I see it in very sick patients who had spontaneous ventilatory effort, assisted or unassisted, who following muscle relaxation (even with minimal or no induction agent) develop profound hypotension. And in my own experience it tends to be minimally responsive to fluid blouses, which I think is strange given that most of these patients are intravascularly volume depleted.
My best guess is the loss of negative intrathoracic pressure and transition to a positive pressure ventilation is impeding venous return. But, I'm unconvinced.
This is the definitely current thinking, at least as I understand it. I've seen significant hypotension on intubation & initiation of PPV, but it's really not an issue of NMB vs. no NMB, is it? It's more an issue of the transition from spont. V or PSV to full PPV. If you can avoid PPV, then by all means do. But if you can't avoid PPV, then I don't see how choosing to use NMB is going to make the PPV-induced hypotension worse.
To add one more: a lot of medications are given in the operating room. A whole regimen of drugs for each case. And of all those drugs, the number one culprit for anaphylaxis in the OR : muscle relaxants.
Definitely things to add to your risk:benefit ratio before you pull out paralytics needlessly.
The anaphylaxis thing is interesting. I just came across a few things about this recently. Very broad estimates of incidence, I've seen anything from 1:1000 up to 1:100,000, with 1:10,000-1:20,000 being the most common range cited, with severe reactions of course being much less common than less severe ones. Also the most common offending agents vary, with some sources saying roc, some saying vec, but most blaming sux as most likely to cause a problem.
Yet, we still give the stuff out like candy in the OR....in my training I never once heard someone say "let's try to avoid the NMB so that we don't cause any increased risk of a reaction". Not that such nonchalance is evidence that we shouldn't consider it, of course, but the point is I think we generally accept the small risk of a severe reaction as much lower than the likely benefits of the drug. As we generally do with all the drugs we give routinely. If the drug is indicated, it's probably well worth the risk. If it isn't indicated, then it's not. With the NMB-in-transport issue, it all comes down to whether we consider them "indicated" or not.
There was an interesting article published in PEC a few years ago that retrospectively looked at HEMS transports of intubated patients in Ontario, CA. It was more of a descriptive article but they did some post-hoc analysis, too. Found a 17% incidence of "critical events" during transport, mostly hypotension that appeared to be mostly related sedative dosing. I think NMB was given in something like 20% of the transports.