How should I perform spinal (subarachnoid) anesthesia in an adult with severe obesity (BMI ≥ 40 kg/m²) scheduled for lower‑extremity surgery, including pre‑operative assessment, optimization of comorbidities, technique, drug dosing, and postoperative management?

Spinal Anesthesia for Severely Obese Patients Undergoing Lower Extremity Surgery

Spinal anesthesia is the preferred anesthetic technique for lower extremity surgery in severely obese patients (BMI ≥40 kg/m²) because it avoids the catastrophic airway complications that occur more frequently and rapidly in this population, though you must anticipate higher technical failure rates and have a backup airway plan immediately available. 1, 2

Pre-operative Assessment and Optimization

Cardiovascular Evaluation

• Screen all severely obese patients for coronary artery disease, structural heart disease, and pulmonary hypertension using the algorithm: comprehensive history/physical examination → 12-lead ECG (looking for right ventricular hypertrophy suggesting pulmonary hypertension or left bundle branch block suggesting occult CAD) → chest X-ray if hypoventilation suspected → consider arterial blood gas. 1
• Patients unable to achieve 4 METs functional capacity or with intermediate cardiac risk factors require formal cardiac stress testing before proceeding with surgery. 1, 3
• For patients with one or more CAD risk factors who cannot exercise, use pharmacological stress testing or transesophageal dobutamine stress echocardiography. 1

Respiratory Assessment

• Assume all severely obese patients have some degree of sleep-disordered breathing and screen accordingly using validated tools (STOP-BANG questionnaire). 1, 2, 4
• Patients with known obstructive sleep apnea must bring their CPAP device to the hospital and use it postoperatively. 1
• Obtain baseline oxygen saturation on room air for comparison postoperatively. 1

Metabolic Optimization

• Optimize HbA1c to <7.5% before elective surgery and target perioperative glucose <140 mg/dL, as this reduces surgical site infection risk by 50%. 3
• Check for diabetes mellitus and ensure adequate glycemic control, as postoperative hyperglycemia significantly increases infection risk. 1

Risk Stratification

• Calculate the Obesity Surgery Mortality Risk Score (OS-MRS): patients with scores of 4-5 must be managed by an anesthetist experienced in obese patient care. 2
• Recognize that severely obese patients undergoing lower extremity surgery have 2-7 fold increased risk of surgical site infections depending on BMI severity (BMI 30-35: 2-3x; BMI 35-40: 4-5x; BMI >40: 6-7x). 3

Equipment Preparation

Essential Equipment (Must Be Immediately Available)

• Extra-long spinal needles (120-150mm length) are mandatory, as standard needles will be inadequate. 1, 2
• Ultrasound machine with low-frequency curvilinear probe for pre-procedural scanning. 5, 6
• Full difficult airway cart including video laryngoscope, supraglottic airways, and emergency cricothyrotomy equipment. 1, 2
• Appropriately sized blood pressure cuff (standard cuffs give falsely elevated readings). 1

Positioning and Technique

Patient Positioning

• Use the sitting position for spinal anesthesia, as this provides superior anatomical landmark identification and higher success rates compared to lateral decubitus. 2, 7, 5
• Ensure adequate assistance (multiple staff members) for positioning, as obese patients require extra support. 1, 2
• Allow extra time for positioning—do not rush this critical step. 2

Ultrasound-Guided Approach

• Perform pre-procedural ultrasound scanning to identify the midline, intervertebral spaces, and measure skin-to-dura distance before attempting needle insertion. 5, 6
• Use the transverse view to identify the posterior dura, transverse processes, and posterior vertebral body below adipose tissue. 5
• Mark the optimal insertion point on the skin after ultrasound identification. 5, 6

Needle Insertion Strategy

• Target the L4-5 interspace rather than L3-4 to avoid excessive cephalad spread of block, as obesity causes more extensive spread of plain local anesthetic solutions. 8
• Insert the needle in the midline using a paramedian approach if midline landmarks are obscured. 6
• Advance slowly with frequent aspiration to confirm cerebrospinal fluid. 7

