Thomas J. Conahan

Remifentanil Compared with Alfentanil for Ambulatory Surgery Using Total Intravenous Anesthesia

The purpose of this study was to test the hypothesis that using a 1:4 ratio of remifentanil to alfentanil, a remifentanil infusion would provide better suppression of intraoperative responses and comparable recovery profiles after ambulatory laparoscopic surgery than an alfentanil infusion, as part of total intravenous anesthesia. Two hundred ASA physical status I, II, or III adult patients participated in this multicenter, double-blind, parallel group study. Patients were randomly assigned 2:1 to either the remifentanil-propofol or alfentanil-propofol regimens. The anesthesia sequence was propofol (2 mg/kg intravenously [IV] followed by 150 micro g [centered dot] kg (-1) [centered dot] min-1), and either remifentanil (1 micro g/kg IV followed by 0.5 micro g [centered dot] kg-1 [centered dot] min-1) or alfentanil (20 micro g/kg IV followed by 2 micro g [centered dot] kg-1 [centered dot] min (-1)), and vecuronium. After trocar insertion, infusion rates were decreased (propofol to 75 micro g [centered dot] kg-1 [centered dot] min-1; remifentanil to 0.25 micro g [centered dot] kg-1 [centered dot] min-1; alfentanil to 1 micro g [centered dot] kg-1 [centered dot] min-1). Alfentanil and propofol were discontinued at 10 and 5 min, respectively, before the anticipated end of surgery (last surgical suture); remifentanil was discontinued at the end of surgery. Recovery times were calculated from the end of surgery. The median duration of surgery was similar between groups (39 min for remifentanil versus 34 min for alfentanil). A smaller proportion of remifentanil patients than alfentanil patients had any intraoperative responses (53% vs 71%, P = 0.029), had responses to trocar insertion (11% vs 32%, P < 0.001), or required dosage adjustments during maintenance (24% vs 41%, P < 0.05). Early awakening times were similar. Remifentanil patients qualified for Phase 1 discharge later and were given postoperative analgesics sooner than alfentanil patients (P < 0.05). Actual discharge times from the ambulatory center were similar between groups (174 min for remifentanil versus 204 min for alfentanil) (P = 0.06). In conclusion, remifentanil can be used for maintenance of anesthesia in a 1:4 ratio compared with alfentanil, for total IV anesthesia in ambulatory surgery. This dose of remifentanil provides more effective suppression of intraoperative responses and does not result in prolonged awakening. (Anesth Analg 1997;84:515-21)

Hemodynamic effects of high-frequency jet ventilation

The hemodynamic effects of high-frequency jet ventilation (HFJV) and conventional ventilation were compared in normovolemic and functionally hypovolemic dogs. In normovolemic animals, no differences in hemodynamic function were found among spontaneous ventilation, conventional ventilation, and HFJV. When venous return was impaired by 15 cm H2O PEEP, cardiac index and stroke index were 25% higher with HFJV than with conventional ventilation (P < 0.05). In another study with PEEP, conventional ventilation was compared to spontaneous ventilation, HFJV synchronized to five different parts of the cardiac cycle, and asynchronous HFJV. Heart rate was 15% lower and mean arterial pressure was 26% lower with conventional ventilation than with HFJV modes (P < 0.05). There were no differences between synchronous and asynchronous HFJV. These results indicate that hemodynamic dysfunction may be less likely with HFJV than conventional ventilation. No advantage of synchronizing jet pulsations to a specific part of the cardiac cycle could be demonstrated.

Cardiovascular effects of protamine sulfate in man.

The effect of protamine sulfate on several cardiovascular and biochemical variables was studied in man under clinical conditions. This study was performed to quantitate these effects in 15 adult patients who had undergone cardiopulmonary bypass for coronary artery bypass grafting. Protamine was administered in typical clinical doses (3 mg/kg) at typical clinical rates (total dose infused over 5 minutes). This infusion rate is greatly in excess of the 50 mg/10 min suggested in the protamine package insert. No statistically significant changes in mean arterial blood pressure, cardiac output, central venous pressure, total or ionized calcium, Pao2, Paco2, pH, Na+, or K+ were found during or after administration of protamine sulfate. Hypotension was observed after administration of protamine to one patient, but no etiologic mechanism was apparent. Previous reports suggest cardiovascular depression by protamine in the dog, a species highly susceptible to these effects. Data obtained in man in this study do not corroborate the canine studies.

