Some aspects of respiratory function in anaesthetised dogs
Mohammed, Najat Rafique
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Some respiratory and circulatory parameters were established for dogs during routine clinical anaesthesia. The possible effects on these parameters of positioning the animals in left lateral, right lateral and dorsal postures were also studied.One hundred clinical dogs and eight experimental dogs were included in this study. The clinical dogs were categorised into two equal groups; group A, for which complete physiological and biochemical data were available and group B, for which the biochemical data were not available. All dogs were anaesthetised by a minimal dose of thiopentone sodium followed by nitrous oxide and halothane throughout the anaesthetic period.PHYSIOLOGICAL PARAMETERS: The respiratory and circulatory parameters which were measured were minute volume, respiratory rate and pulse rate. A review of the literature of the workers noted a decrease in during anaesthesia associated with respiratory rate and tidal volume. showed that a majority minute volume of dogs a decrease in both In man, increases in tidal volume and respiratory rate were frequently observed but the net effect on minute volume was variable, depending on the degree of change in the two components.In this study, a significant positive corelation was found between tidal volume and body weight, and between minute volume and body weight, whilst a significant inverse corelation was established between respiratory rate and body weight.Changes in tidal volume during anaesthesia were not significant. However, respiratory rate and in consequence minute volume showed a significant increase.Minute volume values recorded in dorsal recumbency were found to be significantly higher than the values obtained in lateral postures. In dogs maintained in left lateral and right lateral postures respiratory rate values were comparable, but were significantly lower than the values recorded in dorsal posture. Thus the differences in the minute volume values were determined predominately by the differences observed in the respiratory rates.Pulse rate was found to fall during the interval from ten minutes following induction of anaesthesia until the end of the anaesthetic period. No significant corelation was evident between the pulse rate and body weight or between the pulse rate values obtained in left lateral, right lateral and dorsal postures.BIOCHEMICAL PARAMETERS: The biochemical parameters examined were pH, PaCO2 and Pa02' A review of the literature showed that all authors found an increase in the arterial blood carbon dioxide tension with a moderate to a severe respiratory acidosis in both man and animals.In the present study arterial carbon dioxide tension rose significantly above the preanaesthetic level, and this was associated with a considerable fall in arterial pH. The changes in the standard bicarbonate levels were not significant. The mean PaCO2 value recorded after ix sixty minutes of anaesthesia in dorsal posture was signifi- cantly higher than the mean values obtained in either of the left or right lateral postures. The fall in pH was proportionate to the retention of carbon dioxide. The mean pH values were significantly lower in dogs maintained in dorsal posture than in either of the lateral postures. The standard bicarbonate values recorded in dogs maintained in the three postures were not significantly different.Arterial oxygen tension rose significantly above the control levels. The mean Pa°2 values obtained in the left and right lateral postures were comparable and they were higher than those recorded in dorsal posture.Haemoglobin concentration decreased significantly during anaesthesia, but the extent of this fall did not differ in different postures.As a result of this investigation it is concluded that the exchange of respiratory gases is impaired in spontaneously breathing dogs under anaesthesia and the concomitant progressively developing acidosis is attributed mainly to this cause. The efficiency of ventilation is further reduced in the dorsal posture compared with the lateral.It is recommended that the respiratory blood gases should be intermittently monitored during the anaesthesia of clinical patients, particularly those having hypoxia or acid base abnormalities as part of their clinical syndrome.Adequate oxygenation can be maintained by administering oxygen rich mixtures with the anaesthetic gases but the elimination of carbon dioxide from those patients having high PaCO2 levels will require the animals to be artificially ventilated.It is suggested that the component of respiration most requiring supplementing is the tidal volume, an increase of rate by itself having been shown in the clinical cases which were breathing spontaneously to be inefficient as a compensatory measure.