Airflow measurements in small animals

Monitoring respiratory airflow is extremely important in pharmacological studies of the respiratory system. However, in mechanically ventilated small animals (e.g. rats and guinea pigs) the use of a commercial pneumotachometer, attached directly to the tracheal cannula, substantially increases the equipment dead space (the volume shared by inspired and expired gases). Since apparatus dead space must be added to the tidal volume (determined on the basis of ventilatory rate and animal body weight) necessary to meet the respiratory needs of the animal, the resulting stroke volume is greatly increased, specially in very small animals. This results in an increase in intrathoracic pressure which is potentially associated with a decrease in venous return, cardiac output, and arterial pressure. The author investigated the possibility of avoiding the potential problems of the added instrumental dead space by software-summing the separate flows measured on the inflation and deflation limbs of the breathing circuit. This study shows that the summation of the flow signals obtained from separate pneumotachometers on the inspiratory and expiratory arms of the breathing circuit is not dissimilar to the total flow as measured by a pneumotachometer directly attached to the tracheal cannula and that the method here described can be an advantageous alternative to the employment of a single pneumotachometer.