Sir,End-tidal CO2 (EtCO2) is used as a surrogate to assess adequacy of ventilation since it provides an estimate of the arterial CO2 (PaCO2). The PaCO2 is normally higher than EtCO2 by 2-5 mmHg. However, in conditions where there is ventilation-perfusion mismatch, the EtCO2 may not accurately reflect the PaCO2. Reversal of the normally positive PaCO2-EtCO2 gradient may occur also termed as negative arterial to EtCO2 gradient. We encountered a similar situation in a 2 year old child weighing 10 kg posted for thoracoscopic excision of a left bronchogenic cyst.The patient had a history of fever and cough on and off for 1 year. Computed tomography scan showed a 31 mm × 28 mm × 38 mm fluid density lesion seen in posterior mediastinum causing severe compression of the left main bronchus with resultant obstructive emphysema. The air entry was diminished on the left side. All other investigations were within normal limits. After premedication, anaesthesia was induced with sevoflurane and injection Atracurium 0.6 mg/kg. Right endobronchial intubation was done with 4.5 mm ID endotracheal tube (ETT). The absence of air entry on the operative left side was confirmed by auscultation. The patient was given right lateral position and was ventilated using pressure control mode achieving tidal volume of 6 ml/kg. After CO2 insufflation in the thorax, there was rise in the EtCO2 and the inspiratory pressure requirement. EtCO2 reached up to 60 mmHg after which we started manually ventilating the patient. However, EtCO2 continued to rise till 113–115 mm Hg. Endotracheal suction was done to rule out the blockage of ETT. Air entry was present on the right side with no adventitious sounds. The oxygen saturation was around 94%. An arterial blood gas (ABG) was sent which revealed pH: 7.298, PCO2:45.6, PO2: 223, HCO3
-: 21.8 and SO2: 96%. Thus, there was reversal of PaCO2 - EtCO2 gradient. Due to the rising EtCO2, deroofing of the cyst was done instead of excision, insufflation was stopped, and the patient was made supine after closure of ports. The tube was withdrawn into the trachea, and bilateral air entry was confirmed. The EtCO2 gradually came down up to 45 mm Hg. ABG at this point showed pH: 7.252, PCO2: 53, PO2: 105.1, HCO3-: 23, SO2: 99.6%. The patient was extubated after reversal of neuromuscular blockade. Post-operatively, subcutaneous emphysema was present around the port sites and intercostal drain site.The PaCO2 - EtCO2 gradient is largely dependent on the physiological dead space and the slope of the alveolar plateau in phase 3 of the capnograph. An increase in the dead space can result in an increase in the gradient. Negative PaCO2 - EtCO2 values were first observed during anaesthesia more than 50 years ago by Nunn and Hill. Reversal of the gradient can be seen normally in 50% of infants, pregnant, and obesepatients. Other causes are mechanical ventilation with large tidal volumes and low frequency, increased cardiac output and CO2 production, low functional residual capacity (FRC) and total lung compliance.[1] Thoracoscopy involves CO2 insufflation in the thorax for better visualisation and access. If there is a preexisting communication between the pleura and the bronchial tree, there can be direct absorption of CO2 and falsely high EtCO2 readings.[2] This can lead to negative PaCO2 - EtCO2 gradient where PaCO2 may be only mildly elevated. There are reported cases of negative arterial to end-tidal gradient in cases of malignant hyperthermia (MH).[13] The PCO2 of most alveolar gas is less than PaCO2, but in the terminal part of the expirate, the alveolar PCO2 may increase rapidly towards mixed venous PCO2 and exceeds PaCO2 in the presence of MH because a large amount of CO2 is discharged into the lungs.[3] In our case, the reasons could be multiple, i.e., bronchopleural communication, and low FRC under anaesthesia. In addition, the presence of subcutaneous emphysema suggests extensive CO2 tracking and absorption contributing to the hypercarbia.[4] CO2 desufflation and resumption of two lung ventilation led to the normalisation of EtCO2 without any deleterious effects.In the event of severe intraoperative increase in the EtCO2, we need to correlate the EtCO2 value with PaCO2 value using an ABG. Rising EtCO2 may not always translate to rising PaCO2. Various causes of negative gradient should be ruled out.