100 degrees from the trunk, legs abducted 30-45 degrees, and knees flexed until the lower legs are parallel to the torso.
| Risk | Mechanism | Mitigation |
|---|---|---|
| Peroneal nerve injury | Compression of nerve at fibular head against leg support | Careful padding of lateral knee; avoid direct pressure at fibular head |
| Finger crush injury | Hands near hinge point when foot section raised | Arms on armrests away from table hinge; confirm hand position before moving table |
| Lumbar torsion | Asymmetric leg raising | Raise and lower both legs simultaneously, flexing hips and knees together |
| Compartment syndrome | Elevated lower limbs reduce perfusion; prolonged lithotomy | Limit duration; avoid excessively elevated position; monitor for calf firmness |
| Aortocaval compression | Supine component of lithotomy | Maintain left lateral displacement despite lithotomy position |
| FRC reduction | Uterus and lithotomy both restrict diaphragm | Be vigilant for increasing airway pressures; optimise ventilation settings |
The lithotomy position may cause significant haemodynamic changes: elevation of the legs increases venous return and cardiac preload. In the pregnant patient with a compromised cardiovascular reserve, this transient preload increase followed by the reduction when legs are lowered must be managed carefully.
Lateral Decubitus Position
Occasionally required for thoracic or renal procedures in pregnancy.
- Dependent brachial plexus: at risk of pressure injury; an axillary roll (positioned caudal to the axilla, never within it) protects the dependent shoulder and axillary contents
- Dependent ear and eye: must be checked regularly intraoperatively for pressure
- Pulse oximetry on the dependent arm detects early axillary vascular compression, hypotension measured in the dependent arm or low $SpO_2$ may indicate arterial compression
- Ventilation-perfusion mismatch worsens in lateral position under general anaesthesia; this is further exaggerated in pregnancy because the uterus applies cephalad pressure on the dependent diaphragm
Trendelenburg and Steep Trendelenburg
Used during laparoscopic pelvic surgery including in early pregnancy.
- FRC is decreased by laparoscopy and further decreased by steep Trendelenburg, with a combination of abdominal contents and positioning pushing up on the diaphragm
- Peak and plateau airway pressures may rise by up to 50%
- Facial and airway oedema can develop; document endotracheal tube air leak at start and end of procedure as an indicator of developing oedema
- Shoulder braces are associated with brachial plexus injury and should be avoided or used with extreme caution; non-slip mattresses and chest straps are preferable methods to prevent the patient sliding
- The head should be kept in the midline to minimise brachial plexus stretch
Peripheral Nerve Injury: Mechanisms and Prevention
Peripheral nerve injuries represent a serious and medicolegally significant complication. The fundamental mechanisms are stretch, compression, and ischaemia.
| Nerve | Common Mechanism | Prevention |
|---|---|---|
| Brachial plexus | Arm abduction >90°; shoulder brace in Trendelenburg; head rotation away from arm | Limit abduction to ≤90°; head midline; avoid shoulder braces; padded armboards |
| Ulnar nerve | Pressure on postcondylar groove; elbow flexion | Pad elbows; supinate or neutral forearm; arm abduction <90° |
| Common peroneal nerve | Compression at fibular head from lithotomy leg support | Pad lateral knee; avoid direct pressure on fibular head |
| Radial nerve | Compression at spiral groove; arm hanging over table edge | Pad arm; do not allow arm to hang freely |
| Sciatic nerve | Stretch with hip flexion in lithotomy | Appropriate degree of hip flexion; coordinate movement |
The brachial plexus has two points of fixation (cervical vertebrae and axillary fascia) and a long superficial course. It is susceptible to stretch when:
- The arm is abducted beyond 90 degrees
- The head is rotated away from the side of concern
- The shoulder girdle is depressed
- Shoulder braces compress or displace the shoulder against the thorax
In pregnancy, ligamentous laxity means that normal anatomical limits may be reached at smaller angles of displacement than in the non-pregnant patient.
Perioperative Management
Preoperative Assessment
- Identify pre-existing neurological symptoms, peripheral neuropathy, diabetes, peripheral vascular disease
- Ask whether the patient can comfortably tolerate the anticipated position while awake, in pregnant patients this should include discussion of uterine displacement requirements
- Document baseline neurological status of the extremities
- Review gestational age: aortocaval compression risk becomes significant from approximately 20 weeks
- Assess for oedema, which increases nerve compression vulnerability
Intraoperative Safety Checklist Framework
| Domain | Action |
|---|---|
| Cardiovascular | Left lateral uterine displacement maintained at all times in supine; monitor maternal blood pressure and fetal heart rate where applicable |
| Airway | Secure ETT carefully; re-confirm position after any repositioning; document cuff leak at start and end if Trendelenburg used |
| Upper extremity | Arms abducted ≤90°; elbows padded; forearms supinated or neutral; head midline |
| Lower extremity | Legs raised and lowered together in lithotomy; peroneal nerve padded; hands confirmed safe before moving table sections |
| Pressure areas | Padded mattress; foam or gel pads at bony prominences; heels, sacrum, occiput |
| Duration | Minimise extreme positions; document time in position; reposition or reassess padding for prolonged procedures |
| Monitoring | $SpO_2$ on dependent arm in lateral position; blood pressure bilaterally if vascular compromise suspected |
| Documentation | Record all specific positioning actions; note padding placed; record position changes with times |
Intraoperative Vigilance
- When the foot section of the operating table is raised or lowered in lithotomy, confirm hand and finger position before moving, crush injury to fingers is a preventable catastrophe
- In steep Trendelenburg, trial the position before docking robotic equipment to confirm the patient does not slide and can tolerate the physiologic changes
- Signs of upper extremity neurovascular compromise (inconsistent blood pressure readings, poor $SpO_2$ signal) should prompt immediate reassessment of arm and shoulder positioning
- In lateral decubitus, the dependent axilla must be free; use axillary roll caudal to the axilla and confirm pulse in dependent arm
Postoperative Assessment
- Perform a focused neurological assessment of extremities before discharge from recovery
- Early identification of peripheral neuropathy allows appropriate investigation and management
- Postoperative ulnar nerve palsy can occur even when padding and positioning have been carefully managed, this should be explained to patients as part of the consent process for prolonged procedures
- Any patient reporting eye pain, visual changes, or loss of light perception must be referred urgently to ophthalmology
Documentation
Meticulous documentation of all perioperative positioning actions is both a standard of care and important medicolegally. Documentation should include:
- Specific position used and time intervals
- Padding placed and where
- Uterine displacement measures applied
- Any position changes made intraoperatively and the reason for them
- Postoperative neurological assessment findings
Summary
The pregnant patient undergoing anaesthesia faces the same spectrum of positioning injuries as any surgical patient, nerve stretch and compression, pressure injuries, vascular compromise, compounded by the unique physiology of pregnancy: aortocaval compression, reduced FRC, ligamentous laxity, and oedema. Minimising risk requires a systematic preoperative assessment, meticulous intraoperative technique with left lateral uterine displacement maintained throughout, careful padding and limb positioning according to ASA advisory principles, vigilant monitoring for early signs of neurovascular compromise, and thorough documentation. These measures collectively reduce, but do not eliminate, the risk of positioning-related injury; patients should be informed of residual risk particularly for prolonged procedures.
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