Tuesday, February 25, 2014

Masking A Patient. The Magic Number Is Fifteen.

In anesthesia residency, we were all taught how to gently induce anesthesia and intubate a patient. First we put the patient to sleep with the induction agent of choice, usually propofol. Once the patient was asleep we ventilated him with a face mask to be sure it could be done easily. When that was ascertained, then we gave our muscle relaxant to paralyze the patient for intubation. Masking the patient usually took about 90 to 120 seconds because we avoided using succinylcholine unless a rapid sequence intubation was desired. The reason we tried not to use sux was because of all its possible deleterious side effects, include hyperkalemia, malignant hyperthermia, and increased intracranial pressures. Plus patients frequently complained of whole body muscle aches during post op checks.

Now during those two minutes it took for the nondepolarizing muscle relaxants to work sufficiently for intubation, we had to mask the patient to prevent hypoxia. While doing this, we kept an eye on the peak airway pressure that was being applied to the patient. The magic number was 20 cm H2O. It was taught that using any more pressure to ventilate the lungs will also drive air into the stomach, distending it and possibly causing an aspiration. So we vigilantly looked at the pressure gauge to keep it just below the magical 20 line. Yet, no matter how careful I was, once the surgeon looked inside the abdomen during a procedure like a laparoscopic cholecystectomy, he would complain that the stomach is distended and I needed to drop in an orogastric tube to suction the air out. This isn't always easy to accomplish. The tube can be difficult to pass into the esophagus in the first place, coiling around itself in the mouth. Or it can get obstructed by a hiatal hernia in the lower esophagus. Or it can cause gastric mucosal bleeding from the suction during the case. Therefore placing an OG tube should not be taken lightly and, as with everything else we do, can be fraught with complications.

I always wondered how accurate that teaching of 20 cm H2O was. Now there is a study that lends credence to that age old dogma. In the February issue of Anesthesiology, researchers induced volunteers with propofol then ventilated them at peak airway pressures of 10, 15, 20, and 25 cm H2O. At the same time they used a stethoscope to listen for gastric air and an ultrasound to look for gastric distension and measure the size of the antrum.

What they found was that the stomach did distend in proportion to the pressures applied to the face mask. By listening to the stomach, the researchers found incidences of gastric insufflation ranging from 0% in the P10 group up to 41% in the P25 group. When visualized using the ultrasound, gastric insufflation went from 19% in P10 up to 59% in P25. The antral area when measured by the ultrasound did not significantly increase in size in the P10 and P15 groups while it was significantly increased in the P20 and P25 groups.

How well did the patients ventilate at these different pressures? As you would expect the greater the driving pressure, the higher the tidal volumes. With P10, the tidal volumes were only about 6 ml/kg. Tidal volumes were measured between 8 ml/kg and 12 ml/kg for the P15 and P20 volunteers. When the masking reached P25, the volumes shot up to 14 ml/kg.

Based on these results, the authors concluded that masking a patient up to 15 cm H2O gives a patient the greatest lung volumes with the smallest risk for gastric distension and aspiration. So my old attendings' lectures were not that far off. However, all of this would be moot if we could discover a nondepolarizing muscle relaxant that works as fast as sux without its side effects and dissipated just as quickly. Rocuronium currently comes the closest in rapidity of response. However it cannot be used in short cases due to its prolonged action. That is why the FDA's continued refusal to approve roc's reversal agent sugammedex is so disappointing. It has been used in Europe for years yet we can't get our gloves on it here. I wonder how many MH and other complications of sux could have been avoided in all these years if sugammedex had been approved for use in a timely manner. So in the meantime, keep masking those patients. Just keep it at 15 cm H2O or lower.


  1. The answer will be gantacurium.

  2. Nice article!
    In Spain we use bridion (sugammadex) all the time, but is very expensive! Every time we use it we have to justify it to pharmacy services.
    But it is really good, safer, faster than neostigmine!