Effect Of Mean Airway Pressure Research Articles

Right ventricular function during high-frequency oscillatory ventilation in adults with acute respiratory distress syndrome

To evaluate the effect of mean airway pressure under high-frequency oscillatory ventilation on right ventricular function. Prospective randomized study. Intensive care unit of a tertiary care hospital. Sixteen consecutive patients within the first 48 hrs of mainly pulmonary acute respiratory distress syndrome. After a 6-hr-period of protective conventional mechanical ventilation, patients were submitted to three 1-hr periods of high-frequency oscillatory ventilation (+5, +10, +15) in a randomized order, with a mean airway pressure level determined by adding 5, 10, or 15 cm H2O to the mean airway pressure recorded during conventional mechanical ventilation. Mean airway pressure was 18±3 cm H2O during conventional mechanical ventilation and was increased until 33±3 cm H2O at high-frequency oscillatory ventilation+15. Right ventricular function was assessed using transesophageal echocardiography. During conventional mechanical ventilation, nine patients presented a right ventricular dysfunction (right ventricular end-diastolic area/left ventricular end-diastolic area ratio>0.6) of whom four patients had a right ventricular failure (right ventricular end-diastolic area/left ventricular end-diastolic area ratio>0.9). High-frequency oscillatory ventilation+10 and +15 further worsened right ventricular function, resulting in about a 40% increase in right ventricular end-diastolic area/left ventricular end-diastolic area ratio and a 30% increase in end-diastolic eccentricity index when compared with conventional mechanical ventilation or high-frequency oscillatory ventilation+5 periods. At high-frequency oscillatory ventilation+15, 15 patients had right ventricular dysfunction and nine had right ventricular failure. High-frequency oscillatory ventilation did not improve oxygenation whatever the mean airway pressure level. A significant redistribution of tidal variation to the posterior parts of the lung was observed on electrical impedance tomography measurements when increasing mean airway pressure. However, this redistribution was not observed in patients who presented a worsening of right ventricular function (right ventricular end-diastolic area/left ventricular end-diastolic area increase>40%) at high-frequency oscillatory ventilation+15. In patients with mainly pulmonary acute respiratory distress syndrome, using high mean airway pressure under high-frequency oscillatory ventilation can worsen right ventricular function when compared with protective conventional mechanical ventilation, notably in patients in whom high-frequency oscillatory ventilation produced less alveolar recruitment of the posterior parts of the lungs. This study highlights the interest of monitoring right ventricular function during high-frequency oscillatory ventilation.

Changes in mean airway pressure during HFOV influences cardiac output in neonates and infants.

Changes in mean airway pressure affect cardiac output during conventional positive pressure ventilation. The effect of high-frequency oscillation ventilation (HFOV) on cardiac output is less studied. A prospective study in a university hospital pediatric intensive care unit. Fourteen patients aged <1 year and weighing <10 kg who were on HFOV were included. All patients had been on HFOV for >12 h and were considered to be in a stable condition. In the study group (n = 9) the mean proximal airway pressure (Paw) was increased and decreased by +5 and -3 cmH2O, respectively, from baseline in each patient. Measurements were made at each level including baseline settings between each change. In a control group (n = 5) no changes in ventilatory parameters were made. Cardiac output was assessed with echocardiography and the Doppler technique at each level of Paw and at similar intervals in the control group. Cardiac output changed significantly when Paw was changed in the study group (P = 0.02), with the greatest change at the highest Paw at -11% (range: -19 to -9) compared with baseline. We found no significant changes over time in the control group. This study shows that CO is affected by changes in mean airway pressure during HFOV in concordance with the known effects of mean airway pressure during conventional positive pressure ventilation. The mean changes are smaller than expected compared with earlier studies of conventional mechanical ventilation. Further studies are needed to better understand these relationships.

Effects of mean airway pressure on lung volume during high-frequency oscillatory ventilation of preterm infants.

