MONITORING INTERACTIONS BETWEEN SPONTANEOUS RESPIRATION AND MECHANICAL INFLATIONS IN PRETERM NEONATES

Objectives: To determine the value of a new bedside monitor in assessi ng the interactions between spontaneous respiratory activity and venti lator inflations in preterm infants; and to monitor continuously the d egree of patient ventilator synchrony and the stability of spontaneous respiratory effort during different modes of ventilation and in respo nse to care procedures. Design: A prospective, observational study of physiologic variables recorded by a computerized monitoring system. Se tting: A neonatal intensive care unit in a teaching hospital. Patients : Thirty-one neonates (median gestational age of 28 wks [range 24 to 3 6]; median birth weight of 942 g [range 624 to 2940]) were monitored d uring conventional mandatory ventilation at rates ranging from 47 to 1 08 inflations/min, and 22 infants (median gestational age of 27.5 wks [range 25 to 40]; median birth weight of 1345 g [range 510 to 3490]) w ere monitored during patient triggered ventilation. All infants were s edated as part of the routine care policy. Interventions: Spontaneous respiration (abdominal pressure capsule) and ventilator inflations (ai rway pressure) were recorded continuously for periods of up to 3 days in mechanically ventilated preterm infants. Measurements and Main Resu lts: The monitoring system uses the Frequency Tracking Locus method to derive the Interaction Score, which quantifies the degree of entrainm ent of the spontaneous respiratory pattern by the ventilator. This ana lysis was applied to airway pressure and abdominal capsule signals. A perfect 1:1 interaction between spontaneous inspirations and mechanica l inflations returns an Interaction Score of 1.00, and irregular inter actions return a score of >1.5. During conventional mandatory ventilat ion, a total of 53,074 16-sec epochs (representing 782,811 spontaneous breaths) were studied in 31 preterm infants: 27.4% of epochs showed a 1:1 interaction, 60.5% a non 1:1 interaction, and 12.1% indicated a p assive (i.e., infant apneic) response by the infant, despite excluding periods when paralyzing agents were used. The median Interaction Scor e value during 1:1 interactions was 1.2, whereas for non 1:1 interacti ons the Interaction Score was 2.2. One to one entrainment occurred at conventional mandatory ventilation rates between 50 and 85 inflations/ min: for many infants, such entrainment was achievable over a range of conventional mandatory ventilation rates, while in some infants respi ration was unstable at all rates of conventional mandatory ventilation . During passive ventilation, the median Interaction Score was 1.0. Du ring patient-triggered mechanical ventilation, similar to 67,150 spont aneous respiratory cycles, represented by 3,592 16-sec epochs, were st udied in 22 infants. Overall, 19.5% (702) of epochs showed the criteri a for ideal triggering by spontaneous inspiration and 19.6% (703) show ed autotriggering. In 60.9% (2187) of epochs, a non 1:1 interaction wa s noted. During ideal patient-triggered mechanical ventilation, the me dian Interaction Score was 1.14; during passive (autotriggered) ventil ation, the median Interaction Score was 1.05; and during non 1:1 venti lation, the median score was 1.74. ''Autotriggering'' was found freque ntly in infants of less than or equal to 28 wks gestation. The monitor was able to distinguish between stable and unstable interactions and apnea during conventional mandatory ventilation and patient-triggered mechanical ventilation by reference to the Interaction Score value. Co nclusions: We describe a new kind of bedside monitor for the interpret ation of respiratory data. Unlike other methods, it is able to give th e clinician a continuous measure of patient ventilator interaction whi ch is easy to interpret. It appears to have widespread application in neonatal intensive care nurseries where the babies' own breathing effo rts can affect the efficiency of respiration and cause unwanted physio logic instability. The monitor can be used to determine the optimal ve ntilatory settings to achieve synchrony and to assess the effects of s edative and muscle paralyzing agents.