Evaluation of the Humphrey A.D.E. breathing system

Alan A. Artru 0 1 Ross A. Katz 0 1 0 From the Department of Anesthesiology, University of Washington School of Medicine , Seattle, Waghington. Anesthesiology, RN-I0 , University of Washington School of Medicine , Seattle, WA 98195 1 Artmand Katz: H U M P H R E Y A . D . E , B R E A T H I N G SYSTEM A new breathing circuit (the Humphrey A.D.E., double lever model) was evaluated in adults to determine (1) the fresh gas flow (FGFJ neaded to achieve normocapnio daring controlled ventilation and to just induce rebreathing during spontaneous ventilation, (2) end-expired COz (P~C02) at those FGF values, (3J the standard deviation of FGF requirements for controlled and spontaneous breathing (reliability of recommended FGF settings) and (4) the magnitude of change in P,~C02 produced by varying FGFfrom the recommended values (sensitivity of rite system) The FGFs rharprovided normocapnia with controlled ventilation andjust induced rebreathing wilh spontaneous ventilation were 67 +- I0 and52 + 7 ml'kg I'min~! (mean +- SD), respectively. P~CO~ values were 36_d "~ 0.3 and41.6 +- -?.9 nunHg respectively. During controlled ventilation low reliability was offset by low sensitivity so that P~CO~_changed little when FGF was raised or lowered from recommended values (0.2 mml4glml, kg- t. rain- i). In contrast, during spontaneous yen. tilation low reliability was additive with high sensitivity wken using FGFs lower than the mean value that just induced rebreathing. A tkreshold was reached where towering FGF from recommended values caused large changes in PC.C02 ( t I mnfftg/ml'kg -t'min Jl. It is concluded that the FGF recommended by Humphrey for controlled ventilation is satisfactory. However, the FGF recommended by Humphreyfor spontaneous ventilation may result in hypercapnia in some patients. This can be prevented either by using a higher FGF of 66 ml. kg I. rain- j routinely in atl patients or by using lowerflows with C02 monitoring. - A r t r u a n d K a t z : H U M P H R E Y A . D . E . B R E A T H I N G SYSTEM reliability of recommended FGF settings is of interest because if the variability of FGF requirements is large, routine use of a mean value for FGF will result in a large proportion of patients being hypo- or hypercapnic. The sensitivity of the system is of interest because if large changes in PI~CO2 accompany an alternation of FGF, routine use of a mean value for FGF again will result in a large proportion of patients being hypo- or hypercapnie. Methods This study was approved by the Human Subjects Committee of the University of Washington, Seattle. Thirty-three adult patients undergoing elective, non-thoracic surgery and of ASA physical status I or 1[participated in the study. Patients received either no premeditation or diazepam 10mg PO 90 min prior to induction of anaesthesia. Anaesthesia was induced with thiopentone 3 - 4 mg.kgIV, the trachea was intubated using zuccinylcheline 1.5 mg.kg t IV to enhance muscle relaxation, and anaesthesia was maintained with isoflurane (1.5 per cent, inspired) and nitrous oxide (60 per cent, inspired) in oxygen. Narcotics were not administered. Pancuronium -> O. 1mg-kg-i IV was given to maintain muscle relaxation with rio visible muscle twitch in response to percutaneons stimulation of the ulnar nerve. During this time ventilation was controlled using the double lever, noncoaxial version of the Humphrey A.D.E. system. A tidal volume of 12-15 nll.kg -~ and a ventilator rate of 9-12 breaths-min -~ were used to achieve the recommended ventilatury volume of 135-140 ml.kg-I .rain -t . t-,* Flowmeters on the anaesthesia machine were set to deliver a FGF of > 80ml.kg-l-min -l. To insure the accuracy of FGF delivery, the flowmeters on the anaesthesia machine were calibrated each day to a paediatric turbine-vane flowmeter (Boehringer Laboratories, Wynnewood, PA 19096). Peak end-expired isoflurane concentrations and inspired and expired CO2 values were determined at 0.5-1.5min intervals using mass spectroscopy (The Perkin-Elmer Corp., Pomona, CA 91767). Gas samples were obtained from the endotracheal tube at 6cm distal (relative to anaesthesia machine) from the "Y" of the non-coaxial breathing circuit. At these ventilator settings P~CO~ was < 34 mmHg in all patients. Controlled ventilation FGF and P~CO2 values for controlled ventilation were examined in all patients. Values were determined after patients had been receiving isoflurane for -> 25 rain and after Ihe inspired concentration of isoflurane had been adjusted to provide a stable end-expired concentration of isofluranc of 1.0 per cent. All data were collected during a period of moderate surgical stimulation of deeper FGF PROVIDING NORMAL P~CO2 To determine the normocapnie FGF value, ventilatory volume was continued at 135-140ml.kg-l.min -j and FGF was decreased from --- g0 ml.kg- I.min-t , in 0.25 L. rain-~ steps (5-6min at each setting) until P~CO: stabilized at 3 6 m m H g For the purposes of this study PI~CO2 - 36 mmHg was considered to be normocapnia. Arterial blood samples were obtained for blood gas analysis in 8/33 patients to confirm normocapnia. FGF at normocapnia was recorded for each patient and the values averaged to determine the mean FGF needed to provide normocapnia during controlled ventilation for this group. The reliability of the system was determined from the variability (standard deviation) of the FGF values providing normocapnia. SENSITIVII'Y OF P~CO2 TO ALTERED FGF The FGF then was set at + 10 and - 1 0 ml.kg-l,min -~ from the normocapnic FGF (8-10 rain at each setting), and P~CO2 values recorded. Mean PI~CO2 at +10 and 10ml.kg-l.min -I from normocapnic FGF was used to calculate the magnitude of the change in Pf~CO2produced by varying the FGF from the recommended value during controlled ventilation. Spontaneous ventilation In 19/33 patients the FGF was continued at the high values used for conlrolled ventilation, and recovery from the effects of pancuronium was permitted. Once full recovery from pancuronium was complete (as assured by a normal muscle response to percutaneous stimulation of the ulnar nerve), patients were allowed to breathe spontaneously until stable P~CO2 values were observed with no rebreathing of COz (minimum inspired CO2 of near zero). As above, all data were collected during a period of moderate surgical stimulation of deeper tissues and a stable endexpired concentration of isoflurane of 1.0 per cent. FGF THAT JUST INDUCES REBREATHtNG To determine the minimum FGF providing stable PI~CO2, FGF was lowered, in 0.25 L.min t steps (5-6 rain at each setting) until rebreathing occurred (minimum inspired CO2 > 2 mmHg) without a detectable increase in PI~CO2. The mean of these FGF values was considered the minimum for providing stable P/~CO2. The reliability of the system was determined from the variability of FGF values that just induced rebreathing. SENSIIIVI rY OF PI~Gr 2 I O A L I E R E D bt.JF FGF then was lowered further, in 0.25L-rain -~ step (...truncated)