Dear Editor,
Sodium nitroprusside (SNP) is an integral part of any hypotensive anaesthetic technique and is being used frequently in cardiac and neuro-anaesthetic practice [1]. Its indications and dosages are well-defined and adverse effects are well-documented. However, because of its extreme potency and instant onset of action, undesirable effects are not uncommon. We wish to document an unusual occurrence with potentially disastrous consequences while using the drug.
A 35-year-old male, a known case of right frontal arterio-venous malformation (AVM), which had bled once, was scheduled to undergo clipping of feeder vessels. He had no neurological deficit and his clinical as well as biochemical profiles were essentially normal. Considering the nature of the surgery, general anaesthesia with controlled hypotension and invasive arterial pressure monitoring was planned. He was given β-blockers preoperatively and on the day of surgery he received general anaesthesia using thiopentone, pancuronium and isoflurane. When the surgeon was ready to handle the AVM, SNP microdrip (D-1 in Fig) was started with a view to bring down the mean arterial pressure (MAP) to 50 mmHg. The drip was connected to a previously secured vein (D-2 in Fig) by means of a three-way stopcock and was adjusted to deliver SNP at a rate of 0.5 µg/kg/min. Initially there was difficulty in obtaining a smooth flow which was overcome by adjusting the three-way stopcock and placing D-1 higher than D-2. After the feeder vessels had been clipped, the drip was gradually tapered off and finally closed. Ten minutes later the previous drip (D-2) was restarted. Within seconds the MAP fell to 30 mmHg. The infusion had to be stopped and a thorough revaluation of the anaesthetic technique was carried out. However no implicating causes could be found and MAP returned to normal almost immediately. Same sequence of events recurred on 2 occasions as we attempted to restart drip D-2. Hence it was decided to replace the entire bottle and infusion set. Following this there were no further incidences of hypotension and subsequent course of anaesthesia and surgery was absolutely uneventful.
Fig.
Showing likely position of threeway stopcock leading to retrograde flow
Retrospectively, while auditing the event, the function of the three-way stopcock used was analyzed for any leaks or flow in unintended direction. The stop-cock was found to be normal. However the possibility of SNP solution finding its way into D-2 while connecting it to the three-way could not be ruled out. Therefore it was thought that the venous end of the three-way was turned off accidentally thereby bringing D-1 and D-2 in continuity and allowing retrograde flow in D-2. To confirm this we carried out a simple experiment using the two drip sets and simulating the circumstances (Fig). It was evident that the moment D-1 was placed at a height of 10 cm above D-2, the solution began to flow in to D-2 and within 8–10 seconds the entire tubing and air chamber of D-2 was filled. We also measured this volume to be 12–15 mL approximately. Thus it appears rational to believe that there indeed was transfer of about 1200 to 1500 µg of SNP in D-2 which could have occurred anytime during controlled hypotension. This also explains precipitous fall in MAP each time the infusion was started.
We conclude by emphasizing that a potent drug infusion like SNP should always be set up separately and should not be used for any other purpose in order to avoid treading on a path to disaster.