Comparison of high-frequency jet ventilation with conventional mechanical ventilation in saline-lavaged rabbits

A surfactant-depletion lung-injury model was produced in 37 New Zealand white rabbits by saline lavage. During the next 2 to 3 h, rabbits were ventilated with conventional mechanical ventilation (CMV, group 1), high-frequency jet ventilation (HFJV, group 2), or CMV for 1 h followed by HFJV for 2 h (CMV/HFJV, group 3). Survival until planned termination of the protocol was 56%, 77%, and 63% in groups 1, 2 and 3, respectively. Causes of early demise were usually pneumothorax or metabolic acidosis. There were no statistically significant differences among the groups with respect to survival, incidence of pneumothorax or metabolic acidosis. Arterial oxygenation was more efficient with HFJV (group 2) (P[A-a]O2 = 372 ± 51 torr [mean ± SE] at 2 h) than with CMV (group 1) (P[A-a]o2 = 512 ± 18 torr at 2 h, p < .01). Furthermore, oxygen gas exchange in 3 of 5 group 3 rabbits improved after institution of HFJV. In contrast to previous findings with high-frequency oscillation (HFO), there were no qualitative histologic differences between lungs ventilated with HFJV vs. CMV. Thus, although HFJV produced more efficient gas exchange in this model, it did not improve pulmonary pathology. HFO may be preferable to HFJV in infant respiratory distress syndrome.

Airway movement in dogs during high-frequency jet ventilation.

Cine tantalum bronchograms were recorded from 7 pentobarbital-anesthetized dogs during spontaneous ventilation (SV), high-frequency jet ventilation (HFJV) at 3 frequencies, and intermittent positive-pressure ventilation (IPPV) at 3 combinations of tidal volume (VT) and rate. During SV and the 3 IPPV conditions, the percent inspiratory increase in the diameter of airways greater than 3 mm was the same as in airways less than 3 mm. With HFJV, the percent increase in the diameter of airways greater than 3 mm was twice that of smaller airways. Increases in airway diameter are proportional to transmural, and hence intraluminal airway pressure. These data, therefore, indicate that the contribution of intraluminal pressure changes to intrapulmonary gas transport in small airways during HFJV is less than with either SV or IPPV, and that mechanisms responsible for intrapulmonary gas transport in small conducting airways during HFJV are different than those associated with either SV or IPPV.

Effects of High Frequency Jet Ventilation Design and Operational Variables upon Arterial Blood Gas Tensions

High frequency jet ventilation (HFJV) is but one mode of high frequency ventilation (HFV) that has been utilized successfully to provide respiratory support. In HFJV, a small pulsating jet of gas flowing from a regulated high pressure source is introduced into the airway. Pulsations result from precise regulation of the gas stream by either fluidic or electromechanical control systems.

Effect of alterations in frequency, inspiratory time, and airway pressure on gas exchange during high frequency jet ventilation in dogs with normal lungs

High frequency jet ventilation (HFJV) is becoming increasingly useful for providing respiratory support in patients with normal lungs during operative procedures, and also has been advocated as a technique for ventilating patients during cardiopulmonary resuscitation. We studied the effect of frequency, percent inspiratory time (I/E ratio), peak airway pressure, and airway pressure difference (peak-PEEP) during HFJV as operational variables on the efficacy of gas exchange in dogs with normal lungs. We observed that at a constant peak airway pressure and percent inspiratory time, PaCO2 generally increases as frequency rises above 100/min. In contrast, PaCO2 generally decreases as percent inspiratory time is reduced at a constant frequency and peak airway pressure. In addition, increasing peak airway pressure and airway pressure difference are associated with lower levels of PaCO2. Arterial oxygenation was adversely affected by frequencies above 300/min, but was otherwise not influenced by alterations in frequency, percent inspiratory time, or airway pressure.

Cardiovascular Consequences of High Frequency Ventilation

The potential adverse cardiovascular effects of positive pressure ventilation (decreasing venous return to the right heart leading to a fall in cardiac output) are well known to modern physicians. Since the magnitude of these hemodynamic effects is directly related to the amount of positive pressure applied to the airway, efforts have generally been made to maintain airway pressures as low as possible during mechanical ventilation. High frequency ventilation (HFV) can provide adequate alveolar ventilation at much lower peak airway pressures than conventional ventilation. Consequently, it has been widely expected that HFV would have less adverse effects on the cardiovascular system than conventional ventilation. Early work by Eriksson, et al (1) in healthy dogs supported this view. They found a higher cardiac output and stroke volume, lower peripheral resistance, and lower peak and mean airway pressures with high frequency positive pressure ventilation (HFPPV) compared to conventional ventilation. More recent investigators have reported improved, unchanged, and impaired hemodynamic function with HFV under a variety of experimental conditions (Table 1).