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The Possible Mechanism of Apnea After Institution of Cardiopulmonary Bypass in Conscious Patients

      To the Editor:
      A few authors have reported performing conscious cardiac surgery with thoracic epidural anesthesia (TEA) as the sole anesthetic.
      • Chakravarthy M.
      • Jawali V.
      • Patil T.A.
      • et al.
      High thoracic epidural anesthesia as the sole anesthetic for performing multiple grafts in off-pump coronary artery bypass surgery.
      • Aybek T.
      • Dorgan S.
      • Neidhart G.
      • et al.
      Coronary artery bypass grafting through complete sternotomy in conscious patients.
      Recently, 2 reports of operations requiring cardiopulmonary bypass (CPB) with epidural anesthesia have been published.
      • Stritesky M.
      • Semrad M.
      • Kunstyr J.
      • et al.
      On-pump cardiac surgery in a conscious patient using a thoracic epidural anesthesia—An ultra-fast-track method.
      • Chakravarthy M.
      • Jawali V.
      • Manohar M.V.
      • et al.
      Conscious cardiac surgery with cardiopulmonary bypass using thoracic epidural anesthesia without endotracheal general anesthesia.
      Various benefits such as ultra–fast-tracking, reduced morbidity, and improved neurologic monitoring have been cited by these authors. In the authors’ article,
      • Chakravarthy M.
      • Jawali V.
      • Manohar M.V.
      • et al.
      Conscious cardiac surgery with cardiopulmonary bypass using thoracic epidural anesthesia without endotracheal general anesthesia.
      a unique phenomenon was observed after institution of CPB in these patients. Ten of the 11 patients developed decreasing inspiratory efforts 1 to 2 minutes after establishing CPB, which culminated in apnea. Because the patients were on CPB, no treatment was undertaken. Despite the apnea, the patients were able to perform inspiration on request. Before discontinuing CPB, assisted respiration was commenced and spontaneous respiratory activity resumed 5 to 7 minutes after termination of CPB.
      The authors have performed about 150 such cases requiring CPB and continue to see this phenomenon. A case of mitral valve replacement that was being conducted under TEA is reported; the patient continued to have spontaneous respiration after institution of CPB. The spontaneous ventilation ceased soon after insertion of a pulmonary artery (PA) vent. This finding suggests that the mechanism of spontaneous respiration after institution of CPB in patients undergoing conscious cardiac surgery may be related to the presence or absence of blood circulating in the pulmonary arterial tree.
      A 47-year-old woman with severe mitral stenosis was scheduled to undergo mitral valve replacement. She had undergone closed mitral valvotomy 20 years ago. At the time of admission to the hospital, she had dyspnea on exertion, heart rate was 135 beats/min (atrial fibrillation), and blood pressure was 120/75 mmHg. An echocardiogram showed a mitral valve area of 0.55 cm2, with a clot in the left atrial appendage. Her treatment included digoxin, 0.25 mg once daily, and furosemide, 20 mg twice daily. Her coagulation profile was within normal limits. At this institution, TEA is offered to patients who do not have contraindications, and the possibility of performing the procedure without general anesthesia is suggested. The patient agreed. On the eve of the surgery, a 16-G epidural catheter (Epidural Minipack; Portex Ltd, Kent, UK) was inserted by midline approach, the space was identified by the loss-of-resistance technique, and 3 cm of the catheter were inserted.
      The patient was premedicated with 150 μg of fentanyl intramuscularly. Epidural analgesia was achieved with 6 mL of 0.5% bupivacaine, and 6 mL of 2% lidocaine with 1:200,000 epinephrine. The patient breathed spontaneously with 100% oxygen through the circle absorber. The operating room temperature was maintained above 20°C, and the patient was kept warm by a warming blanket. Warm (38°C) fluid was used for intravenous infusion and for clearing the surgical field.
