RESUMO
OBJECTIVE: To measure airflow resistance in prongs of nasal CPAP, making use of different gas admission flow (GAF) in the ventilation circuit, in different internal diameters of the nasal prongs, besides verifying whether a GAF responding only to the demand of three times the minute-volume(MV) is enough to the circuit not to be cause of CO(2) retention. METHODOLOGY: Nasal prongs, assembled in the original circuits, were used, having their prongs kept open to the atmosphere. Pressure was read at a pressure monitor, in water centimeters, connected to the appropriate entrance of the circuit. A flowmeter balanced to the pressure was used, gauged at 50 psi, installed to the oxygen net of the Hospital, connected to the assessing set of the CPAP circuit. Initially, making use of the 8 l/min flow and keeping the exhaling set of the circuit closed, it was possible to eliminate the nasal prongs larger than two once the measured resistance was equal to zero. Having nasal parts number zero, 1 and 2 selected for this study, the system was then assembled as for the neonate: the inhaling set to the gas source and the exhaling set sunk into different depths in the water seal (2, 4, 6 and 8 centimeters). At the level of patient analysis, in order to assess the CO(2) retention, a mechanical pulmonary ventilation device was used as gas source and a nasal CPAP circuit was assembled to the device in adequate places. GAF values and FiO(2) were determined in the commands of the mechanical ventilation device. The assessment of gas concentration in the ventilation circuit was made while assisting two newborns. Gas samples were obtained within the ventilation circuit in the system assessing set (samples A), and right after the distal prong to the gas entrance (samples B). To determine MV the Tidal Volume (considered 10ml/kg) was multiplied by the respiratory frequency of the patient; GAF was three times MV. RESULTS: To a maximum GAF of 8 litres/min, only prongs sized zero, 1 and 2 showed resistance to the flow, measurable by the method used. There was an increase in resistance in proportion to the raise of GAF and proportionally opposite to the internal diameter of some prongs. Maximum difference in CO(2) partial pressure obtained from the gas given to the ventilation circuit and the one obtained from the nasal prongs was, in average, 0.43mmHg (p <0.5). CONCLUSIONS: Taking into account that during ventilation assistance through nasal CPAP there is the possibility of GAF incurring in the increase in resistance, what would involve a greater effort from the newborn to overcome such resistance during exhaling into the system (generating possibly an unexpected CPAP), and minimum GAF determined is that sufficient to meet no more than three times the MV, the conclusion is that prongs with the largest internal diameter possible and GAF only the necessary to meet, at least, the needs of the demand three times the MV should be used during this therapeutic procedure.
