Artificial Increase of Dead Space for the Improvement of Post-Operative Ventilation — A Review

A reduction in post-operative pulmonary ventilation is almost invariably present amongst patients undergoing upper abdominal surgery*12). It is very common amongst those having lower abdominal surgery and very rare following orthopaedic operations*1). Nichols and Howell (1970/l> analysing their results conclude that there is no significant difference in venti­ latory capacity changes between patients receiving the classical preand post-operative physiotherapy and between those patients who did not receive this treat­ ment. However, they excluded from this trial patients with pre-existing acute or chronic obstructive lungdisease because, for this type of patient, there is general agreement about the therapeutic value of classical physiotherapeutic procedures. From their trial it is evident that a post-operative decrease of pulmonary ventilation is not correlated with an obstructive pattern of respiration. It is, however, clearly correlated with a restrictive pattern of ventilation i.e. shallow breathing, as a result of diaphragmatic inhibition. Further evidence to their conclusions is promulgated by the work of FinerV'* who studied the variability in expiratory efforts before and after cholecystectomy. This author stresses the importance of encouragement — “the elicitation of the latent encouragement potential,” in order to over­ come diaphragmatic inhibition and to improve pul­ monary ventilation. However, he also stresses the clinical value of post-operative pain relief, which he effects by different and mutually compared kinds of nerve blocks (intercostal, epidural and splanchnic). Controlled measurements of vital capacity (V.C.), forced expira­ tory volume (F.E.V.) and peak expiratory flow showed that the best results were obtained by a combination of encouragement and intercostal block. A somatic and visceral pain block combination did not' appear to be superior to a somatic pain block alone. The author’s opinion is that the relief of somatic pain by analgesic therapy, following cholecystectomy, is the more im­ portant factor in the improvement of post-operative expiratory efforts. Diaphragmatic inhibition causes a consequent reduc­ tion of lung volume. This reduced lung volume is a con­ tributory cause of segmental and massive lung collapse, of raised diaphragm and of pleural effusion. In the early stages of their development these complications frequently cannot yet be diagnosed by clinical symp­ toms and have to be detected by radiological changes. It is thus clear that, in order to prevent the clinical complications of reduced lung volume, diaphragmatic inhibition must at all costs be overcome. Because of the inadequacy of classical physiotherapeutic measures for the improvement of post-operative pulmonary ventilation, a new therapeutic approach is needed, by which at will an increase of ventilation, especially after abdominal surgery, can be obtained. The next problem posed is the method of choice, because different methods have been proposed and evaluated. A recent comparison has been made by Jones (1968)*3) in a clinical study of the rather small number of 15 patients who had recently undergon thoracotomy or laparotomy operations. H e compared five commonly employed methods for enhancing ventila tion, using them sequentially in each patient for two to five days post-operatively and estimating their effective­ ness towards the increase of the V t. All these methods were applied without the administration of narcotics for four hours previous to the study. In six of the patients involved pre-operative as well as post-operative studies were done. From this study it is apparent, as it is from the aba ' literature, that following abdominal or thoracic surgl there is a diminished ventilatory ability, the post-opera­ tive breathing values averaging 60% of the pre-opera­ tive values. The comparison between the five different clinical methods for enhancing ventilation had the following results: 1. Rebreathing with the Adler rebreather (1967)<‘> was relatively ineffective in increasing depth of respira­ tion. 2. I.P.P.B. (Intermittent Positive Pressure Breathing) with the conventionally employed inspiratory pres­ sure of 15 cm H 20 and the inhalation of 5% C 02 were methods of intermediate value. They were, however, infrequently equal or superior to voluntary efforts in producing significant increases in V t 3. Voluntary deep breathing was frequently quite effective. Increases in V t of 80% or more occurred in two-thirds of the subjects, while V t as large or larger than those produced by any other method were observed in about 50% of the patients. 