      The surgery commenced, allowing access to the airway. Heparin, 20,000 U, was administered intravenously and activated coagulation time was 450 seconds. Normothermic CPB was established slowly over 5 minutes with a flow of 2.4 L/min/m2; fresh gas flow was maintained to achieve a PaO2 in the range of 200 to 300 mmHg and PaCO2 in the range of 35 to 40 mmHg. After establishing CPB, it was observed that complete bypass and reduction in the size of heart were not achieved. Spontaneous ventilatory efforts of the patient persisted, which decreased the surgical access. The surgeon inserted a PA vent; activation of the vent resulted in the heart emptying. Soon after this, spontaneous ventilatory efforts of the patient ceased, and the valve was replaced. After release of the cross-clamp on the aorta, the heart rhythm returned spontaneously to atrial fibrillation. Spontaneous ventilatory efforts returned with the discontinuation of PA venting. Assisted ventilation was performed until complete discontinuation of CPB. Discontinuation from CPB did not require any inotropic support; the surgery was completed in 130 minutes, with a cross-clamp time of 42 minutes and CPB time of 86 minutes. About 5 minutes after CPB, assisted ventilation was terminated, and the patient breathed spontaneously for the rest of the procedure. The patient was transferred to the intensive care unit and to the ward on the next day. She was discharged on the fifth postoperative day.
      This relatively new method of performing cardiac surgery requiring CPB under TEA has been reported recently.
      • Stritesky M.
      • Semrad M.
      • Kunstyr J.
      • et al.
      On-pump cardiac surgery in a conscious patient using a thoracic epidural anesthesia—An ultra-fast-track method.
      • Chakravarthy M.
      • Jawali V.
      • Manohar M.V.
      • et al.
      Conscious cardiac surgery with cardiopulmonary bypass using thoracic epidural anesthesia without endotracheal general anesthesia.
      • Schachner T.
      • Bonatti J.
      • Balogh D.
      • et al.
      Aortic valve replacement in the conscious patient under regional anesthesia without endotracheal intubation.
      • Chakravarthy M.
      • Jawali V.
      • Patil T.A.
      • et al.
      Cardiopulmonary bypass in conscious patients undergoing cardiac surgery.
      Stritesky et al
      • Stritesky M.
      • Semrad M.
      • Kunstyr J.
      • et al.
      On-pump cardiac surgery in a conscious patient using a thoracic epidural anesthesia—An ultra-fast-track method.
      claim superiority of this technique over conventional techniques by using endotracheal general anesthesia in patients with pulmonary morbidities and those undergoing endoscopic cardiac surgeries. Schachner et al
      • Schachner T.
      • Bonatti J.
      • Balogh D.
      • et al.
      Aortic valve replacement in the conscious patient under regional anesthesia without endotracheal intubation.
      commented that “surgical progress was slightly disturbed by the spontaneous breathing pattern.” However, in the authors’ series, it was observed that patients ceased to have voluntary respiratory efforts after establishment of CPB.
      • Chakravarthy M.
      • Jawali V.
      • Patil T.A.
      • et al.
      Cardiopulmonary bypass in conscious patients undergoing cardiac surgery.
      It was hypothesized that the causes of this apnea were sudden hemodilution at the onset of CPB, absence of pulsatile flow, acute hypocapnea, and/or absence of blood flow in the pulmonary artery. The authors are now of the opinion that the cause of cessation of spontaneous respiration may be absence of blood in the pulmonary vascular tree and may operate via stretch receptors in the pulmonary artery.

      References

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        • et al.
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        Coronary artery bypass grafting through complete sternotomy in conscious patients.
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        • Kunstyr J.
        • et al.
        On-pump cardiac surgery in a conscious patient using a thoracic epidural anesthesia—An ultra-fast-track method.
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        Conscious cardiac surgery with cardiopulmonary bypass using thoracic epidural anesthesia without endotracheal general anesthesia.
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        • Schachner T.
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        Aortic valve replacement in the conscious patient under regional anesthesia without endotracheal intubation.
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