SUMMARY Doppler ultrasound measurements of pulmonary blood flow in 20 babies with severe respiratory distress syndrome treated in a randomised controlled trial of surfactant replacement showed that the immediate improvement of oxygenation was not associated with a significant increase in pulmonary blood flow. Reduction in ventilator settings and increases in the extent of chest wall movements measured by a cardiorespiratory monitor suggested that the improvement after surfactant had been given was a result of alveolar stabilisation and increased pulmonary compliance. Further simultaneous studies of pulmonary blood flow and pulmonary compliance are needed to confirm these findings.Natural surfactant has been used to reverse the effects of respiratory failure in animals. 1-5 These studies showed an almost instantaneous increase in arterial oxygen tension followed by a more gradual change in pH and arterial carbon dioxide tension. This rapid improvement in oxygenation was reminiscent of the action of tolazoline in babies with pulmonary hypertension, and we wondered if surfactant acted like a pulmonary vasodilator drug.In lambs changes in pulmonary compliance can be detected 15-30 minutes after giving surfactant -3 but pulmonary blood flow either does not change4 or increases after a delay of about two to three hours.5 In immature baboons both compliance and pulmonary blood flow increase after instillation of surfactant, pulmonary blood flow by threefold.6 As far as we are aware there have been no studies of pulmonary blood flow changes in babies after surfactant treatment. Patients and methodsBased on the threefold increase in pulmonary blood flow in immature baboons after surfactant treatment6 we estimated that to detect a similar change in babies with a power of 80% and a probability of <0-05 (two tailed test) would require a sample of 20 patients.Serial measurements of pulmonary blood flow were made in 20 consecutive babies who were taking part in a randomised controlled trial of surfactant replacement.7 All babies entering the study had severe respiratory distress syndrome requiring 13 mechanical ventilation with >60% oxygen when less than 15 hours old.7The surfactant was isolated from porcine lungs by chloroform-methanol extraction and further purified by liquid-gel chromatography8; it was given in a total dose of 200 mg/kg (2.5 ml/kg) through the endotracheal tube. Treated babies were disconnected from the ventilator and 1-25 ml/kg of surfactant was instilled into each main bronchus through a sterile feeding tube. After each instillation the baby was manually ventilated at a rate 40-60/ minute for one minute using the same oxygen concentration and pressure of <35 cm H20 before being reconnected to the ventilator without altering its settings. In the babies who acted as controls, manual ventilation without instillation of liquid was carried out. After enrolment into the study blood gases were measured at five, 15, 30, and 60 minutes, and appropriate changes in the ventilator settings were made to keep arterial...

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