Thoracic epidural anesthesia as an adjunct to general anesthesia for cardiac surgery: Effects on ventilation-perfusion relationships

Abstract

Postoperative hypoxemia remains a serious and frequent complication after cardiac surgery. Development of atelectasis during anesthesia appears to be a major factor in producing the increase in intrapulmonary shunt and consequently impaired gas exchange after cardiopulmonary bypass. Postoperative hypoxemia can produce significant ill effects, including increase in duration of postoperative ventilation, pulmonary complications, myocardial ischemia, and neurologic impairment. These complications are associated with prolonged intensive care unit (ICU) and hospital length of stay, resulting in significant increase in cost. With great interest I have read a study by Tenling et al1Tenling A Joachimsson PO Tyden H et al.Thoracic epidural anesthesia as an adjunct to general anesthesia for cardiac surgery: Effects on ventilation-perfusion relationships.J Cardiothorac Vasc Anesth. 1999; 13: 258-264Abstract Full Text PDF PubMed Scopus (44) Google Scholar that extends the knowledge about the effects of thoracic epidural anesthesia (TEA) on pulmonary function after cardiac surgery. The authors found that TEA when used in conjunction with general anesthesia caused similar changes to intrapulmonary shunt, ventilation-perfusion mismatch, and alveolar arterial oxygen gradient when compared with general anesthesia alone. Patients in the TEA group had a slightly lower PaCO2 on the first postoperative day, indicative of improved ventilation; however, the incidence of postoperative atelectasis was similar in both groups. (Continued) I would like to challenge the conclusions made by the authors—that TEA facilitated the reduction of postoperative intubation time and decreased the need for morphine analgesics after coronary artery bypass graft surgery. First, according to the protocol, one of the measurements had to be taken 4 hours after separation from cardiopulmonary bypass, necessitating compulsory postoperative sedation. This factor may have precluded some patients from earlier extubation in the TEA group. Nevertheless, the duration of intubation in the control group after admission to the ICU was on average 4 hours longer than in the TEA group. Although it was stated that extubation criteria were the same for both groups, the reasons for prolonged intubation in the control group were not identified. The dose of fentanyl was not standardized between the groups (5 to 10 μg/kg v 1 to 2 mg in the TEA and control groups). This difference was clearly an inherent contributing element that prevented patients from early extubation in the control group. Second, the authors showed that opioid requirements were reduced in the epidural group; however, they failed to comment on the clinical significance of this finding (reduced incidence of opioid side effects?, improved respiratory function?, earlier mobilization of patients?, reduced length of stay?, improved patient satisfaction?). The epidural administration of sufentanil infusion (3 to 7 μg/hr) must have largely contributed to morphine-sparing effects in the TEA group and should have been accounted for when average doses of postoperative opioid analgesics were calculated. It has been confirmed that early extubation is safe and cost-effective in cardiac surgery.2Cheng DCH Karski J Peniston C et al.Morbidity outcome in early versus conventional tracheal extubation after coronary artery bypass graft surgery: A prospective randomized controlled trial.J Thorac Cardiovasc Surg. 1996; 112: 755-764Abstract Full Text Full Text PDF PubMed Scopus (308) Google Scholar, 3Cheng DCH Karski J Peniston C et al.Early tracheal extubation after coronary artery bypass graft surgery reduces costs and improves resource use: A prospective randomized controlled trial.Anesthesiology. 1996; 85: 1300-1310Crossref PubMed Scopus (344) Google Scholar Most studies to date that have explored TEA in cardiac surgery have been limited to patients with good left ventricular function undergoing elective coronary artery bypass graft surgery. The rates of serious complications after this type of surgery are rather low, making it difficult to show that one anesthetic technique is better than the other. Future research trials should target the particular subgroups of patients (eg, with chronic obstructive airway disease, obesity, elderly, poor left ventricular function), as well as patients scheduled for off-pump procedures to investigate the potential improvement in outcome with application of regional techniques in cardiac surgery.