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Neurology India, Vol. 59, No. 5, September-October, 2011, pp. 777-778 Letter to Editor Venous air embolism during release of tethered spinal cord in prone position Charu Mahajan, Girija P Rath, Vivek B Sharma, NS Ajai Chandra Department of Neuroanaesthesiology, All India Institute of Medical Sciences, New Delhi, India PMID: 22019677 DOI: 10.4103/0028-3886.86569 The occurrence of venous air embolism (VAE) in position other than sitting is a less-recognized entity. Few subclinical cases may even go unnoticed. VAE has been reported in adult patients undergoing major spine surgeries in prone position. [1] However, no reference for its occurrence could be found in children undergoing corrective surgery for neural tube defects. A 2-month-old male, weighing 6 kg, was posted for excision and repair of cervico-dorsal myelomeningocele (MMC), and release of tethered spinal cord at L 3-4 level. Anesthesia was induced with sevoflurane in O 2 and IV access was secured. Tracheal intubation was facilitated with 3.0 cuffed tracheal tube after IV fentanyl 10 mg and rocuronium 4 mg. Anesthesia was maintained with O 2 , N 2 O, sevoflurane, fentanyl and rocuronium. Positive pressure ventilation was provided with pressure control mode, and the child was placed prone. Excision and repair of MMC was uneventful, which was followed by laminectomy and release of tethered spinal cord. At the time of dural opening, a sudden drop of end-tidal CO 2 (EtCO 2 ) from 40 to 18 mmHg was observed [Figure - 1]. Subsequently, the arterial oxygen saturation (SpO 2 ) decreased from 99 to 80%, heart rate increased from 128 to 148 beats/min, and the BP dropped down from 84/46 to 64/36 mmHg. The ventilation was checked to rule out possible dislodgement of tracheal tube. Simultaneously, N 2 O was switched off, surgeon was informed, and the surgical site was flooded with saline. There was no central venous catheter in situ; hence, air could not be aspirated. The child was ventilated with 100% O 2 and the SpO 2 improved to 99% within 1 min. The EtCO 2 gradually increased to 34 mmHg over 8 min. Hemodynamic parameters were improved with fluid infusions, which remained unchanged even after 15 min of correction. Hence, it was decided to continue with the surgery. Rest of the procedure was unremarkable. At the end of surgery, residual neuromuscular blockade was reversed and the trachea was extubated. The postoperative arterial blood gases (ABG), chest X-ray and ECG were within normal limits. The child was discharged on 5 th postoperative day without any fresh neurological deficit. The air entrainment into the venous system includes two risk factors: Direct communication between air and non-collapsible veins, and a pressure gradient favoring the air entry. [2] VAE, being less common in prone position, is logistically explained by the fact that there is almost absence of any gravitational gradient between the site of surgery and the right atrium. However, it has been explained how a significant gradient may still exist with the patient in prone position. [1] In our case, the child was placed prone on bolsters with abdomen hanging free, thereby lowering caval pressures, and hence, increasing the chance of air entrainment into the portal circulation. In sitting position, the incidence of VAE is lower in children than in the adults. [3] The finding has been attributed to the relatively high dural sinus pressure in children. [4] In this child, the excision of cervico-dorsal MMC resulted in acute loss of CSF. Though the CSF is not in direct continuity with the systemic circulation, still it might have lowered the dural sinus pressure. Subsequent dural incision for release of tethered cord might have led to the air entrainment via the exposed venous sinuses. The child experienced significant hemodynamic changes along with desaturation, thus indicating that a large amount of air embolism must have taken place. [5] Our literature search revealed this infant to be the youngest patient experiencing significant VAE in prone position. Nevertheless, for all surgical procedures performed in horizontal position, even if the risk of VAE is low, a close visual monitoring of surgical steps, EtCO 2 , and hemodynamics should be done, keeping occurrence of VAE in mind. A false sense of safety that the patient is not operated in sitting position should not be relied upon. References
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