4. I.P.P.B. at the relatively high inspiratory pressure of 25 cm H 30 was clearly the most superior method. When V t greater than those due to voluntary deep breathing were observed, they were almost always produced by I.P.P.B. of 25 cm H 20 . However, the author stresses the limitation of study. A comparative evaluation of these methods inv. larger series of patients, wherein the augmentation of V t is the only variable, is recommended. “The voluntary deep breathing” mentioned in 3 can be compared with “the elicitation of the latent en­ couragement potential” of Finer (1970)(2>. In a department of physiotherapy one needs for the average daily hospital practice “efficient” methods for the improvement of post-operative ventilation. A t the same time, the methods applied should be as simple as possible, require a minimum of routine instrumentation and a minimum of patient conveyance. Inhalation of C 0 2 requires C 0 2 supply, oxygen supply, a gas flow measuring device and a well-fitting gas ad­ ministration set. I.P.P.B. requires a rather expensive and complicated equipment for artificially assisted respiration. One wonders whether Jones (1968)*3> has paid sufficient attention to the clinical possibilities of rebreathing methods for the improvement of post-operative pul­ monary ventilation. In experimental physiology the principle of a simple procedure, by which at the same time pulmonary ventila-

A reduction in post-operative pulm onary ventilation is alm ost invariably present amongst patients undergoing upper abdominal surgery*1-2). It is very common amongst those having lower abdominal surgery and very rare following orthopaedic operations*1).
Nichols and Howell (1970/l> analysing their results conclude that there is no significant difference in venti latory capacity changes between patients receiving the classical pre-and post-operative physiotherapy and between those patients who did not receive this treat ment. However, they excluded from this trial patients with pre-existing acute or chronic obstructive lungdisease because, for this type of patient, there is general agreement about the therapeutic value of classical physiotherapeutic procedures. F rom their trial it is evident that a post-operative decrease of pulm onary ventilation is not correlated with an obstructive pattern of respiration. It is, however, clearly correlated with a restrictive pattern of ventilation i.e. shallow breathing, as a result of diaphragmatic inhibition. Further evidence to their conclusions is prom ulgated by the w ork of FinerV'* who studied the variability in expiratory efforts before and after cholecystectomy. This author stresses the importance of encouragement -"the elicitation of the latent encouragement potential," in order to over come diaphragmatic inhibition and to improve p u l m onary ventilation. However, he also stresses the clinical value of post-operative pain relief, which he effects by different and mutually compared kinds of nerve blocks (intercostal, epidural and splanchnic).
Controlled measurements of vital capacity (V.C.), forced expira tory volume (F.E.V.) and peak expiratory flow showed that the best results were obtained by a combination of encouragement and intercostal block. A somatic and visceral pain block com bination did not' appear to be superior to a somatic pain block alone. The author's opinion is that the relief of somatic pain by analgesic therapy, following cholecystectomy, is the m ore im portant factor in the improvem ent of post-operative expiratory efforts.
Diaphragmatic inhibition causes a consequent reduc tion of lung volume. This reduced lung volume is a con tributory cause of segmental and massive lung collapse, of raised diaphragm and o f pleural effusion. In the early stages of their development these complications frequently cannot yet be diagnosed by clinical symp toms and have to be detected by radiological changes.
It is thus clear that, in order to prevent the clinical complications of reduced lung volume, diaphragmatic inhibition must at all costs be overcome. Because of the inadequacy of classical physiotherapeutic measures for the improvement of post-operative pulm onary ventilation, a new therapeutic approach is needed, by which at will an increase of ventilation, especially after abdominal surgery, can be obtained.
The next problem posed is the m ethod of choice, because different methods have been proposed and evaluated. A recent comparison has been m ade by Jones (1968)*3) in a clinical study of the rather small num ber o f 15 patients w ho had recently undergon thoracotom y or laparotom y operations. H e compared five comm only employed methods fo r enhancing ventila tion, using them sequentially in each patient for two to five days post-operatively and estimating their effective ness towards the increase of the V t. All these methods were applied w ithout the adm inistration of narcotics for four hours previous to the study. In six of the patients involved pre-operative as well as post-operative studies were done.