Drug Dosing

Local Anesthetic Selection and Dosing

• Use standard doses of hyperbaric bupivacaine (9-12 mg of 0.5% solution) despite obesity, as dose reduction is not necessary for spinal anesthesia. 2, 7
• Do NOT calculate local anesthetic doses based on total body weight—use standard dosing or lean body weight calculations to avoid toxicity. 2
• Add fentanyl 10-20 μg as an adjunct to reduce postoperative opioid requirements without limiting mobility. 1, 7

Expected Block Characteristics

• Anticipate more extensive cephalad spread in obese patients (approximately 4 dermatome levels higher than non-obese patients with the same dose). 8
• When injecting at L4-5, expect sensory block to reach T8-T10 in severely obese patients versus T11-T12 in normal BMI patients. 8
• Good surgical anesthesia for lower extremity procedures is reliably achieved with these doses. 8

Intraoperative Management

Hemodynamic Monitoring

• Anticipate and aggressively treat hypotension, as obese patients tolerate supine positioning and Trendelenburg poorly due to increased intra-abdominal pressure and aortocaval compression. 1, 2
• Have vasopressors (phenylephrine, ephedrine) drawn up and immediately available before inducing the block. 1
• Monitor for tachycardia, which may be the only sign of complications such as intra-abdominal pathology or inadequate anesthesia. 1

Sedation Strategy

• Minimize or completely avoid sedation, as obese patients with undiagnosed sleep-disordered breathing can develop airway obstruction with even minimal sedation. 1, 2, 4
• If sedation is absolutely necessary, use minimal sedation only (patient responds normally to verbal commands) and have an anesthesiologist present. 4
• Never attempt solo operator-sedator procedures in obese patients. 1

Airway Management Plan

• Have a detailed airway management plan in place even when using spinal anesthesia alone, as conversion to general anesthesia may be necessary. 1, 2
• Recognize that obese patients have markedly reduced functional residual capacity and safe apnea time (as short as 2-3 minutes versus 8-10 minutes in normal patients). 1, 4

Postoperative Management

Recovery Room Monitoring

• Continue supplemental oxygen until baseline oxygen saturations are achieved without support. 1
• Monitor with continuous pulse oximetry until oxygen saturations remain at baseline without supplemental oxygen AND parenteral opioids are no longer required. 1
• Observe the patient while unstimulated for at least one hour for signs of hypoventilation, apnea, or hypopnea with oxygen desaturation before discharge from PACU. 1

Analgesia Strategy

• The spinal opioid adjunct (fentanyl) provides excellent postoperative analgesia with reduced systemic opioid requirements, which is ideal for obese patients. 1
• Use multimodal analgesia with acetaminophen and NSAIDs as first-line agents. 4
• Avoid patient-controlled analgesia (PCA) systems unless the patient is monitored in a level-2 unit, as there is increased risk of respiratory depression in those with undiagnosed sleep-disordered breathing. 1

Extended Monitoring Criteria

• Patients with poorly controlled OSA who received any opioids require level-2 care with continuous monitoring. 4
• Obese patients (BMI >35) should be monitored 3+ hours beyond standard recovery time. 4
• Any ongoing hypoventilation requires anesthetic reassessment for possible need for respiratory support and level-2 care. 1

Mobilization and VTE Prophylaxis

• Early mobilization is vital—patients should be out of bed on the day of surgery. 1
• All severely obese patients require pharmacological VTE prophylaxis with weight-adjusted low molecular weight heparin dosing: for patients >150 kg, use enoxaparin 60 mg twice daily or dalteparin 7500 units twice daily. 1
• Mechanical compression devices should be used but can be disconnected for mobilization. 1

Common Pitfalls to Avoid

• Failure to have extra-long spinal needles immediately available leads to procedure failure and potential conversion to general anesthesia with its attendant risks. 1, 2
• Using the L3-4 interspace instead of L4-5 results in excessively high blocks (T4 versus T8) in obese patients. 8
• Calculating local anesthetic doses based on total body weight causes toxicity—use standard doses for spinal anesthesia. 2
• Providing sedation without adequate monitoring or airway backup can lead to catastrophic airway obstruction. 1, 2, 4
• Discharging patients from PACU before observing them unstimulated for adequate duration misses delayed respiratory depression. 1
• Ignoring tachycardia as a potential sign of complications delays recognition of serious postoperative problems. 1