Case of the month October 2007

Topic:  Surgery for a parotidectomy

An 11 year old child is having surgery for a parotidectomy (possible tumor).  She had general anesthesia once before without any difficulties noted.  Following uneventful endotracheal intubation facilitated with rocuronium, the anesthetic continues with sevoflurane and oxygen.  Breath sounds were equal and clear, but her end-tidal CO₂ rose to 68, despite increasing the minute ventilation to 14 L/ min.  Her axillary temperature was 36.5, her heart rate was 70-110 and blood pressure was 110/64.  A blood gas was obtained.  Arterial pH 7.16, pCO₂ 71, pO₂ 461, normal bicarbonate by calculation.  While calling the MH hotline, the caller noted that the inspired CO₂ was rising by capnography.  The CO₂ scrubbing canisters were changed, but not noted to be exhausted.  They are mixing dantrolene when the MH hotline consultant is reached on the phone:

1. Is this MH?
1. Definitely MH
2. Probably MH
3. Not MH

2. If not MH, what else could this be?
1. Thyrotoxicosis
2. Sepsis
3. Pneumothorax
4. Respiratory circuit/equipment problem

3. What action should be taken now?
1. Administer dantrolene 2.5 mg/kg i.v.
2. Actively cool the patient
3. Change to a total intravenous anesthetic and discontinue sevoflurane
4. Examine the patient, airway and circuit for compression, anesthesia machine

4. Should this child be referred for MH biopsy testing?
1. No
2. Yes
3. Get more information from the family

Narrative:

            This child developed intraoperative hypercarbia, without evidence of hypoxia, rigidity, hypermetabolism or fever.  Not only was the end-tidal CO₂ climbing, but the inspired CO₂ was increasing.  This indicates inadequate ventilation and accumulation of exhaled gas in the airway circuit.  The first concern is the appropriate location of the endotracheal tube, presence or lack of bronchospasm, and prevention of hypoxia.  Then the endotracheal tube is examined for compression or kinking resulting in failure to deliver the tidal volume to the alveolar space.  Also, the tidal volume on the ventilator could be too low, or if using pressure controlled ventilation, then a change in compliance could diminish the delivered tidal volume.

The table was turned away from the usual position for the surgeons to have complete access to the child’s head.  In examining the anesthesia circuit, it was discovered that a piece of extension tubing had been inserted between the ‘Y’ of the circuit and the patient’s endotracheal tube.  This resulted in an ineffectively delivered tidal volume (too much dead space), and the accumulation and rebreathing of CO₂.  This is an error which did not result in major harm to this patient.  This situation can occur when someone not familiar with the anesthetic equipment interspaces a connection or extension which results in very large dead space.  I have also seen this when a patient (infant) was taken for an MRI under anesthesia using a semi-open system (Mapleson), but the provider interspaced an extension tubing between the elbow with the fresh  gas flow and the endotracheal tube.

Once the circuit issue was identified, the explanation was clear.  The dead space was removed, and the CO₂ promptly responded.  No cooling, dantrolene or other laboratory testing is required.   The child should not be referred for a caffeine halothane contracture testing, as there is insufficient concern for malignant hyperthermia.

Answers:

1.      C.  This is not MH.

2.      D.  There is no evidence of hypermetabolism here consistent with A, B, or C.  This is an equipment problem with rebreathing of CO₂.

3.      D.  There is no evidence of MH crisis here, only hypercarbia with an increasing inspired concentration of CO₂.  This indicates rebreathing and the anesthesia equipment and ventilator need to be inspected. 

4.      A.  This child should not be referred for biopsy testing.

Joseph R. Tobin, M.D.

Professor of Anesthesiology

Director, MH Biopsy Testing Center

Wake Forest University School of Medicine