F rom this study it is apparent, as it is from the a b a ' literature, th at following abdom inal or thoracic surgl there is a diminished ventilatory ability, the post-opera tive breathing values averaging 60% of the pre-opera tive values.
T he com parison between the five different clinical methods fo r enhancing ventilation had the following results: 1. Rebreathing with the A dler rebreather (1967)<'> was relatively ineffective in increasing depth of respira tion. 2. I.P.P.B. (Interm ittent Positive Pressure Breathing) with the conventionally employed inspiratory pres sure of 15 cm H 20 and the inhalation of 5% C 0 2 were methods of interm ediate value. They were, however, infrequently equal or superior to voluntary efforts in producing significant increases in V t 3. V oluntary deep breathing was frequently quite effective. Increases in V t of 80% o r m ore occurred in two-thirds of the subjects, while V t as large or larger than those produced by any other method were observed in about 50% of the patients. 4. I.P.P.B. at the relatively high inspiratory pressure o f 25 cm H 30 was clearly the m ost superior method. W hen V t greater than those due to voluntary deep breathing were observed, they were alm ost always produced by I.P.P.B. of 25 cm H 20 . However, the author stresses the lim itation of study. A com parative evaluation of these methods inv. larger series of patients, wherein the augm entation of V t is the only variable, is recommended. "The voluntary deep breathing" m entioned in 3 can be com pared with " the elicitation o f the latent en couragem ent potential" of Finer (1970)(2>.
In a departm ent of physiotherapy one needs for the average daily hospital practice "efficient" methods for the im provem ent of post-operative ventilation. A t the same time, the methods applied should be as simple as possible, require a minim um of routine instrumentation and a minim um of patient conveyance.
Inhalation of C 0 2 requires C 0 2 supply, oxygen supply, a gas flow measuring device and a well-fitting gas ad m inistration set. I.P.P.B. requires a rath er expensive and complicated equipm ent for artificially assisted respiration.
One wonders w hether Jones (1968)*3> has paid sufficient attention to the clinical possibilities of rebreathing m ethods for the im provem ent o f post-operative pul m onary ventilation.
In experimental physiology the principle of a simple procedure, by which at the same time pulm onary ventila-DESEMBER 1973 FISIOTERAPIE 3 could be increased and a norm al P a C 0 2 could t l 0 I1m a i n t a i n e d , has been described by G ad (1880). H e be m te(j a large tube with a diam eter o f 0,4 cm and con"lume of about 108 ml to the trachea o f a rabbit thus 3 vnking by this m anoeuvre an increase o f respiratory Pr? me and respiratory rate. Liljestrand (1918) nlied tubes with a diam eter of 1,8 cm and a volume of ^740-1950 ml to hum an beings. H e discovered that the CO content of the inspiratory a ir would increase to the same degree, as the am ount of C 0 2 present in the hvsiological dead space from the previous expiration. 13 S c o tt and Cutler (1928)<s) described the use of hyper ventilation by rebreathing of carbon dioxide from a per bag for the treatm ent of post-operative massive atelectasis.
Ouomacio and D iaz-Rom ero (1937)(6) described the artificial increase of respiratory dead space as a practical method for the im provem ent of pulm onary ventilation. The artificial dead space is increased to a volume, w here by it may still be compensated by the corresponding in c re a se of V t . U nder these conditions no hypocapnia occurs because, notw ithstanding the increase of Respirag y Minute V olum e (R.M.V.), the P A C 0 2 is kept 'constant by the increase in physiological dead space.
