R. Giuliani

AUTO–POSITIVE END-EXPIRATORY PRESSURE AND DYNAMIC HYPERINFLATION

PEEP is indicated in patients with COPD only to unload the respiratory muscles from the auto-PEEP resulting from expiratory flow limitation. If auto-PEEP is not caused by flow limitation, application of PEEP will cause further hyperinflation, worsening respiratory mechanics, muscle activity, and hemodynamics. To assess the presence of expiratory flow limitation correctly, to measure auto-PEEP correctly, and to identify the maximal PEEP level to be used, measurements of flow and opening pressure must be obtained during a brief period of suspended respiratory muscle activity (obtained by sedation) with the patients own breathing pattern reproduced accurately.

Lung regional stress and strain as a function of posture and ventilatory mode

During positive-pressure ventilation parenchymal deformation can be assessed as strain (volume increase above functional residual capacity) in response to stress (transpulmonary pressure). The aim of this study was to explore the relationship between stress and strain on the regional level using computed tomography in anesthetized healthy pigs in two postures and two patterns of breathing. Airway opening and esophageal pressures were used to calculate stress; change of gas content as assessed from computed tomography was used to calculate strain. Static stress-strain curves and dynamic strain-time curves were constructed, the latter during the inspiratory phase of volume and pressure-controlled ventilation, both in supine and prone position. The lung was divided into nondependent, intermediate, dependent, and central regions: their curves were modeled by exponential regression and examined for statistically significant differences. In all the examined regions, there were strong but different exponential relations between stress and strain. During mechanical ventilation, the end-inspiratory strain was higher in the dependent than in the nondependent regions. No differences between volume- and pressure-controlled ventilation were found. However, during volume control ventilation, prone positioning decreased the end-inspiratory strain of dependent regions and increased it in nondependent regions, resulting in reduced strain gradient. Strain is inhomogeneously distributed within the healthy lung. Prone positioning attenuates differences between dependent and nondependent regions. The regional effects of ventilatory mode and body positioning should be further explored in patients with acute lung injury.

Right ventricular ejection fraction measurement in moderate acute respiratory failure (ARF). Effects of PEEP

Eight patients mechanically ventilated for acute respiratory failure were submitted to increasing levels of PEEP, from 0 to 15 cm H2O. Right ventricular ejection fraction (RVEF) and end-diastolic volume index (RVEDVI) were measured using the fast response thermistor Swan-Ganz catheter. PEEP induced a linear decrease of cardiac index, while the pulmonary artery pressure increased. In three patients (group A) with a RVEDVI larger than 120 ml at ZEEP, RVEF decreased and RVEDVI increased with PEEP. In the other five patients (RVEDVI>120 ml, group B), RVEF was unchanged and RVEDVI decreased at PEEP 15 cm H2O. This study suggest that RV changes induced by PEEP are probably a function of the initial RVEF and RVEDVI.

Effects of positive end-expiratory pressure on right ventricular function in COPD patients during acute ventilatory failure

ObjectiveTo examine the effects of external positive end-expiratory pressure (PEEP) on right ventricular function in chronic obstructive pulmonary disease (COPD) patients with intrinsic PEEP (PEEPi).DesignProspective study.SettingGeneral intensive care unit in a university teaching hospital.PatientsSeven mechanically ventilated flow-limited COPD patients (PEEPi=9.7±1.3 cmH2O, mean±SD) with acute respiratory failure.InterventionHemodynamic and respiratory mechanic data were collected at four different levels of PEEP (0-5-10-15 cmH2O).Measurements and resultsHemodynamic parameters were obtained by a Swan-Ganz catheter with a fast response thermistor. Cardiac index (CI) and end-expiratory lung volume (EELV) reductions started simultaneously when the applied PEEP was approximately 90% of PEEPi measured on 0 cmH2O (ZEEP). Changes in transmural intrathoracic pressure (PEEPi,cw) started only at a PEEP value much higher (120%) than PEEPi. The reduction in CI was related to a decrease in the right enddiastolic ventricular volume index (RVEDVI) (r=0.61;p<0.001). No correlation between CI and transmural right atrial pressure was observed. The RVEDVI was inversely correlated with PEEP-induced changes in EELV (r=−55;p<0.001), but no with PEEPi,cw (r=−0.08; NS). The relationship between RVEDVI and right ventricular stroke work index, considered an index of contractility, was significant in three patients, i.e., PEEP did not change contractility. In the other patients, an increase in contractility seemed to occur.ConclusionsIn COPD patients an external PEEP exceeding 90% of PEEPi causes lung hyperinflation and reduces the CI due to a preload effect. The reduction in RVEDVI seems related to changes in EELV, rather than to changes in transmural pressures, suggesting a lung/heart volume interaction in the cardiac fossa. Thus, in COPD patients, application of an external PEEP level lower than PEEPi may affect right ventricular function.