Measurement of mean lung volume (MLV) in high-frequency oscillatory ventilation (HFO) may be useful for optimizing the high lung volume strategy, but has not been available until now. We have measured MLV by means of the sulfur hexafluoride (SF6) washout method in 13 premature infants ventilated with HFO because of respiratory distress syndrome (gestational age, 23 to 31 wk [median, 25 + 6/7 wk]; birthweight, 630 to 1,140 g [790 g]; age at measurement, 2 to 10 d [4 d]; weight, 675 to 1,250 g [850 g]). To evaluate the relationship between MLV and mean airway pressure (MAP), the latter was systematically varied between the measurements. With clinically selected MAP, MLV was between 23.3 and 41.9 ml/kg (median, 33.5 ml/kg) and was strongly dependent on MAP in each patient; linear regression analyses resulted in slope factors between 1.0 and 6.9 ml/cm H2O/kg (median, 2.83 ml/cm H2O/kg), with correlation coefficients between 0.77 and 0.99 (median, 0.94). Stabilization of MLV after modification of MAP took 2 to 25 min (median, 9 min). We conclude that the selection of MAP on a clinical basis leads to a wide range of different MLVs. Despite the strong dependence of MLV on MAP, the prediction of MLV solely based on MAP was impossible because of large patient to patient variability of linear regressions. Alveolar recruitment and derecruitment may take up to 25 min after MAP changes.

Effects of mean airway pressure and tidal volume on lung and chest wall mechanics in the dog

Dependencies of the dynamic mechanical properties of the respiratory system on mean airway pressure (Paw) and the effects of tidal volume (VT) are not completely clear. We measured resistance and dynamic elastance of the total respiratory system (Rrs and Ers), lungs (RL and EL), and chest wall (Rcw and Ecw) in six healthy anesthetized paralyzed dogs during sinusoidal volume oscillations at the trachea (50-300 ml; 0.4 Hz) delivered at mean Paw from -9 to +23 cmH2O. Changes in end-expiratory lung volume, estimated with inductance plethysmographic belts, showed a typical sigmoidal relationship to mean Paw. Each dog showed the same dependencies of mechanical properties on mean Paw and VT. All elastances and resistances were minimal between 5 and 10 cmH2O mean Paw. All elastances, Rrs, and RL increased greatly with decreasing Paw below 5 cmH2O. Ers and EL increased above 10 cmH2O. Ecw, Ers, Rcw, and Rrs decreased slightly with increasing VT, but RL and EL were independent of VT. We conclude that 1) respiratory system impedance is minimal at the normal mean lung volume of supine anesthetized paralyzed dogs; 2) the dependency of RL on lung volume above functional residual capacity is dependent on VT and respiratory frequency; and 3) chest wall, but not lung, mechanical behavior is nonlinear (i.e., VT dependent) at any given lung volume.

Effect of mean airway pressure on bronchial mucosal blood flow in rabbits with oleic acid-induced injury.

The effects of mean airway pressure (MAP) on bronchial mucosal blood flow at the carina in nine anesthetized adult rabbits undergoing high frequency ventilation were studied. The animals were given oleic acid to diminish lung compliance and to exclude the influences of increasing MAP on systemic venous return. Mucosal blood flow decreased significantly when MAP exceeded 14 cmH2O. Mucosal blood velocity tended to decrease as higher levels of MAP were applied. Mucosal blood volume decreased significantly when MAP exceeded 20 cmH2O. Hemodynamics, arterial blood gas values and pH were not changed significantly. Our data demonstrate that bronchial mucosal blood flow decreases at high levels of MAP even if cardiac output remains constant. Hence this decrease of bronchial mucosal blood flow may be due to the transmission of pressure to submucosal bronchial capillaries.

Effect of mean airway pressure and frequency on blood gases during high frequency oscillation : V. Chan and A. Greenough, Department of Child Health, King's College Hospital, London SE5 9RS (UK)

Effect of mean airway pressure and frequency on blood gases during high frequency oscillation : V. Chan and A. Greenough, Department of Child Health, King's College Hospital, London SE5 9RS (UK)

Effect of ventilatory strategy on cardiac output during high frequency jet ventilation

The aim was to study the effects of changes in ventilation variables on cardiac function during high frequency jet ventilation. Controlled changes in mean airway pressure (2-10 cm H2O), tidal volume (4 and 8 ml), and frequency (4-8 Hz) were used to assess the effects of ventilatory variables on cardiac output in cats with normal and reduced lung compliance during high frequency jet ventilation. Cardiac output was continuously measured with an electromagnetic flow probe placed around the aorta. Respiratory compliance was reduced by lung lavage. Seven anaesthetised adults cats, weight 3.0(SEM 0.2) kg, were used in the study. Cardiac output fell as mean airway pressure was raised from 2 to 10 cm H2O at all frequencies and tidal volumes. At identical levels of minute ventilation and mean airway pressure, cardiac output was improved with combinations of smaller tidal volumes and higher frequencies. Reducing respiratory compliance by lung lavage dampened the effect of mean airway pressure on cardiac output, but maintained the relationship between ventilatory variables observed before lavage. At identical settings of minute ventilation and mean airway pressure, cardiac output may be improved with smaller tidal volumes during high frequency jet ventilation.

