Malignant Hyperthermia Part II
The perfusionist’s preparation to deal with malignant hyerthermia (MH) and its variants should begin with the education of other health care professionals. Because the resuscitation of a patient enduring MH can be lengthy, sufficient time is usually available for a perfusionist to set-up an extracorporeal circuit for cardiopulmonary support of the MH patient. Since most MH cases occur in the perioperative period, anesthesia, operating room, and recovery room personnel can be educated to notify the perfusionist to standby if MH or a variant is diagnosed. However, since NMS and non-anesthesia related rhabdomyolysis patients are often admitted to the ICU, the ICU personnel should also be educated to the perfusionist’s potential role in a failing resuscitation effort. For these reasons, the perfusionist, along with anesthesia personnel, should provide a resource for training in MH and its variants by participating in regularly scheduled updates to the perioperative and ICU personnel.
Since MH and its variants are very rare, few perfusionists will have an opportunity to participate in the resuscitation efforts. The few perfusionists who do participate in the care of such a patient will probably only have a single opportunity to rescue a child or adult who is dying from MH. So, an accurate assessment of the patient’s condition is vital in quickly planning an extracorporeal perfusion strategy. For example, is the patient experiencing cardiac or respiratory failure or both? Is the hyperthermia being well controlled by topical cooling and lavaging or will cooling by extracorporeal circulation be necessary? Is there hemolysis from the elevated temperature? Is rhabdomyolysis present with severe potassium and myoglobin levels? Is the patient hyperosmolar? How severe is the base deficit? Is muscle rigidity going to affect the cannulation site? With these facts in mind, the perfusionist can plan a strategy to best support the patient until the hypermetabolic episode dissipates.
First, the appropriate extracorporeal circuit size should be chosen, particularly if the patient is a child. Neck and femoral muscle rigidity may preclude those sites for cannulation and make sternotomy appropriate. If the hyperthermia is uncontrolled, then the perfusionist must have equipment available to cool the blood, as most conventional ECMO and CPS systems do not have cooling capacity.
If severe hemolysis or rhabdomyolysis is present, then hemodilution of the patient’s blood volume with the circuit prime is appropriate. The choice of oxygenator is critical because the patient will be in a hypermetabolic state with increased oxygen consumption and excessive C02 excretion. If the patient is a child, then a borderline sized oxygenator should not be used. If previous hemodilution has already been performed, then a crystalloid prime might be inappropriate if the hematocrit is already low. A Catch-22 situation may occur wherein hemodilution with normal saline is necessary to reduce the potassium and myoglobin concentration, but in a hypermetabolic state a reduced hematocrit might result in tissue hypoxia and worsening of the acidosis. Since a hypermetabolic state is usually present, pump flow should be based on an increased cardiac index, the sweep gas should be kept high until the carbon dioxide is well controlled and 100% oxygen should be used to maximize tissue oxygenation, even if arterial hemoglobin is 100% saturated.
The type of prime is important and depends upon the presence of elevated potassium, myoglobin or free hemoglobin. It also depends upon the presence of a hyperosmolar state. If serum electrolytes are normal, then a balanced electrolyte, calcium free saline solution can be used. Sodium bicarbonate should be added to this prime in the appropriate amount to prevent the hemodilution of the patient’s remaining inherent bicarbonate level. If there is no hyperosmolar state, then mannitol can be added to stimulate urine output. In the presence of hyperosmolar state, no mannitol should be used. In addition, any saline solution should be diluted with the appropriate amount of water such that the addition of sodium bicarbonate does not result in a hyperosmolar prime solution. If the potassium is elevated, then a normal saline solution plus bicarbonate should be used to dilute the patient’s inherent potassium level without increasing the base deficit further.
Heparinization is dependent upon several things also. In the trauma patient, particularly head trauma, with rhabdomyolysis, heparinization may be a relative contraindication. If available, heparin-bonded circuits are indicated. Trauma may also be associated with DIC, and depending on the opinions of the individual health care professionals involved, heparinization may be beneficial or detrimental. Heparinization of post-surgery patient may also result in increase blood loss from the surgery site.
Once on extracorporeal support, ancillary interventions should be considered, depending on the patient’s condition. If the hematocrit low, the base deficit is extreme, or bleeding excessive, the transfusion of washed, low potassium red blood cells and other blood products may be appropriate. If potassium, myoglobin, or free hemoglobin is high, then high flux, zero-balanced hemofiltration should be attempted. Some myoglobin and free hemoglobin proteins can be forced across a hemofiltration device and success is readily seen by a pink colored ultrafiltrate. While hemodialysis may be capable of removing potassium, the urgency of the situation may preclude its relatively slow correction using hemodialysis. And hemodialysis will not remove myoglobin or free hemoglobin.
The resolution of the hypermetabolic state, the resolution of the elevated potassium, the resolution of calcium metabolism and resolution of the hyperthermia can be accomplished within a few hours of extracorporeal support. However, cardiac or pulmonary damage may result in the need to prolong extracorporeal cardiopulmonary support. And failure to adequately reduce myoglobin or free hemoglobin levels may result in the need for hemodialysis for many days or weeks before resolution of the acute renal failure.