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Canadian Journal of Anesthesia, Vol 5, 288-322, Copyright © 1958 by Canadian Anesthesiologists' Society

Evaluation of a Ventilator with Fixed Volume Control and Variable Regulated Pressure

ALLEN B. DOBKIN M.D., F.A.C.A., D.A.(AM.BD.)1

1 Department of Anaesthesia and the Cardiopulmonary Laboratory, University of Saskatchewan, College of Medicine and University Hospital, Saskatoon

Pulmonary ventilation was controlled mechanically and automatically in over 300 selected major operative procedures in order to decide upon the efficacy of regulating such control by a predetermined volume and pressure amplitude. The volume selected was derived from Radford's nomogram, modified according to the "normal" weight for the patient as noted in a table of heights and weights related to the age group, and corrected for abnormal postures and the presence of pulmonary emphysema. The pressure amplitude was maintained as uniform as possible and was selected on the basis of previous experience with similar mechanical respirators. A three-phase cycle, lasting about three seconds, consisting of an initial accelerated rise to peak inspiratory pressure lasting approximately one-third of the cycle, followed by a short period of subatmospheric pressure and the remainder at atmospheric pressure was found to approach the ideal clinical condition most closely (15, 78, 79).

These characteristics of the ventilation cycle were applied mainly to the three commonly employed anaesthetic circuits: the non-rebreathing system employing a series of non-return valves, including the Fink-type valve; the Magill or Mapleson A type of semi-closed system; and the closed circle system. Nitrous oxide was the primary anaesthetic maintenance agent, with supplements of trichlorethylene, meperidine (intravenously) or occasionally a trace of ether, when the first two system were used. Cyclopropane was the primary maintenance agent, usually with a trace of ether, when the closed circle system was employed. In every case, non-distensible tubing connected the anaesthetic machine to the patient and to the ventilator in order to ensure the delivery of the volume as set. This volume was checked with a ventilation meter, and was found to agree within the permissible volume error of the meter.

The first criterion for deciding whether such control of pulmonary ventilation was satisfactory depended upon data derived from arterial blood samples, drawn anaerobically from a representative number of patients after premedication, during the course of the anaesthetic and in the immediate postanaesthetic period. These were compared with data derived from several normal healthy adults, and from several patients whose respiration was controlled by manual compression of the rebreathing bag. The blood data measured directly included pH, haematocrit, total carbon dioxide content, oxygen capacity, oxygen content and oxygen saturation. From these were derived the haemoglobin content, plasma CO2 content, plasma bicarbonate content, carbon dioxide tension and oxygen tension.

The second criterion depended upon the clinical observations during each individual anaesthetic as to the general condition of the patient during and after the operation, and the facility of maintaining cardiovascular homeostasis.

Whenever the non-rebreathing system was employed, anaesthesia was easy to maintain at levels of hypnosis, analgesia and muscle relaxation which were satisfactory to the surgeon and the anaesthetist. The general condition of the good risk patients was not affected adversely; when the patients were seriously ill on arrival for operation, their condition did not deteriorate and in most instances improved. Pulmonary ventilation, according to the blood data, was adequate in every case but one, in which an error in the mechanics of the non-rebreathing valve was overlooked. Although a slight respiratory alkalosis developed in the majority of the patients managed with this system, there was no evidence of a compensatory metabolic alteration in the fixed acids, the respiratory alkalosis disappeared promptly, immediately after anaesthesia, and the return of spontaneous respiration was never delayed because of the method of ventilation. These conditions prevailed in every type of operation that was studied with the non-rebreathing system.

When the semi-closed system was employed, with nitrous oxide, meperidine and trichlorethylene, pulmonary ventilation was adequate in those patients whose chest mobility was not impaired or hampered by abnormal postures. In the prone jackknife position and in the head low position, there was a slight but definite trend toward respiratory acidosis which continued into the postanaesthetic period. This alteration was not accompanied by change in fixed acids. Moderate but persistent hypotension and a small pulse pressure were usually seen with the prone jackknife position. The hypotension extended to the postoperative period only when this system was used.

The closed circle system employed with cyclopropane and a trace of ether, provided satisfactory operating conditions, and pulmonary ventilation in each case maintained acid base homeostasis. Only with moderately deep ether anaesthesia could a mixed respiratory and metabolic acidosis be overlooked in this system (80).

It was evident from these data that respiratory disturbances of acid base balance and disturbances of cardiovascular homeostasis need not occur during clinical anaesthesia, if adequate pulmonary ventilation is provided. It appears evident also that metabolic acidosis does not always accompany respiratory acidosis, and does not appear when trace amounts of diethyl ether are used with nitrous oxide or cyclopropane. With a non-rebreathing system, all non-return valves and the expiratory valve must function perfectly; a semi-closed system is suitable only if high gas flows can be provided and abnormal postures are not required; and the closed circle system is highly efficient if the carbon dioxide absorbent is packed properly in the canister and the valves function with very low resistance. The dictum of Leigh that "Careful observation both of the respiration and of the signs of its inefficiency is the most important function of the anaesthetist" (81) bears re-emphasis, but to it one must now add the statement: in major operative procedures, pulmonary ventilation should be controlled according to a predetermined volume and pressure amplitude according to the individual patient, to the anaesthetic circuit employed, and to the posture adopted.

Note:

Preliminary report was presented at the Canadian Anaesthetists' Society Annual Meeting in Saskatoon, Saskatchewan, June 26, 1957.

This investigation was assisted by equipment and a grant-in-aid from the Ohio Chemical and Surgical Company, Madison, Wisconsin.






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