Effects of sustained pressure application on compliance and blood gases in healthy porcine lungs

Intercomparison of recruitment maneuver efficacy in three models of acute lung injury*

OBJECTIVE

To compare the relative efficacy of three forms of recruitment maneuvers in diverse models of acute lung injury characterized by differing pathoanatomy.

DESIGN

We compared three recruiting maneuver (RM) techniques at three levels of post-RM positive end-expiratory pressure in three distinct porcine models of acute lung injury: oleic acid injury; injury induced purely by the mechanical stress of high-tidal airway pressures; and pneumococcal pneumonia.

SETTING

Laboratory in a clinical research facility.

SUBJECTS

Twenty-eight anesthetized mixed-breed pigs (23.8 +/- 2.6 kg).

INTERVENTIONS

The RM techniques tested were sustained inflation, extended sigh or incremental positive end-expiratory pressure, and pressure-controlled ventilation.

PRIMARY MEASUREMENTS

Oxygenation and end-expiratory lung volume.

MAIN RESULTS

The post-RM positive end-expiratory pressure level was the major determinant of post-maneuver PaO2, independent of the RM technique. The pressure-controlled ventilation RM caused a lasting increase of PaO2 in the ventilator-induced lung injury model, but in oleic acid injury and pneumococcal pneumonia, there were no sustained oxygenation differences for any RM technique (sustained inflation, incremental positive end-expiratory pressure, or pressure-controlled ventilation) that differed from raising positive end-expiratory pressure without RM.

CONCLUSIONS

Recruitment by pressure-controlled ventilation is equivalent or superior to sustained inflation, with the same peak pressure in all tested models of acute lung injury, despite its lower mean airway pressure and reduced risk for hemodynamic compromise. Although RM may improve PaO2 in certain injury settings when traditional tidal volumes are used, sustained improvement depends on the post-RM positive end-expiratory pressure value.

Changes in Splanchnic Circulation During an Alveolar Recruitment Maneuver in Healthy Porcine Lungs

Recruitment maneuvers (RM) are advocated as a complement to mechanical ventilation during anesthesia and in acute lung injury. However, they produce high intrathoracic pressures and volumes that may compromise hemodynamics. Our aim was to analyze the effect of a RM on hemodynamics in 10 anesthetized pigs. We assessed carotid, pulmonary, femoral, and hepatic arterial pressures, hepatic and portal venous pressures, total splanchnic (celiac trunk + superior mesenteric artery), hepatic, splenic, renal, and carotid arterial flows, and portal venous flow. We recorded hemodynamics, respiratory mechanics and blood gases before and at 8 min after RM (sustained inflation to 40 cm H(2)O of airway pressure lasting 20 s). Hemodynamics were also measured during RM, and at 1, 3, and 5 min after RM. All flows (P = 0.030) and arterial pressures (P < or = 0.048) decreased during RM, whereas venous pressures increased (P = 0.030). Flows and pressures returned to 75%-109% of baseline immediately after RM. Total splanchnic, renal and portal flows remained decreased at 8 min after RM (P < or = 0.042). Oxygenation did not change, and respiratory mechanics improved after the RM. RM produced a marked, though transitory, impairment of blood flow in all studied vessels. Despite prompt partial recovery, total splanchnic circulation remained reduced at 8 min after RM. This residual decrease may present a risk in conditions with markedly compromised circulatory reserves.

IMPLICATIONS

Recruitment maneuvers (RM) produce high intrathoracic pressures and volumes that may compromise hemodynamics. We found a marked transient impairment of hemodynamics during a RM in 10 anesthetized pigs. At 8 min after RM, blood flow remained reduced in the celiac trunk, superior mesenteric, and renal arteries, as well as in the portal vein. This residual decrease may present a risk in conditions with markedly compromised circulatory reserves.

Effect of forced deflation maneuvers upon measurements of respiratory mechanics in ventilated infants

OBJECTIVE

To determine the effect of forced deflation maneuvers on respiratory mechanics and to assess the reproducibility of such measurements in intubated infants with lung disease.

DESIGN AND SETTING

Prospective study in the pediatric intensive care unit of a university children's hospital.

PATIENTS

Ten clinically stable infants requiring mechanically assisted ventilation for acute pulmonary disease, mean age 5.9 months (1-18), mean weight 5.8 kg (3.2-13).

INTERVENTIONS

Two sets of measurements of compliance (Crs) and resistance (Rrs) were obtained at 20-min intervals both before and after +40/-40 cmH(2)O forced deflation maneuvers. Forced deflation measurements were repeated at the end of the study.

RESULTS

. Forced deflation caused a significant increase in Crs from 0.53+/-0.09 and 0.58+/-0.11 ml/cmH(2)O/kg to 0.71+/-0.11 and 0.68+/-0.11 ml/cmH(2)O/kg. Rrs measurements did not differ. The low coefficients of variation for repeated measures of the baseline measurements (Crs 4.2+/-0.5%, Rrs 7.1+/-0.8%, for forced vital capacity 8.6+/-2.5%, maximum expiratory flows at 25% vital capacity 16.0%+/-3.3%) confirmed the good reproducibility during stable conditions.

CONCLUSIONS

Inflation and deflation maneuvers affect subsequent measurements of respiratory system compliance but not measurements of maximum expiratory flow-volume relationships in intubated infants, probably through recruitment of lung volume. Careful interpretation and planning of the sequence of infant pulmonary function testing is necessary to reassure that changes are not related to short-term alterations in volume history.