Effect of mean airway pressure on gas exchange during high-frequency oscillatory ventilation

We studied the effect of mean airway pressure (Paw) on gas exchange during high-frequency oscillatory ventilation in 14 adult rabbits before and after pulmonary saline lavage. Sinusoidal volume changes were delivered through a tracheostomy at 16 Hz, a tidal volume of 1 or 2 ml/kg, and inspired O2 fraction of 0.5. Arterial PO2 and PCO2 (PaO2, PaCO2), lung volume change, and venous admixture were measured at Paw from 5 to 25 cmH2O after either deflation from total lung capacity or inflation from relaxation volume (Vr). The rabbits were lavaged with saline until PaO2 was less than 70 Torr, and all measurements were repeated. Lung volume change was measured in a pressure plethysmograph. Raising Paw from 5 to 25 cmH2O increased lung volume by 48-50 ml above Vr in both healthy and lavaged rabbits. Before lavage, PaO2 was relatively insensitive to changes in Paw, but after lavage PaO2 increased with Paw from 42.8 +/- 7.8 to 137.3 +/- 18.3 (SE) Torr (P less than 0.001). PaCO2 was insensitive to Paw change before and after lavage. At each Paw after lavage, lung volume was larger, venous admixture smaller, and PaO2 higher after deflation from total lung capacity than after inflation from Vr. This study shows that the effect of increased Paw on PaO2 is mediated through an increase in lung volume. In saline-lavaged lungs, equal distending pressures do not necessarily imply equal lung volumes and thus do not imply equal PaO2.

Cardiovascular Effects of High-Frequency Oscillatory and Jet Ventilation

To determine the effects of mean airway pressure on hemodynamics during high-frequency ventilation, we ventilated five cats (wt 2.8 +/- 0.6 kg) using both HFOV (frequencies 3 to 20 Hz) and HFJV (frequencies 4 to 8 Hz) at Paw values ranging from 2 to 12 cm H2O. Combinations of frequency and tidal volume that maintained normocapnia were employed in random order before and after reduction of static compliance of the respiratory system by lung lavage. Heart rate was comparable during both modes of high-frequency ventilation. During both HFOV and HFJV, the cardiac output decreased and PVR increased in normal and surfactant-deficient cats as Paw was elevated (all p less than 0.01, ANOVA). For both HFOV and HFJV linear regression of Paw and cardiac output yielded comparable slopes and y-intercepts. Lung lavage reduced the effect of Paw but did not eliminate it. Changes in ventilatory frequency did not affect cardiac output or PVR. We conclude that the interaction between high-frequency ventilation and cardiovascular function is largely determined by Paw and compliance and is independent of ventilator frequency and the type of ventilator used.

Effects of mean airway pressure on gas transport during high-frequency ventilation in dogs.

In 10 anesthetized, paralyzed, supine dogs, arterial blood gases and CO2 production (VCO2) were measured after 10-min runs of high-frequency ventilation (HFV) at three levels of mean airway pressure (Paw) (0, 5, and 10 cmH2O). HFV was delivered at frequencies (f) of 3, 6, and 9 Hz with a ventilator that generated known tidal volumes (VT) independent of respiratory system impedance. At each f, VT was adjusted at Paw of 0 cmH2O to obtain a eucapnia. As Paw was increased to 5 and 10 cmH2O, arterial PCO2 (PaCO2) increased and arterial PO2 (PaO2) decreased monotonically and significantly. The effect of Paw on PaCO2 and PaO2 was the same at 3, 6, and 9 Hz. Alveolar ventilation (VA), calculated from VCO2 and PaCO2, significantly decreased by 22.7 +/- 2.6 and 40.1 +/- 2.6% after Paw was increased to 5 and 10 cmH2O, respectively. By taking into account the changes in anatomic dead space (VD) with lung volume, VA at different levels of Paw fits the gas transport relationship for HFV derived previously: VA = 0.13 (VT/VD)1.2 VTf (J. Appl. Physiol. 60: 1025-1030, 1986). We conclude that increasing Paw and lung volume significantly decreases gas transport during HFV and that this effect is due to the concomitant increase of the volume of conducting airways.