(Chest. 1999;116:26S-27S.)
© 1999
American College of Chest Physicians
Closing Pressure Rather Than Opening Pressure Determines Optimal Positive End-Expiratory Pressure and Avoids Overdistention*
Kevin M. Creamer, MD;
Laryssa L. McCloud, PhD;
Lyle E. Fisher, MD and
Ina C. Ehrhart, PhD
*
From the Medical College of Georgia, Vascular Biology Center and Pediatric Critical Care, Augusta, GA.
Correspondence to: Kevin Creamer, MD, Pediatric Critical Care, Children's Medical Center, 1446 Harper St, Augusta, GA 30912
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Introduction
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Ventilator-induced
lung
injury can result from either low lung volume ventilation with its
associated cyclic alveolar collapse1
or from lung
overdistention2
3
and volutrauma. Positive end-expiratory
pressure (PEEP) is the main determinant of end-expiratory lung volume
(EELV).4
If PEEP is set too low, alveolar and bronchiolar
collapse can occur. Tidal volume in conjunction with PEEP determines
end-inspiratory lung volume (EILV), which is a key determinant in
volutrauma.5
When PEEP is determined by the inflation limb
inflection point, or the "opening pressure" (OP), of the
pressure-volume (P-V) curve, it can be used to recruit collapsed
alveoli, improving oxygenation as well as compliance.6
7
However, OP, an inspiratory parameter, may overestimate the least
amount of PEEP required to maintain alveolar stability and result in
overdistention.8
We decided to look at expiratory
parameters to help determine optimal PEEP. We set out to find a
mechanical ventilation strategy that would utilize optimal PEEP for
alveolar recruitment but not result in overdistention.
We hypothesized that PEEP should be optimized to closing pressure (CP),
the pressure at closing volume (CV), rather than OP. We also
hypothesized that PEEP determined by CP would avoid overdistention, and
allow ventilation on the most compliant portion of the P-V curve in
both healthy and injured lungs.
We used mongrel dogs that were anesthetized and ventilated. After a
left thoracotomy, they were then heparinized and exsanguinated. The
left lower lobe was removed and the pulmonary vein, bronchus, and
pulmonary artery were cannulated. The lobe was then perfused with blood
using a roller head pump set at 600 mL/min, and pulmonary vein
pressure was set at 5 cm H2O. The pulmonary artery,
pulmonary vein, and airway pressures were measured continuously. We
inflated and deflated the lobes with a syringe attached to the
bronchial catheter and measured ventilatory parameters under the four
conditions described below. We measured OP, CP, CV, and the deflation
limb deflection point (Pdef), which represents the upper boundary of
the compliant portion of the P-V curve. The EELV, if OP were used as
PEEP, was also determined. The separate conditions these measurements
were obtained under included the following: inflated and deflated from
complete collapse (atelectasis), with deliberate hyperinflation
(hyperinflated), after mechanical ventilation with (injured), and
without injury (ventilated). Airway pressures were recorded after each
stepwise volume change. The hyperinflated group was inflated beyond
total lobe capacity, while the other three groups were inflated to
total lung capacity.9
The injured lobes were injected with
50 µg of phorbol myristate acetate to simulate ARDS. Added volumes in
the injured group were lower because of the significant edema and air
trapping that resulted from the injury.
As shown in Table 1
,
if OP were used as PEEP, the resultant EELV would be greater than the
Pdef under all conditions. Since Pdef represents the upper volume of
the compliant portion of the P-V curve, all subsequent ventilation
would occur on the noncompliant portion of the curve. However, if the
pressure at CV, or CP determined PEEP, all ventilation would take place
entirely on the most compliant portion of the P-V curve.
The findings in this isolated blood-perfused dog lung model allow us to
conclude that the use OP as PEEP results in overdistention beyond the
compliant portion of the P-V curve. This occurred under all conditions
tested, including postinjury. We also conclude that the use of CP to
determine PEEP would keep tidal ventilation in the optimal lung zone
for both maintaining patency and avoiding overdistention. The pressures
required to open the lung were not the pressures required to keep it
open. "Open lung" strategies10
using PEEP greater than
OP for the recruitment of collapsed alveoli can lead to overdistention,
because they do not take into account the entire P-V curve. In
situations in which P-V hysteresis is increased, the use of OP to
determine PEEP will result in higher EELV than necessary during
mechanical ventilation. Our model demonstrates two scenarios in which
P-V hysteresis is increased: atelectasis and acute lung injury. We
speculate that a mechanical ventilation strategy that includes PEEP to
keep the lung above CV, and tidal volume limitation to keep EILV below
the noncompliant portion of the P-V curve, would diminish the risk of
ventilator-induced lung injury.
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References
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Muscedere, JG, Mullin, JBM, Gan, K, et al (1994) Tidal ventilation at low airway pressures can augment lung injury. Am J Respir Crit Care Med 149,1327-1334[Abstract]
-
Carlton, DP, Cummings, JJ, Scheerer, RG, et al (1990) Lung over expansion increases pulmonary microvascular protein permeability in young lambs. J Appl Physiol 69,577-583[Abstract/Free Full Text]
-
Hernandez, LA, Peevy, KJ, Moise, AA, et al (1989) Chest wall restriction limits high airway pressure induced lung injury in young rabbit. J Appl Physiol 66,2364-2368[Abstract/Free Full Text]
-
Katz, JA, Ozanne, GM, Zinn, SE, et al (1981) Time course and mechanics of lung-volume increase with PEEP in acute pulmonary failure. Anesthesiology 54,9-16[ISI][Medline]
-
Dreyfuss, D, Saumon, G (1993) Role of tidal volume, FRC, and end-inspiratory volume in the development of pulmonary edema following mechanical ventilation. Am Rev Respir Dis 148,1194-1203[ISI][Medline]
-
Ranieri, VM, Giuliani, R, Fiore, T, et al (1994) Volume-pressure curve of the respiratory system predicts effects of PEEP in ARDS: `occlusion' versus `constant flow' techniques. Am J Respir Crit Care Med 149,19-27[Abstract]
-
Matamis, D, Lemaire, F, Hart, A, et al (1984) Total respiratory pressure-volume curves in the adult respiratory distress syndrome. Chest 86,58-66[Abstract/Free Full Text]
-
Marini, JJ (1996) Tidal volume, PEEP, and barotrauma: an open and shut case? Chest 109,302-304[Free Full Text]
-
Glaister, DH, Schroter, RC, Sudlow, MF, et al (1973) Bulk elastic properties of excised lungs and the effect of a transpulmonary pressure gradient. Respir Physiol 17,347-364[CrossRef][ISI][Medline]
-
Amato, MBP, Barbas, CSV, Medeiros, DM, et al (1995) Beneficial effects of the `open lung approach' with low distending pressures in acute respiratory distress syndrome: a prospective randomized study on mechanical ventilation. Am J Respir Crit Care Med 152,1835-1846[Abstract]
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Introduction
References
