The Science Journal of the American Association for Respiratory Care

2000 OPEN FORUM Abstracts

RELATIONSHIP OF RAPID SHALLOW MECHANICAL VENTILATION PATTERN (FREQUENCY/TIDAL VOLUME RATIO: F/VT) TO PHYSIOLOGIC DEADSPACE FRACTION (VD/VT) EARLY IN THE ACUTE RESPIRATORY DISTRESS SYNDROME (ARDS)

RH Kallet RRT, J-F. Pittet MD, T Nuckton MD, JA Alonso RRT, B Daniel RRT, MA Matthay MD; University of California, San Francisco; San Francisco General Hospital, San Francisco, CA 94110

Background: Minute ventilation (VE) delivered with a rapid, shallow pattern may worsen carbon dioxide (CO2) gas exchange for 4 reasons: 1.) as VT decreases relative VD increases because anatomic deadspace is fixed;1 2.) at high F, inspiratory time (TI) shortens and decreases CO2 diffusion between alveolar and airway gases;2 3.) tissue resistance in ARDS is F-dependent so that high F may result in ventilation inhomogeneity;3 4.) high F decreases expiratory time (TE), which may cause dynamic hyperinflation and intrinsic positive end-expiratory pressure (PEEPI).3 Therefore, we evaluated the effect of a rapid shallow pattern (F/VT) and PEEPI on VD/VT and in early ARDS.

Methods: VD/VT was measured on Assist/Control ventilation in 110 patients on Day 1 of ARDS. An arterial blood gas was obtained while measuring mean expired CO2 concentration with a Deltatrac metabolic monitor.4 Mean expired CO2 was corrected for compressible volume dilution.4 VT and VE were corrected for compressible circuit loss. Regression analysis was done comparing F, VT, TI, PEEPI and F/VT to VD/VT. Alpha was set at 0.05.

Results: Both F/VT and F showed as moderate positive correlation with VD/VT (r = .48?). TI showed a moderate negative correlation with VD/VT (r = -.41?). In contrast, VT showed a weak negative correlation to VD/VT (r = -.32?). PEEPI could be measured in 29 patients and was detected in 23 patients (3.09 ± 3.16 cm H2O). PEEPI correlated poorly with VD/VT (r = .17). Substituting TE as an indirect marker for dynamic hyperinflation showed a similar poor correlation with VD/VT (r = .12).

Conclusions: A rapid shallow mechanical ventilation pattern is associated with increased VD/VT with F as the predominant factor. The moderate correlation between both F and TI with VD/VT (in contrast to VT, TE and PEEPI) suggests that ventilation inhomogeneity may be the most important ventilation-related factor influencing deadspace in early ARDS.
1. Shapiro BA. Clinical application of blood gases. St Louis: Mosby; 1994 P29-30.
2. Fowler WS. The respiratory deadspace. Am J Physiol. 1948; 154: 405-416.
3. Broseghini C, Brandolese R, et al. Respiratory resistance and intrinsic PEEP in patients with ARDS. Eur Respir J 1988; 1: 726-731.
4. Lum L, Saville A, et al. Accuracy of physiologic deadspace measurement in intubated pediatric patients using a metabolic monitor. Crit Care Med. 1998;26: 760-754.
(?p < 0.05).

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