AIR-FLOW MEASUREMENTS IN SMALL ANIMALS

Monitoring respiratory airflow is extremely important in pharmacologic al studies of the respiratory system. However, in mechanically ventila ted small animals (e.g. rats and guinea pigs) the use of a commercial pneumotachometer, attached directly to the tracheal cannula, substanti ally increases the equipment dead space (the volume shared by inspired and expired gases). Since apparatus dead space must be added to the t idal volume (determined on the basis of ventilatory rate and animal bo dy 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 i s potentially associated with a decrease in venous return, cardiac out put, 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 d eflation limbs of the breathing circuit. This study shows that the sum mation of the flow signals obtained from separate pneumotachometers on the inspiratory and expiratory arms of the breathing circuit is not d issimilar to the total flow as measured by a pneumotachometer directly attached to the tracheal cannula and that the method here described c an be an advantageous alternative to the employment of a single pneumo tachometer.