| Concept | High-yield detail | Exam trap |
|---|---|---|
| BMI formula | BMI = body weight kg / height² m | Need to convert height to meters and square it. |
| BMI categories | <18.5 underweight; 18.5–24.9 normal; 25–29.9 overweight; 30–34.9 class I; 35–39.9 class II; ≥40 class III/morbid; ≥50 super; ≥60 super-super. | ≥40 = morbid; ≥50 = super; ≥60 = super-super. |
| Weight formulas | IBW male = height cm − 100. IBW female = height cm − 105. LBW = IBW × 1.3. TBW = actual. | IBW may underdose some meds; TBW may overdose others. |
| Metabolic syndrome | Diagnosis if >3/5: waist >40” men/>35” women, TG >150, HDL <40 men/<50 women, HTN >130/85 or meds, fasting glucose >100 or meds. | ↑ risk diabetes, CV events, stroke, periop CV/pulm/renal/wound events. |
| Hepatic | Fatty infiltration/NAFLD, NASH, focal necrosis, cirrhosis; abnormal LFTs in ~1/3, ↑ ALT most common. | Obesity is not just airway — liver metabolism/function matters. |
| Endocrine/renal | ↑ T2DM, impaired glucose tolerance, insulin resistance, glomerular hyperfiltration, ↑ renal blood flow, ↑ GFR, hypothyroidism ~25%. | Renal clearance may be increased in obesity. |
| Positioning risk | Pressure injury, rhabdomyolysis, nerve injuries; carpal tunnel most common. | Pad everything and position deliberately. |
| Drug/Class | Dose scalar | Reason/comment | Trap |
|---|---|---|---|
| Propofol | Induction: LBW; Maintenance: TBW | ↑ Vd; longer elimination half-life. | Do not use TBW for induction bolus. |
| Succinylcholine | TBW | ↑ pseudocholinesterase activity associated with ↑ weight. | “Totally sucks” = TBW. |
| Nondepolarizing muscle relaxants | IBW | Hydrophilic drugs dosed to IBW → shorter duration/predictable recovery. | “I rock” = IBW for roc/NDMR. |
| Fentanyl/Sufentanil | LBW | TBW overestimates dose requirements. | Opioid respiratory depression risk. |
| Remifentanil | IBW infusion rates | Distribution/elimination similar to non-obese. | Do not overrun on TBW. |
| Dexmedetomidine | TBW, start lower | Less respiratory depression but hypotension/bradycardia risk. | TBW but cautious dosing. |
| Neostigmine | TBW; max 5 mg noted | Prompt early reversal; delayed full recovery. | Ceiling/max issue. |
| Sugammadex | TBW | Rapid/complete reversal of steroidal NMBs in obese population. | Helpful but dosing based on depth of block. |
| Issue | High-yield detail | Exam cue |
|---|---|---|
| Airway | Obesity itself is not an automatic difficult airway predictor, but obesity causes more rapid desaturation. | Preoxygenation is critical. |
| Ramped/head-elevated position | Use for preoxygenation, intubation, emergence, extubation. | Align external auditory meatus with sternal notch if using ramped positioning. |
| OSA | ↑ difficult airway risk and pulmonary complications. | Semi-upright, continuous pulse ox, opioid-sparing, O₂ ± CPAP. |
| Ventilation | Use lung-protective strategy; obese patients desaturate quickly due to low FRC. | Reverse Trendelenburg/semi-sitting improves mechanics. |
| Phentermine | Common anti-obesity med; amphetamine-like; half-life 24 hr; stop 4–7 days before elective surgery. | Risk refractory hypotension; phenylephrine > ephedrine; pregnancy category X; can cause false + drug screen; postpone if not stopped per notes. |
| GLP-1 receptor agonists | If withheld + NPO + no GI symptoms → continue. If continued ± symptoms → POCUS. | Full or inconclusive stomach → postpone or RSI. Consider metoclopramide/famotidine, OG before emergence, awake extubation. |
| Bariatric surgery | Often laparoscopic; combine obesity + pneumoperitoneum physiology. | DVT prophylaxis, aspiration risk, ramping, opioid-sparing, nausea prevention. |
