The Science Journal of the American Association for Respiratory Care
Case Summary: A 1190 gm male infant was delivered by C-section at 27 weeks gestation secondary to placental abruption 9 weeks post maternal rupture of membranes. The infant was intubated at birth and received 4 doses of Survanta. Development of pulmonary interstitial emphysema ensued subsequent to lung hypoplasia and mechanical ventilation. Fifteen days after birth. HFV was initiated with an Infant Star ventilator for worsening respiratory acidosis and to prevent air leak syndromes. Heliox with conventional ventilation was instituted 9 days later following a rise in PaCO2 to 107 mmHg. Following initial improvement in ventilation and oxygenation with heliox, IV Decadron, and aerosolized Albuterol, desaturations with bradycardia occurred with increasing frequency. Eight days following the start of heliox the infant required 2 minutes of CPR for prolonged desaturation with severe bradycardia. Four days later, FDA approval for compassionate use of inhaled NO was obtained. NO was administered with the Ohmeda INOVENT delivery system. NO was started while heliox administration continued. An initial set dose of 5.4 ppm delivered a dose of 10 ppm because of the effect of helium on the accuracy of the INOVENT's flow injector module. Within 15 minutes the FiO2 was decreased from .80 to .60 which required an adjustment of the set NO dose to 2.6 ppm. The NO dose was subsequently titrated up to 15 ppm with a set dose of 3.5 ppm. This allowed a further reduction in FiO2 to .50 and required an adjustment of the set NO dose to 3.2 ppm to maintain dose delivery at 15 ppm. As FiO2 was reduced, the subsequent increase of the helium concentration, allowed a reduction in both ventilator rate and pressure. The respiratory status of the infant stabilized. Five days later heliox was discontinued. After three more days of NO inhalation, the dose was weaned over a three day period and discontinued. The infant was managed with conventional therapies and was weaned from the ventilator and extubated fifty six days later.
Discussion: The flow injector module of the INOVENT utilizes a heated wire ananometer to measure gas flow. The high thermal conductivity of helium cools the heated wire element more rapidly and causes calculations of higher flowrates than are actually delivered. This effect increases proportional to the concentration of helium in the gas mixture and results in higher NO dose delivery at any set dose. To maintain a constant NO dose delivery, any changes in FiO2 (and therefore the helium concentration), required adjustments in the set NO dose. The following chart quantifies this effect. Measurements were made on a test lung at baseline pressures=33/4, RR=44, insp, time=.44 seconds, and with a rate change to RR=20, and pressure change=20/4.
Conclusion: NO dose delivery of the Ohmeda INOVENT is affected by simultancous use of heliox but appears to remain stable while varying rate and pressure during heliox delivery.
(See Original for Figure)