Schwartz and D ale (1955)<7> describe the addition of dead space in order to produce hyperventilation fo r the prevention of atelectasis. They give evidence that there is a direct relationship between the size of P a C 0 2 and the V t i.e. the P a C O s and VT -curves closely parallel each other. The greatest increm ent in "stim ulated Tidal Volume" occurred during the first two minutes: in this time the V t increased from a control value of 596 to 1178 ml. During the third m inute there was another significant increase from 1178 to 1334 ml. From the 3rd to the 10th m inute there was little change. Simi larly the greatest rise in P a C 0 2 from a control value of 39,44 to 44,17 mm of Hg occurred during the first two minutes. A second increase from 44,17 to 47,20 mm Hg was noted between the 2nd and the 5th minutes. A concomitant decrease of P a 02 was also dem onstrated during the first tw o minutes of hyperventilation (100,5 mm Hg control value, sharply decreasing to 77,61 mm Hg); over the following 8 min P a 02 rose steadily and gradually to 88,93 mm Hg. A lthough the lowered P A 0 2 was generally well tolerated by the patients, this eventual P a 02 decrease could be prevented by directing a flow of oxygen into the distal end o f the dead space tube. A dead space of 1 000 ml gave a m ore than 100% in crease of V t in 14 patients during the first three postjerative days w ithout any significant alteration of Rpiratory rate. The same study was repeated, but this time a flow of 3 4 litres of oxygen per m inute was introduced into the distal end of the rebreathing tube. In the three patients involved the P a O z was m aintained above the control value of 100,5 mm Hg. T he average increases in VT and P A C 0 2 were sim ilar to the changes noted in 15 patients breathing room air. This clearly indicates that while subjects rebreathe through the 1 000 ml dead space tube, the elevation of P a C 0 2 is the critical factor that effects the increase of V T Schwartz-Dale-Rahn (1957)('> evaluated their clinical experience in over 1 000 patients with a dead space rebreathing tube as a post-operative routine. The tube was used fo r a period of five minutes at tw o hourly intervals. Its use caused an apparent decrease in the incidence of post-operative atelectasis and was thus established as an effective therapy in the treatm ent of atelectasis. N o difficulties due to hypoxia were observed, although the occasional patient had to receive additional oxygen through the distal end of the tube. Giebel ( As long as alveolar ventilation does not decrease, respiratory rate does n o t increase sig nificantly. However, as soon as respiratory rate in adults increases to m ore than 20-24 per m inute, dead space ventilation increases and alveolar ven tilation decreases, leading to consequent hypercapnia. 2. In all groups of patients there is a decrease of A a D 0 2 (alveoloarterial oxygen difference) during respiration with artificial increase of dead space. This m ay only be explained as an overall im prove m ent and norm alisation of the pre-existing ventila tion/perfusion disturbances. T he increase of V t in duced by the addition of dead space therefore not only improves the pulm onary ventilation, but also the total pulm onary perfusion. 3. T he decrease of A a D 0 2 parallels closely the in crease in P a 0 2. 4. The P a C 0 2 rem ains within norm al limits during the efficient addition of volum e to th e artificial dead space. T he increase in pulm onary ventilation as a result of the increased P C 0 2 in the inspired air is autoregulated in such a way th a t the P a C 0 2 rem ains within norm al limits. 5. D uring respiration with artificial increase of dead space there is neither increase o f the blood pressure n o r of the pulse rate. 6. The best criterion fo r testing w hether the volume added to the dead space is correct is the behaviour o f the respiratory rate. In adult patients the respiratory rate should not surpass 20-24 p er minute. 7. If correctly applied, the m ethod of artificial in crease of dead space provided an efficient treatm ent of subacute and chronic atelectasis. Edelist and Orkin (1967)(1"> produced, w ith dead space increase of 750 ml and an oxygen flow of 300 m l per minute in to the rebreathing tube, an increase of the inspiratory P C 0 2, which was sufficient to increase pul monary ventilation at least 2 | times. T he authors' as sum ption was, th at by the significantly increased pul m onary ventilation areas of m iliary atelectasis w ould be opened and thus venous adm ixture w ould decrease. They could, however, no t support these assumptions with their results. In only two of the eight patients was the post rebreathing Pa 0 2 elevated in comparison with pre-rebreathing P a 02. In the rem ainder, there was no change, or an actual decrease of Pa 0 2. This seems illogical, but it m ust be rem em bered th at their patients were not suffering from any know n cardio pulm onary disease. V enous adm ixture, with the ex clusion of actual anatom ical shunts from the right to the left heart, may be caused by tw o pulm onary mechanisms: 1. Atelectasis: In this case the atelectatic areas o f the lungs are perfused, but not ventilated. H ald and Johanson (1970)*11) exposed 23 patients to inhalation of 2% C 0 2 in room air fo r 20 m inutes after the com pletion o f surgery. D uring this adm inistration period an increase in P a 0 2 and pulm onary ventilation was observed, whereas the P a C 0 2 rem ained constant.