| System/effect | What happens with CO₂ pneumoperitoneum | Exam trap |
|---|---|---|
| Why CO₂? | Highly soluble and nonflammable. | Still causes hypercarbia/acidosis. |
| Hemodynamics | ↑ catecholamines, ↑ RAAS, ↑ vasopressin; ↑ SVR; ↓ venous return with IVC compression; CO variable/decreased. | PnP can cause hypertension/vasoconstriction OR decreased preload/hypotension. |
| Vagal/celiac reflex | Rapid stretch/insufflation can trigger severe bradycardia. | Treatment: stop insufflation/decompress, atropine/epi if needed. |
| Respiratory | ↓ lung compliance, ↓ FRC, ↑ PIP, V/Q mismatch, atelectasis, hypercarbia. | Carina can shift cephalad → ETT endobronchial risk. |
| Renal | ↓ renal perfusion and UO. | Low UO may be PnP pressure, not just volume status. |
| Hepatic | ↓ hepatic blood flow. | Worse with low CO/MAP and high IAP. |
| CNS | ↑ ICP/CBF, especially with Trendelenburg and hypercarbia. | Concern with steep Trendelenburg and neuro risk. |
| Max IAP | Typically ≤15 mmHg. | Higher pressure worsens preload/organ perfusion/respiratory mechanics. |
| Position | Physiology | High-yield complication |
|---|---|---|
| Steep Trendelenburg | ↑ venous return initially, ↑ ICP/CVP/IOP, airway/facial edema, ↓ lung compliance. | Airway edema; delayed extubation concern. |
| Reverse Trendelenburg | ↓ venous return/preload, may improve respiratory mechanics. | Hypotension + DVT/venous pooling risk. |
| Lateral/jackknife | Dependent pooling; V/Q mismatch; lower extremities can become dependent. | Bottom lung gets blood flow, top lung over-ventilated; axillary roll just caudal to dependent axilla. |
| Robotic docking | Limited patient/airway access after docking. | Secure airway, lines, padding before docking. |
| Complication | Signs/symptoms | Treatment | Trap |
|---|---|---|---|
| Refractory hypoxia | ↓ SpO₂ | Confirm 100% O₂, verify ETT/EtCO₂, immediate PnP release, neutral positioning. | Always check tube and circuit first. |
| Refractory hypercarbia | ↑ EtCO₂ | Vent changes; rule in/out MH. | Hypercarbia is common in lap, but MH must be considered if severe/unexplained. |
| Subcutaneous emphysema | Unexplained hypercarbia ± acute hypotension, crepitus; may need CXR/CT postop. | Deinsufflation, reinsufflate at lower pressure, ventilator changes to normalize CO₂, supportive care, may remain intubated. | Crepitus + hypercarbia. |
| Capnothorax | SubQ emphysema upper torso, severe ↑ CO₂, ↓ EKG amplitude, ↓ breath sounds, ↓ chest excursion. | Immediate peritoneal deinsufflation, hyperventilation, PEEP. | CO₂ thorax may resolve faster than air but can be severe. |
| Tension capnothorax | ↑ peak airway pressure, hypoxia, severe hypotension. | Supportive care; emergent needle decompression or chest tube (chest tube preferred in OR); consider open conversion. | High PIP + hypoxia + hypotension. |
| Venous gas embolism | Acute EKG changes, ↑ HR/arrhythmias/QRS widening, hypoxia, hypotension, ↓ EtCO₂, ↑ ET nitrogen, mill-wheel murmur. | Terminate PnP/decompress abdomen, supportive care, rapid IV fluids, 100% O₂ and stop N₂O, hyperventilate, consider Trendelenburg + left lateral, aspirate gas via CVC. | TEE most sensitive; precordial Doppler most sensitive noninvasive. |
| Urologic injury | ↓ UO, hematuria, pneumaturia. | Inform surgeon of changes in UO. | Not every low UO is volume: could be injury or pressure effect. |
| Occult blood loss/vascular injury | Rapid hemodynamic changes, poor visualization, concealed bleeding. | Vigilance; prepare for conversion to open. | Lap blood loss can be underestimated. |
