SUSCEPTIBILITY TO PERIODIC BREATHING WITH ASSISTED VENTILATION DURINGSLEEP IN NORMAL SUBJECTS

Assisted ventilation with pressure support (PSV) or proportional assis t (PAV) ventilation has the potential to produce periodic breathing (P B) during sleep. We hypothesized that PB will develop when PSV level e xceeds the product of spontaneous tidal volume (VT) and elastance (VTs p.E) but that the actual level at which PB will develop [PSV(PB)] will be influenced by the Delta PCO2 (difference between eupneic PCO2 and CO2 apneic threshold) and by Delta RR [response of respiratory rate (R R) to PSV]. We also wished to determine the PAV level at which PB deve lops to assess inherent ventilatory stability in normal subjects. Twel ve normal subjects underwent polysomnography while connected to a PSV/ PAV ventilator prototype. Level of assist with either mode was increas ed in small steps (2-5 min each) until PB developed or the subject awa kened. End-tidal PCO2, VT, RR, and airway pressure (Paw) were continuo usly monitored, and the pressure generated by respiratory muscle (Pmus ) was calculated. The pressure amplification factor (PAF) at the highe st PAV level was calculated from [(Delta Paw + Pmus)/Pmus], where Delt a Paw is peak Paw - continuous positive airway pressure. PB with centr al apneas developed in 11 of 12 subjects on PSV. Delta PCO2 ranged fro m 1.5 to 5.8 Torr Changes in RR with PSV were small and bidirectional (+1.1 to -3.5 min-l). With use of stepwise regression, PSV(PB) was sig nificantly correlated with VTsp (P = 0.001), E (P = 0.00009), Delta PC O2 (P = 0.007), and Delta RR (P = 0.006). The final regression model w as as follows: PSV(PB) = 11.1 VTsp + 0.3E - 0.4 Delta PCO2 - 0.34 Delt a RR - 3.4 (r = 0.98). PB developed in five subjects on PAV at amplifi cation factors of 1.5-3.4. It failed to occur in seven subjects, despi te PAF of up to 7.6. We conclude that I) a PCO2 apneic threshold exist s during sleep at 1.5-5.8 Torr below eupneic PCO2, 2) the development of PB during PSV is entirely predictable during sleep, and 3) the inhe rent susceptibility to PB varies considerably among normal subjects.