The increasing P a 0 2 contrasts w ith the data of Edelist and O rkin, but once again I should like to refer to ' the im portance o f patient selection. In post anaesthetic patients especially there is a know n high ratio of m icro-atelectatic changes an d consequent arteriovenous pulm onary shunting. T his correlates well with the favourable effect of pulm onary hyperventilation on the Pa 0 2 in these cases.

Summary
F rom the literature reviewed, as regards the value of m ethods used fo r im provem ent o f pulm onary ventilation, several concepts have emerged. 1. Post-operative ventilatory disturbances are prevalent especially in thoracic and abdom inal (both higher and lower) surgery. 2. In patients w ithout pre-existing cardio-pulm onary disease these ventilatory disturbances are never of the obstructive type, b u t always of the restrictive type. 3. Classical physiotherapy procedures are of great value fo r the prevention and treatm ent of pul m onary com plications in the patient suffering from obstructive lung disease. However, the value of these procedures as regards restrictive ventilatory disturbances is questionable. 4. It is of prim e im portance th at post-operative ven tilatory disturbances be treated in order to prevent incipient atelectasis, bronchopneum onia and massive lung-collapse. between the degree o f analgesia and the am ount of respiratory depression. 8. T he simultaneous com bination of an artificial in crease of dead space and adequate analgesia merits further study. A com parison should therefore be m ade between th e effects of rebreathing methods under painless, pre-operative conditions and the effects of rebreathing m ethods under painful, post operative conditions. T hereafter analgesia should be adm inistered and pulm onary ventilation with re breathing m ethods retested. 9. This study could be com pleted b y aiding rebreath ing with an increase of artificial dead space and adequate analgesia-therapy with the psycho-therapy of encouragem ent (Finer: "the elicitation of the latent encouragem ent p o te n tial!"). V oluntary deep, painless breathing supported by the stim ulation of the respiratory centre by an artificially increased dead space w ould then be th e sum total of therapeutic achievem ent tow ards the difficult task of overcom ing post-operative pulm onary ventilation dis turbances. This achievem ent w ould thus have been obtained w ith a m inim um o f costs and w ithout 1T need fo r sophisticated equipm ent. of dead space to produce hyperventilation for pro phylaxis of atelectasis". Surgical Forum 6 , 282. 8 Schwartz, S. /., Dale, W. A . and R ahn, H. (1957) "D ead space re-breathing tube fo r prevention of atelectasis". J.A.M .A. 163, 1248. 9 Giebel, O. (1968) "U b er das V erhalten von Ventila tion, G asaustausch und K reislauf by Patienten mit norm alen und gestorten G asaustausch unter kiinstlicher Totraum -V ergrosserung" . Anaesthesilogie u n d W iederbelebung N o. 41. 10 Edelist, G. and Orkin, L . R . (1967) "Evaluation of a rebreathing tube fo r preventing atelectasis". Anestehesiology 28, 211. 11 Hald, T. and Johansen, S. H . (1970) "C arbon dioxide adm inistration fo r correction of post-operative hypoxaem ia". A cta A naesth. Scandin. 14, 53.