| Topic | High-yield detail | Exam trap |
|---|---|---|
| Preop eval | Airway, chronic pain/opioids, anticoagulants, NSAIDs, comorbidities, neurologic baseline, regional candidacy. | Document preexisting neurologic deficits before blocks/spine cases. |
| Rheumatoid arthritis | Atlantoaxial instability, TMJ limitation, cricoarytenoid arthritis. | Maintain neutral neck; airway obstruction/stridor risk from cricoarytenoid arthritis. |
| Femur fracture / IM nail | Fractures from lesser trochanter to distal femur; antegrade nail common; CBC; thigh may hide ~1 L blood; EBL 250–500 mL but underestimated. | Reaming can displace medullary contents → pulmonary embolic physiology/cardiovascular instability. |
| Fracture table | Traction/positioning concerns. | Hidden blood loss + pressure/nerve injury. |
| ACDF | Evaluate neck mobility and preexisting deficits; ask about paresthesias/deficits during assessment. | Dysphagia most common complication from esophageal retraction; RLN injury, hemorrhage, cord damage, VAE. |
| Scoliosis surgery | Idiopathic 70%; female > male 4:1; surgery correlates with Cobb angle: >50° thoracic, >40° lumbar. | Chronic hypoxia/hypercapnia/pulmonary vascular constriction; extensive blood loss, hypothermia, long surgery. |
| Scoliosis monitoring | A-line, central venous catheter; intraoperative neuro monitoring. | Avoid long-acting paralytic after monitoring baseline if MEPs used. |
| Phase | Physiologic changes | High-yield clue |
|---|---|---|
| Inflation | ↑ BP and ↑ HR from sympathetic response/tourniquet pain. | Tourniquet pain begins around 60 min; C fibers = dull/burning pain. |
| During prolonged inflation | Local ischemia, anaerobic metabolism, lactate/K/CO₂ build up distal to tourniquet. | Max inflation time generally about 2 hr. |
| Deflation | ↓ BP, ↑ EtCO₂, ↑ K⁺, ↓ pH, ↑ lactate. | Deflation = acidosis + hyperkalemia + EtCO₂ bump. |
| Need more time | Notify surgeon, deflate ~15 min for reperfusion, re-exsanguinate limb, reinflate. | Re-exsanguish before reinflating. |
| Complication | Risk/description | Signs | Treatment |
|---|---|---|---|
| Fat embolism syndrome | Within 72 hr of long bone or pelvic fractures; 10–20% mortality in notes. | Classic triad: dyspnea, confusion, petechiae. Intraop: ↓ EtCO₂, ↓ SaO₂, cardiovascular instability. | Supportive care: O₂, pressors, pulmonary vasodilators; stabilize fractures early. |
| Bone cement implantation syndrome | Polymethylmethacrylate in joint arthroplasty; embolization of fat, marrow, air during cementing. | ↓ SpO₂, ↓ BP, ↓ EtCO₂, ↓ CO, arrhythmias. | Increase O₂ before cement, maintain preload/euvolemia, monitor closely during cementing, document cement time. |
| VTE | Risk factors: obesity, age >60, surgery >30 min, tourniquet use, lower extremity fracture, immobility >4 days. | DVT/PE risk; without prophylaxis DVT can develop in 40–80% of patients per notes. | Prophylaxis key: SCDs, anticoagulants such as heparin/Lovenox/warfarin, neuraxial anesthesia considerations. |
| Beach chair cerebral hypoperfusion | Head above heart lowers cerebral MAP. | MAP at brain decreases about 0.75 mmHg per cm or 2 mmHg per inch. | Level A-line at external auditory meatus; consider A-line. |
| Bezold-Jarisch reflex | Can occur in sitting/beach chair with low preload. | Bradycardia + hypotension. | Fluids, ephedrine, glycopyrrolate/atropine as appropriate. |
| Positioning nerve injury | Prone/lateral/traction/obesity increase risk. | Pressure injury, nerve compression, rhabdomyolysis. | Padding, neutral alignment, axillary roll caudal to dependent axilla in lateral. |