clinical investigations
in critical care
The Control of Breathing during Weaning from Mechanical Ventilation* \Villiam f: Dunn, .\I.D.; Steven B. Nelson, .\I.S.; and Rolf D. llubmayr, ;u.D., f:C.C.P.
Using the recruitment threshold technique, we measured the co. responsiveness of the unloaded respiratory pump in 14 mechanically ventilated patients prior to weaning. The co. recruitment threshold (C01 RT) was compared with the arterial co. tension during unassisted breathing (C01SB) and with the PaC02 during mechanical ventilation (C01 MV) at machine settings determined by the primary physician. Based on these comparisons, we tested the hypotheses that (l) patients without weaning-induced respiratory distress (group 1) maintain C01 SB near CO.RT, (2) patients with weaning-induced respiratory distress (group 2) retain C02 SB above C02 RT, thereby manifesting incomplete load compensation, and (3) CO.MV is ventilator setting dependent and provides insufficient information about the ventilatory requirement during weaning. Respiratory distress was prospectively de6ned as sustained tachypnea (rate?30) or intense dyspnea (Borg scale rating)
pump fi1ilure n·sults from an imbalance Respiratory lwtween tlw wntilatory load and the strength of tlw respiratory musdes. 1 The intrinsic load of the respiratory musdes depends largdy on the impedance of tht• respiratory system and the minute volume (\-'..:) that must he maintained to satisf~· the vPntilatory requirement. The detPrminants of\-'E includt• the rate of C0 2 produt·tion by tlw tissues (\'co). the dt•ad spat•e to tidal volume ratio (VJ>/\'T), and the arterial C0 2 tension (PaC0 2 ). Thest' ,·ariahles, although t•asily measurable during mechanical ventilation, oftt>n provide limited insight into the ability of a patient to wean from the ventilator because the ventilatory requirement during spontaneous breathing is not known. A rise in C0 2 during weaning relative to the PaC0 2 during mechanical ventilation either means that the patient had been hyperwntilated or reflects the respiratory pump's response to a large load. 'Til distinguish between these two mechanisms, we *Fnnn tht• Di\'ision of Thorack Dist•ast•s and Seetion of Critkal Can• 1\lt•didm·. !\.lam Clink and Mam Foundation. Rot.'ht•s!t•r, Mimlt'sota. · · JIL-:3Slll7 frnm the 1\ational Institute' of SnpjJorted hy ~-:rant Ilea th. 81'thesda. l\ld. Mannst·ript re: rt•,·i•ion at•t•t•ptt•d Fehrnary 13 &Jllilll reque.vts_. IJr. Dw111. F/88 .\la!/f> Clinic, Roche.vter, .\fimw.mta .5.590.5
754
and limited weaning in nine of 14 patients. The average C01 RT was 40 mm Hg in both groups. All patients in group l maintained C01 SB near C01 RT (p>O.l). Seven of nine patients in group 2 retained C01 by ?3 mm Hg above C01 RT (p
in critical care
The Control of Breathing during Weaning from Mechanical Ventilation* \Villiam f: Dunn, .\I.D.; Steven B. Nelson, .\I.S.; and Rolf D. llubmayr, ;u.D., f:C.C.P.
Using the recruitment threshold technique, we measured the co. responsiveness of the unloaded respiratory pump in 14 mechanically ventilated patients prior to weaning. The co. recruitment threshold (C01 RT) was compared with the arterial co. tension during unassisted breathing (C01SB) and with the PaC02 during mechanical ventilation (C01 MV) at machine settings determined by the primary physician. Based on these comparisons, we tested the hypotheses that (l) patients without weaning-induced respiratory distress (group 1) maintain C01 SB near CO.RT, (2) patients with weaning-induced respiratory distress (group 2) retain C02 SB above C02 RT, thereby manifesting incomplete load compensation, and (3) CO.MV is ventilator setting dependent and provides insufficient information about the ventilatory requirement during weaning. Respiratory distress was prospectively de6ned as sustained tachypnea (rate?30) or intense dyspnea (Borg scale rating)
pump fi1ilure n·sults from an imbalance Respiratory lwtween tlw wntilatory load and the strength of tlw respiratory musdes. 1 The intrinsic load of the respiratory musdes depends largdy on the impedance of tht• respiratory system and the minute volume (\-'..:) that must he maintained to satisf~· the vPntilatory requirement. The detPrminants of\-'E includt• the rate of C0 2 produt·tion by tlw tissues (\'co). the dt•ad spat•e to tidal volume ratio (VJ>/\'T), and the arterial C0 2 tension (PaC0 2 ). Thest' ,·ariahles, although t•asily measurable during mechanical ventilation, oftt>n provide limited insight into the ability of a patient to wean from the ventilator because the ventilatory requirement during spontaneous breathing is not known. A rise in C0 2 during weaning relative to the PaC0 2 during mechanical ventilation either means that the patient had been hyperwntilated or reflects the respiratory pump's response to a large load. 'Til distinguish between these two mechanisms, we *Fnnn tht• Di\'ision of Thorack Dist•ast•s and Seetion of Critkal Can• 1\lt•didm·. !\.lam Clink and Mam Foundation. Rot.'ht•s!t•r, Mimlt'sota. · · JIL-:3Slll7 frnm the 1\ational Institute' of SnpjJorted hy ~-:rant Ilea th. 81'thesda. l\ld. Mannst·ript re: rt•,·i•ion at•t•t•ptt•d Fehrnary 13 &Jllilll reque.vts_. IJr. Dw111. F/88 .\la!/f> Clinic, Roche.vter, .\fimw.mta .5.590.5
754
and limited weaning in nine of 14 patients. The average C01 RT was 40 mm Hg in both groups. All patients in group l maintained C01 SB near C01 RT (p>O.l). Seven of nine patients in group 2 retained C01 by ?3 mm Hg above C01 RT (p
