ANESTHESIA FOR SUSCEPTIBLE PATIENTS
Anesthesia should consist of nitrous oxide, barbiturates, etomidate,
propofol, opiates, tranquilizers, or nondepolarizing muscle relaxants. Potent volatile
agents and succinylcholine must be avoided, even in the presence of dantrolene.
Some human patients have experienced a hypermetabolic state despite these precautions,
but they have always responded favorably to intravenous dantrolene. Preoperative
dantrolene is not needed because the use of nontriggering agents is almost always
associated
with uneventful anesthesia. If used, 1 to 2 mg/kg should be given intravenously
just before induction to avoid the side effects of lengthy pretreatment. In obstetric
cases, it is best given after the cord is clamped to avoid the problems of a floppy
child, because cord blood levels may approach 65% of maternal levels.[125]
Regional anesthesia is safe and may be preferred. Amide anesthetics had been considered
dangerous in susceptible patients because they induce or worsen contractures in vitro
as a result of their effect in increasing calcium efflux from the SR. However, these
effects require millimolar concentrations, far greater than plasma values achieved
in clinical use. Porcine and human studies demonstrated the lack of danger of amide
anesthetics. Intravenous lidocaine was used as long ago as 1970 to treat acute MH
without harm and with apparently good results.[6]
Anesthetic machines may be "cleansed" of potent volatile agents
by removal or sealing of the vaporizers, change of soda lime, perhaps replacement
of the fresh gas outlet hose, and use of a disposable circle with a flow of 10 L/min
for 5 minutes. After flow is reduced, the volatile agent's concentration may again
increase. A gas analyzer demonstrates a continuous volatile vapor concentration.
This is important because some new machines require a constant high flow to minimize
volatile agent concentrations.[126]
The anesthesiologist should confidently discuss the anesthetic
care with the patient, assuring him or her that all will be done to avoid difficulties
with MH and that the appropriate drugs, knowledge, and skills are immediately at
hand if any problems occur. Many of these patients have undergone procedures uneventfully,
such as dental analgesia and obstetric anesthesia, before the diagnosis of susceptibility
was made. The patient can enter the therapeutic environment in a reassured, relaxed,
and comfortable state. Outpatient procedures are feasible in most environments;
the time of discharge depends on usual outpatient criteria. Any facility using MH
triggers on an inpatient or outpatient basis should have dantrolene immediately available.
Several species, such as pigs, dogs, cats, and horses, have suffered
MH episodes.[46]
A canine colony inbred for MH
has the canine RYR1 gene.[127]
Cosgrove and colleagues[128]
examined contractures
in greyhounds in conjunction with a triggering anesthetic challenge; all findings
were normal. Continuing study of the capture myopathy of wild animals, more recently
in feral deer, suggests that this is "overstraining" during the chase of normal animals
rather than a genetic disorder similar to MH.[129]
