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Topic: Outpatient knee arthroscopy - tachycardia, hypercarbia; treatment MH crisis
A 24-year-old 70 kg male semi-professional soccer player is scheduled for an elective right knee arthroscopy with possible menisectomy and ACL repair at 0730 at a freestanding ambulatory surgery center. He denies any family history of serious complications associated with general anesthesia. The surgeon and patient prefer that a femoral nerve block be performed in the preoperative area, and that general anesthesia be employed in the operating room to ensure complete patient comfort.
In the preoperative area, the patient is sedated with 3 mg iv midazolam and 25 mg fentanyl. The femoral nerve block is performed with nerve stimulator localization; 15 cc 0.5 % bupivacaine and 15 cc 1.5% lidocaine with 1:200,000 epinephrine are injected following negative aspiration for blood. The patient is transported to the operating room. He is induced with 150 mg propofol and 50 mg lidocaine; a #4 LMA is easily placed with good chest rise, +ET-CO2, and ability to generate a 500 cc tidal volume with <15 cm positive pressure. Anesthesia is maintained with sevoflurane/nitrous oxide with spontaneous ventilation. A forehead LCD temperature strip is used for temperature monitoring. The patient’s initial respiratory rate is 18/minute with a tidal volume of 300 cc. With the dial concentration 1.0% sevoflurane, the patient has no initial change in heart rate, BP, or respiratory rate when the scope is first inserted into the knee joint.
Over the next 20 minutes, the patient’s respiratory rate increases to 30/minute and the tidal volume also increases to 600 cc. The thigh tourniquet has not been inflated. The sevoflurane concentration is increased briefly to 6%, then maintained at 2.5 % for the next 5 minutes. The respiratory rate increases to 36/minute with no reduction in tidal volume. The end-tidal CO2 reads between 60-65 mm. SpO2 decreases from 99% to 96%.
1) Which of the following could explain the increase in minute ventilation and hypercarbia?
A) Light anesthesia
B) Pulmonary embolus
C) Malignant hyperthermia
D) Incompetent unidirectional valves
E) Ineffective CO2 absorption
The gas analyzer does not show any elevation of inspired CO2. The LCD temperature strip shows a temperature of 37.5° C. The patient’s baseline heart rate was 56; his heart rate is now 95. The anesthesia care professional is unsure about the high ET-CO2 reading.
2) The anesthesia care professional should:
A) Ask for another CO2 analyzer
B) Briefly disconnect the gas sampling tubing from the patient circuit, breath into it and see if it reads ~40 mm
C) Change the CO2 absorbent.
The anesthesia care professional suspects MH. She checks muscle tone in the arms and masseter muscles. No rigidity is detected.
3) True or False: The absence of muscle rigidity rules out MH.
The surgery center does not have a lab on-site.
4) Which of the following actions should NOT be taken?
A) Notify the surgeon, OR nurses and workroom personnel of the MH crisis
B) Arrange urgent transport to the closest hospital with an ICU
C) Have the MH cart brought to the OR
D) Discontinue sevoflurane
E) Bring another anesthesia machine into the room and change machines
F) Use succinylcholine for endotracheal intubation
G) Intubate with propofol with or without a non-depolarizing muscle relaxant
H) Prepare at least 7 bottles dantrolene for rapid iv administration
I) Increase fresh oxygen gas flow to 10 liters/minute
J) Mechanically ventilate the with initial minute ventilation >20 liters/minute
K) Avoid using the ventilator and ventilate by hand
L) Measure temperature in the distal esophagus, nasopharynx, or rectum.
M) Draw an arterial or non-tourniqueted venous blood sample into a heparinized syringe and place on ice for transport with the patient
5) True or False: Paramedics should immediately transport the patient to the hospital before any dantrolene has been given.
6) Dantrolene dissolves best in which of the following:
A) Normal saline for injection
B) Sterile water for injection at 20° C.
C) D5W
D) Sterile water for injection at >30° C.
Temperature probes in the distal esophagus and nasopharynx read 38.5° C. The case is being done on Saturday morning, and there are only four people available to manage the MH crisis. The patient has been intubated and the MH cart has just been brought to the room.
7) Which of the following actions is NOT appropriate:
A) Two people should leave the room to obtain lots of ice in order to rapidly reduce the patient’s temperature.
B) All persons should work together to mix and give 2-3 mg/kg dantrolene ASAP, mechanically hyperventilate the patient, and treat any arrhythmia suggestive of hyperkalemic cardiotoxicity.
The EKG shows a wide QRS complex tachycardia at 120/minute with BP 95/60.
8) Which of the following are appropriate emergency treatment options?
A) 300 mg – 1000 mg iv CaCl2
B) 10 units regular insulin iv + 25 cc D50 iv followed by a dextrose infusion
C) 40 mg Lasix iv
D) 4 mg (>40 puffs via metered-dose inhaler) aerosolized albuterol.
E) 2 mg/kg iv lidocaine
Answers:
1. C, D, E
2. B
3. False
4. E, F, K
5. False
6. D
7. A
8. A, B, D
Narrative:
1) Light anesthesia would be associated with hyperventilation and a decrease in ET-CO2. Hypermetabolism with MH results in hypercarbia despite an increase in minute volume. Both incompetent unidirectional valves and ineffective carbon dioxide absorption result in elevated inspired CO2. Hypercarbia due to problems with carbon dioxide absorption may be temporarily overcome by increasing fresh gas flow to the point where the circuit is effectively non-rebreathing.
2) If an MH crisis is suspected, I think this is the quickest way to determine if the gas analyzer is properly calibrated. If there is no inspired CO2, there is no reason to change the CO2 absorbent.
3) Life-threatening MH may occur without muscle rigidity.
4) One must notify your co-workers, discontinue the triggering anesthetic, get the MH cart, dissolve and rapidly give dantrolene, mechanically hyperventilate the patient, and obtain blood that may be tested once the patient arrives at a hospital (critical labs would be blood gas and potassium level.) Transport to a hospital with critical care capability must be arranged.
Each bottle of dantrolene has 20 mg dantrolene and 3 gm mannitol.
Ventilating by hand typically results in a decrease in delivered tidal volume and ties up an individual who could be performing other tasks. It is not beneficial to change anesthesia machines; increasing fresh oxygen gas flow to at least 10 liters/minute will rapidly decrease the inspired concentration of halogenated vapor.
An LCD strip is not sufficiently accurate for measuring temperature during an MH crisis; skin vasoconstriction will likely result in a skin temperature that is much lower than the core temperature.
5) Dantrolene must be given prior to transport in order to significantly reduce hypermetabolism. Control of hypermetabolism combined with hyperventilation will normalize PaCO2 and anaerobic production of lactic acid.
6) Dantrolene dissolves much more rapidly in sterile water for injection that has been warmed to 30-40° C. Dantrolene will not dissolve in isotonic crystalloid.
7) A temperature of 38.5° C does not pose an acute threat to life.
8) In the setting of MH, this likely represents hyperkalemic cardiac toxicity. Calcium, while not lowering the potassium level, directly and rapidly counteracts cardiac toxicity from hyperkalemia. Both insulin and albuterol lower potassium levels by promoting potassium transport back into cells. Dextrose is given with insulin to avoid hypoglycemia. Lasix does not lower potassium levels rapidly enough to be useful in an emergency. In the setting of acute hyperkalemia, lidocaine administration may result in asystole (hyperkalemia markedly enhances lidocaine’s depressant effects on the conduction system.)
_________________________
MH Hotline Consultant
Professor of Anesthesiology
UCLA School of Medicine
Los Angeles CA 90095-1778
