The Science Journal of the American Association for Respiratory Care

1998 OPEN FORUM Abstracts

STATIC vs DYNAMIC VOLUME-PRESSURE (VP) CURVES IN ARDS

Carl Haas, MLS, RRT: Lauren Stapp, RRT; Robert Bartlett, MD. Critical Care Support Services and the Department of Surgery, University of Michigan Health System, Ann Arbor MI

Introduction Static VP curves help tailor ventilator settings to specific pathophysiology. PEEP is set at or slightly above the lower inflection point (LIP) to prevent collapse and minimize injury from repeated opening and closing. Tidal volume or distending pressure is set below the upper inflection point (UIP) to minimize overdistending lung units and causing a stretch injury. Study Objective To compare LIP & UIP values obtained from static VP curves vs dynamic VP curves from ventilator graphics. Method Nineteen studies were done on 10 paralyzed ARDS patients. Static VP curves were obtained using the super-syringe technique. After disconnecting the ventilator for 15 seconds, a 3 L syringe containing pure O_{2} was attached to the patient. Airway pressure was graphed as 100 mL inflations were injected. Each inflation was held for 2-3 seconds to obtain a plateau. Volume was limited to 1.5 L and pressure to 50 cm H_{2}O. HR, BP, and SpO_{2} were monitored on all; SvO_{2} and continuous CO, if available. LIP was identified as the pressure at the intersection of a tangent to the initial part of the curve with a tangent to the steep part of the curve (LIPii) and the UIP as the pressure at the intersection of a tangent to the steep part of the curve with the tangent to the final portion of the curve. Dynamic VP curves were obtained using a constant flow of 60, 30, and 15 L/m with the ventilator (Nellcor Puritan Bennett 7200) set to a VT =1.5 L, PEEP=0 cm H_{2}O, rate=5, and pressure pop-off at 50 cm H_{2}O. LIP and UIP were obtained in the same manner as described above. The pressure where the tangent to the steep part of the inspiratory limb crossed the X-axis, referred to as the extrapolated LIP (LIPie), was calculated. The pressure at the intersection of the tangents of the initial and final portion of the deflation limb, referred to as the deflation LIP (LIPdi), was calculated. Results LIP: The table presents the data as mean ± SD (cm H_{2}O). All dynamic methods, except LIPie at 15 L/m (p=0.99), were statistically different from the static LIP (* p < 0.05). As flow increased, inflation LIP increased. Deflation LIP was minimally affected by inspiratory flow, but was significantly lower than the static LIP.

LIPii LIPie LIPdi

[cdot] Static 11.5 ± 3.4 -- --

[cdot] Dynamic, 15 L/m 13.5 ± 4.1* 11.5 ± 4.3 6.6 ± 1.9*

[cdot] Dynamic, 30 L/m 18.9 ± 6.0* 17.3 ± 5.5* 6.9 ± 1.8*

[cdot] Dynamic, 60 L/m 27.6 ± 6.8* 25.2 ± 5.6* 5.9 ± 1.5*

UIP: Sixteen of 17 (94%) studies had an UIP on either the static or dynamic curve. Five patients did not have a UIP on the static curve; only 4 did not have an UIP on the 15 L/m dynamic curve. Fewer dynamic curves demonstrated a UIP as flow increased, as the ventilator pressure-cycled before one could be seen. Dynamic UIP increased as inspiratory flow increased.

Static Dynamic: 15 L/m Dynamic: 30 L/m

All UIPs 34.3 ± 6.3 36.3 ± 4.1 38.6 ± 4.0

n/p 13 15/0.29 12/0.016

UIPs with Full 31.3 ± 3.4 35.5 ± 4.2 40.0 ± 3.7

Data Set (n=7) p=0.04 p=0.005

L/m Dynamic: 60 L/m

All UIPs 45.6 ± 4.0

n/p 8/0.0003

UIPs with Full 45.9 ± 4.2

Data Set (n=7) p=0.0003

Conclusion 1) LIP cannot be accurately measured from dynamic VP curves at inspiratory flows commonly used to ventilate patients. 2) Dynamic LIP overestimates static LIP, although the lowest flow studied (15 L/m) was clinically close. Extrapolating back to the X-axis was no different than static LIP. 3) UIP may be overestimated using dynamic VP curves with a constant inspiratory flow.

The 44th International Respiratory Congress Abstracts-On-Disk®, November 7 - 10, 1998, Atlanta, Georgia.

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