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Evaluation of a ventilator with fixed volume control and variable regulated pressure

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Summary and Conclusions

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 selecled 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 va ve, 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 systems 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 abnorma 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 ]ackkmfe 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 respiratorv 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.

Résumé

Sur au-délà de 300 cas, nous avons contrôlé, mécaniquement et automatiquement, la ventilation pulmonaire au cours ďopérations majeures afin ďapprécier ľefficacité ďun tel contrôle dont le volume, la pression et ľamplitude étaient décidés au préalable. On a choisi le volume ďaprès le monogramme de Radford et calculé ďaprès le poids “normal” du malade tel que mentionné dans une table de tailles et poids selon ľâge, et corrigé selon les positions anormales et la présence ďemphysème pulmonaire. Nous avons conservé, autant que possible, un volume et une pression uniformes que nous avons déterminés ďaprès notre expérience acquise avec de semblables respirateurs mécanique. Un cycle à trois phases, ďune durée approximative de trois secondes, consistant en une élévation rapide initiale vers une pression inspiratoire maximale ďune durée égale environ à un tiers du cycle, suivi ďune courte période de press]on inférieure à la pression atmosphérique, et le reste du cycle à une pression égale à la pression atmosphérique nous a paru réaliser à très peu près, la ventilation clinique idéale (15,78,79)

Nous avons étudié ces caractéristiques du cycle de ventilation en employant les trois différents circuits ďusage courant en anesthésie en évitant la réinspiration au moyen ďune série de valves, dont la valve du type de Fink, le système semi-feimé de Magill ou de Mapleson A et le système à circuit fermé. Nous avons employé comme agents avec les deux premiers systèmes de circuit le protoxyde ďazote comme agent prmcipal, avec des traces de trichlorethylène, de la meperidine (1 v) et, à ľoccasion, des traces ďéther. Lorsque nous avons employé le circuit fermé, nous avons utilisé le cyclopropane comme agent principal et, habituellement, un peu ďéther. Dans tous les cas, de façon à assurer que le volume déterminé était porté au malade, nous avons employé des tubes à parois non extensibles entre le malade, la machine et le ventilateur. On à vérifié ce volume par un ventilomètre et on n’a trouvé qu’une marge ďerreur admissible dans de pareilles circonstances.

Comme premier critère pour établir ľefficacité satisfaisante de la ventilation pulmonaire contrôlée, nous avons comparé les résultats obtenus des analyses du sang artériel prélevé à ľabri de ľair chez un bon nombre de malades, après la prémédication, au cours de ľanesthésie et immédiatement après ľanesthésie. Nous avons comparé ces résultats avec ceux obtenus chez des adultes en santé et chez plusieurs malades dont la respiration était contrôlée par compression manuelle sur le ballon. On a cherché dans le sang, le pH, ľhématocrite, le contenu total de gaz carbonique, la capacité en oxygène, le contenu en oxygène et la saturation en oxygène. De ces données, on a trouvé, ľhémoglobine, le gaz carbonique plasmatique, le bicarbonate plasmatique, la tension de gaz carbonique et ďoxygène.

Comme deuxième critère, nous avons accepté les observations cliniques faites durant ľanesthésie de chacun des malades portant sur ľétat général de chacun au cours de ľopération et après ľopération et sur la facilité de maintenir ľéquilibre cardiovasculaire.

A chaque fois que nous avons employé un système sans réinspiration, nous avons réussi à maintenir facilement les niveaux ďanesthésie désirés par le chirurgien et ľanesthésie, soit pour le sommeil, ľanalgésie, ou le relâchement musculaire. Ľétat général des malades en bonne santé n’a pas été touché outre mesure, ľétat des malades sérieusement touchés à ľarrivée à la salle ďopération ne s’est pas aggravé et, dans bien des cas, s’est amélioré. D’après les résultats des analyses de sang, la ventilation pulmonaire est demeurée satisfaisante à ľexception ďun cas où une erreur s’est glissée dans la mécanique des valves. En dépit du fait que nous ayons observé une légère alcalose respiratoire chez la plupart des malades traités de cette façon, nous n’avons pas noté de réact ons métaboliques compensatrices dans les acides fixes, ľalcalose respiratoire est disparue rapidement, aussitôt après ľanesthésie, et le retour de la respiration spontanée n’a jamais été tardif à cause de ce système de ventilation. Cet état de chose a été observé au cours de toutes les opérations où le système de non-rémspiration a été étudié.

Quand nous avons employé un système semi-fermé, avec le protoxvde, la meperidine et le trichlorethylène, chez les malades où la ventilation pulmonaire n’était pas modifiée ou inhibée par les positions anormales qui agissent sur la mobilité thoracique, la ventilation pulmonaire est demeurée adéquate. En position ventrale avec flexion et en position de Trendelenbourg, il est apparu une légère mais constante tendance à ľacidose respiratoire qui a persisté au cours de la période post-anesthésique. Toutefois, cela n’a pas modifié les acides fixes En position ventrale avec flexion, on a noté une hypotension légère mais persistante ainsi qu’une différentielle pincée. Ľhypotension a persisté jusqu’à la période postopératoire seulement lorsque ce système a été employé.

Avec ľemploi du circuit fermé, du cyclopropane et ďun peu ďéther, dans tous les cas, nous avons obtenu une anesthésie satisfaisante au cours des opérations et le maintien de ľéquilibre acide-base. C’est seulement avec une anesthésie à ľéther plutôt profonde que nous avons constaté une acidose respiratoire et métabolique avec ce système (80).

Il devient évident, ďaprès ces résultats, qu’on ne devrait pas observer, durant ľanesthésie clinique, de modifications de ľéquilibre acide-base ni de modifications de ľéquilibre cardio-vasculaire, si ľon maintient une ventilation pulmonaire adéquate. Il appert également que ľacidose métabolique n’accompagne pas toujours ľacidose respiratoire et qu’elle n’apparaît pas loisqu’on emploie des traces ďéther diéthylique avec le protoxyde ou le cyclopropane. Quand on emploie un système de non-rémspiration, toutes les valves empêchant le retour et la valve ďexpiration doivent fonctionner ďune façon parfaile. Il convient ďemployer le système semi-fermé seulement si ľon peut donner un grand débit de gaz et si la chirurgie n’exige pas de position anormale. Quant au circuit fermé, il est tout-à-fait efficace à condition que ľabsorbant du gaz carbonique soit bien tassé dans le contenant et que les valves n’offrent qu’une résistance minime. Le dicton de Leigh, “une observation attentive de la respiration et des signes de son efficacité demeure la fonction la plus importante de ľanesthésiste” (81) mérite qu’on insiste, mais, maintenant, il faut lui ajouter “au cours des opérations majeures, la ventilation pulmonaire devrait être contrôlée à un volume, une pression et une amplitude déterminés ďavance pour chacun des malades, selon la variété de circuit anesthésique employé et la position exigée.”

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Correspondence to Allen B. Dobkin.

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

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Dobkin, A.B. Evaluation of a ventilator with fixed volume control and variable regulated pressure. Canad. Anaesth. Soc. J. 5, 288–322 (1958). https://doi.org/10.1007/BF03014131

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Keywords

  • Nous Avons
  • Respiratory Acidosis
  • Pulmonary Ventilation
  • Blood Data
  • Prone Jackknife Position