(Written with assistance from Thomas Doyle, MS, CCP)
Cardiopulmonary bypass has proven effective in the rewarming of hypothermic or under-ice drowning victims. Oxygenation has not been shown to be a critical factor in these circumstances. However, most groups make use of an oxygenator to provide for efficient heat exchange and the potential need for supplementation of the arterial PO2 if the need arises during rewarming of the victim (now patient).
II. PREDISPOSING FACTORS:
A. Most common predisposing factors relating to Accidental Hypothermia:
1. Impairment from alcohol, drug abuse, or mental illness – 40% of all cases. Conditions that interfere with normal cutaneous vasoconstriction or muscle movement may lead to core hypothermia and possibly ventricular fibrillation.
2. Avalanche and climbing accidents – 28% of all cases.
3. Accidental water immersion – 16% of all cases.
a. Fresh water drowning – Will see profound hypo-osmotic plasma volume expansion as free water is absorbed across the alveolar membrane causing pulmonary edema, congestive heart failure, and hemolysis.
b. Salt water drowning – May be less physiologically disturbing compared to fresh water drowning.
Initially body cooling by conduction (immersion) or convection (air exposure) induces peripheral vasoconstriction and shivering, generating additional heat. Later you will see declining mental status, ataxia, tremulous speech and hyper-reflexia to about 32 Celsius; shivering ceases. Below 32 Celsius stupor progresses and hypo-reflexia appears along with muscular rigidity, hypoventilation, respiratory acidosis and cardiac arrhythmia’s commonly with underlying bradycardia. 28 Celsius is generally considered the fibrillatory threshold. Finally multiple-system organ failure occurs secondary to decreased organ perfusion and cerebral blood flow.
IV. TREATMENTS: CONVENTIONAL WARMING VS. CPB
Depending on the level of hypothermia, either conventional warming techniques or cardiopulmonary bypass may be used. The advantages of bypass include reliable tissue perfusion , circulatory support, and maintenance of oxygenation during rewarming.
In severe exposure hypothermia with deep to profound hypothermic core body temperatures, many unusual problems may be exhibited. These include, but are not limited to, the following:
1. Venous access problems necessitating a cut-down for venous access.
2. Difficulties in establishing electrocardiogram and arterial pressure monitoring.
3. After rewarming, increased vascular permeability and potential coagulopathies.
4. High urinary output or no urine output. A hemoconcentrator should be incorporated into the ECC for better fluid management.
5. High volume requirements during CPB (10-15 liters) to maintain a normal fluid status and rewarming pump flow rates.
A. Cardiopulmonary bypass is indicated/recommended if the core temperature is less than 25 Celsius, if there is ventricular fibrillation, or if there is no rhythm. Either femoral-femoral or atrial-aortic bypass may be used. There is no significant advantage to one over the other as shown by survival rates.
B. If the patient is stable and at 25 Celsius to 28 Celsius, treatment may be by conventional methods or cardiopulmonary bypass. When the patient core temperature is less than 28 Celsius, there is a high risk of cardiac arrest. Peritoneal irrigation may be used along with airway warming, gastrointestinal warming, and pleural irrigation.
C. If the patient is stable and at 28 Celsius to 32 Celsius, conventional warming methods should be used as stated above. There is still an increased risk of cardiac instability at this temperature.
D. Patients with core temperatures above 32 Celsius can be treated with conventional warming methods.
A. Severe brain injury or other severe injuries.
B. Other prognostic indicators which show a grave prognosis include:
1. Profound hyperkalemia with K+ > 10 meq/L
2. Severe acidosis with pH < 6.5
3. Rectal temperature (core) < 12 Celsius
Much research remains to done in this area and ruling out patients due to poor health or degree of hypothermia to attempt rewarming on may be a mistake..
VII. MORBIDITY / MORTALITY RATE:
A. According to a study in the Annals of Thoracic Surgery (1994;58:895-8), a comprehensive literature search was done and data compiled which showed the following:
1. Overall survival rate was 60%, with 90% of those surviving in cardiac arrest prior to resuscitation. The mean core temperature was 21 Celsius in that group of 68 patients.
2. 80% of survivors returned to their previous level of function.
3. Of non-survivors, 67% died due to the inability to establish a cardiac rhythm or unable to wean from cardiopulmonary bypass.
4. There were no survivors with a core temperature less than 15 Celsius.
5. Survival of drug abuse/alcohol victims was 70%. Accidental immersion victims had 64% survive, and avalanche/climbing victims had the lowest survival at 32%.
6. Complications of the post-cardiopulmonary bypass patients were acute respiratory distress syndrome (ARDS) or pulmonary edema (42%), acute renal failure (20%), pneumonia (22%).
A. The perfusion team initiates cardiopulmonary bypass at the patient’s present core temperature and begins to gradually and slowly (4 degrees centigrade/hour).
B. A slow rewarm period insures an overall even distribution of rewarming to all vital organs and is preferred.
C. As core temperature approaches 32C, the patient can be defibrillated.
D. Because of the patient’s prior arrested circulation, metabolic acidosis is always present and contributes to the myocardial irritability.
E. Vigorous and aggressive acid-base and electrolyte management must be performed by the perfusion team.
F. During the rewarm, vascular beds will continue to open up and “wash-out” acid.
REMEMBER: A normal pH on this blood gas DOES NOT mean you will have a normal pH in 2 minutes. You must continually monitor and watch for change.