Falling End-Tidal CO2 and Rising Airway Pressures During Laparoscopy in a Patient With Severe COPD

Framework. ETCO2 tracks pulmonary CO2 excretion divided by alveolar ventilation; the PaCO2-to-ETCO2 gradient widens whenever dead space or V/Q mismatch increases. So ask three questions: did ventilation change (no), did CO2 production change, did pulmonary perfusion change?

Ranked differential

1. Low cardiac output with increased dead space - IAP 15 plus reverse Trendelenburg cut venous return; BP 88/50 with HR 112 fits. Perfusion of non-gas-exchanging lung falls, ETCO2 drops while PaCO2 is unchanged or rising.
2. CO2 embolism - cannot be excluded; typically abrupt, with desaturation, arrhythmia, mill-wheel murmur. A gradual four-minute decline with SpO2 97% makes it less likely but not impossible.
3. Circuit leak/disconnect - effectively excluded: airway pressures rose rather than fell.
4. Falling CO2 production (hypothermia, profound hypoperfusion) - secondary, follows from cause 1.

Next five minutes. Draw an ABG: a normal or high PaCO2 with ETCO2 of 31 proves a widened gradient and confirms dead space rather than hyperventilation. Ask the surgeon to drop insufflation to 10–12 mmHg and level the table while you give a modest crystalloid bolus plus phenylephrine to restore MAP. Recheck pressures, auscultate for subcutaneous emphysema, keep FiO2 high, and do not chase the ETCO2 with minute ventilation.

What the numbers mean. Driving pressure (plateau − PEEP) is the stress actually delivered to aerated lung; values above roughly 15–18 cmH2O associate with pulmonary complications. Here the rise comes from reduced compliance: pneumoperitoneum splints the diaphragm, the chest wall is heavy, and dependent lung is atelectatic.

Order of levers

1. Reduce IAP first - often the single largest and fastest improvement in both compliance and hemodynamics, and it costs the surgeon little.
2. Re-titrate PEEP. In a BMI 44 patient, PEEP 8 may be under-recruiting; a step increase can lower driving pressure by recruiting. But with a plateau of 33 there is also overdistension risk, so titrate to best compliance (or esophageal balloon if available) rather than to a protocol number.
3. Keep tidal volume at 6 mL/kg predicted body weight. Do not shrink below 6 unless driving pressure stays high and hypercapnia is tolerated.
4. Do not raise the rate to normalize ETCO2. With COPD and a baseline PaCO2 of 48, added rate risks air trapping. Check an expiratory-hold total PEEP and the flow-time curve for expiratory limitation; keep I:E at 1:2 or longer.

Targets. Permissive hypercapnia to about pH 7.20, driving pressure under 15–16 if achievable, and hemodynamics as the real endpoint. A recruitment maneuver only if the patient is stable enough to tolerate transient preload reduction.

Readiness. Confirm TOF ratio above 0.9 (not just twitch count), adequate spontaneous ventilation with a normal respiratory pattern on pressure support, and hemodynamics back to baseline after desufflation - venous return and BP usually improve within minutes. Sit the patient up 30–45 degrees and keep PEEP/CPAP during the transition; obesity plus COPD makes derecruitment at extubation rapid.

Hypercapnia trap. His baseline PaCO2 is 48 and his brain is set to that. Judge ventilation by pH and clinical work of breathing, not by ETCO2. Over-ventilating him to an ETCO2 of 35 in the OR can produce a relative hypoventilation/apnea problem later if opioids are added.

Analgesia and PONV. Multimodal: scheduled acetaminophen/NSAID if renal function allows, local anesthetic port-site infiltration or TAP block, opioid-sparing antiemetic prophylaxis. Excess opioid in this body habitus plus residual hypercapnia is a recipe for reintubation.

Postoperative risks to plan for. Atelectasis and pneumonia (incentive spirometry, early ambulation, sitting position, consider prophylactic CPAP or NIV in the first 24 h), opioid-induced hypoventilation if OSA coexists, and delayed recognition of a CO2 embolism that was masked by the stable SpO2. Specify an ICU or step-down disposition rather than a routine ward bed.