Acoustic effects of positive end-expiratory pressure on normal lung sounds in mechanically ventilated pigs.

Computerized lung sounds analysis offers a new technique to monitor regional ventilation during spontaneous breathing. The purpose of the present study was to assess the acoustic behaviour of the respiratory system in healthy pigs during mechanical ventilation when a positive end‐expiratory pressure (PEEP) is applied. Lung sounds were recorded during mechanical ventilation and different PEEP levels of 0, 5, 10, 15 and 20 cmH2O were applied. The increase in end‐expiratory lung volume (EELV) related to the PEEP application was also measured and the correlation between changes in EELV (ΔEELV) and sound amplitude (ΔA) was examined. The amplitude of normal lung sounds was reduced by application of PEEP ≥10 cmH2O (P<0·05). The increase in PEEP from 0 to 20 cmH2O reduced the acoustic energy of lung sounds recorded at ZEEP by 0·3 dB (PEEP 5), 2 dB (PEEP 10), 5 dB (PEEP 15) and 7 dB (PEEP 20), which corresponds to 1%, 6%, 14% and 21% in acoustic attenuation, respectively. The variations in ΔA correlated with changes in lung volume (P<0·05) and with changes in compliance of the respiratory system (P<0·05), but were not correlated with changes of the resistance of respiratory system. The frequency analysis showed a downward shifting of the spectra at frequencies between 150 and 600 Hz for PEEP levels ≥10 cmH2O and frequencies between 75 and 600 Hz for PEEP levels ≥15 cmH2O. The application of increasing levels of PEEP reduced the amplitude and changed the spectral characteristics of normal lung sounds.

Estimating respiratory system compliance during mechanical ventilation using artificial neural networks

In this study we evaluated whether a technology based on artificial neural networks (ANN) could estimate the static compliance (CRS) of the respiratory system, even in the absence of an end-inspiratory pause, during continuous mechanical ventilation. A porcine model of acute lung injury was used to provide recordings of different respiratory mechanics conditions. Each recording consisted of 10 or more consecutive breaths in volume-controlled mechanical ventilation, followed by a breath having an end-inspiratory pause used to calculate CRS according to the interrupter technique (IT). The volume-pressure loop of the breath immediately preceding the one with pause was given to the ANN for the training, together with the CRS separately calculated by the IT. The prospective phase consisted of giving only the loops to the trained ANN and comparing the results yielded by it to the compliance separately calculated by the investigators. Determination of measurement agreement between ANN and IT methods showed an error of −0.67 ± 1.52 mL/cm H2O (bias ± sd). We could conclude that ANN, during volume-controlled mechanical ventilation, can extract CRS without needing to stop inspiratory flow.

Patient-ventilator interactions in the critically ill

Positive-pressure breaths can be categorized by three variables: the trigger, the limit, and the cycle. They interface the ventilator with the three variables of the breathing pattern: ventilatory drive, ventilatory requirements, and duration and ratio of inspiratory time to total breath cycle duration. The inspiratory effort necessary to trigger a breath is a significant part of the total inspiratory effort; optimization of the triggering mechanisms may improve patient to ventilator interaction. Setting ventilator flow as close as possible to patients flow by appropriate setting of peak value and waveform profile will also improve patient to ventilator interactions. During pressure support ventilation, ineffective efforts and uncoupling between effort and ventilator output are due to the asynchrony between ventilator and patients inspiratory time. They may be counterbalanced by adequate peak flow and flow threshold values. Proportional assist ventilation may optimize patient to ventilator interactions, but continuous monitoring of respiratory mechanics should be performed along with its definitive technologic implementation. Its clinical use should be continuously adapted to resistance and elastance measurements.

Respiratory mechanics II

To determine whether i.v.NAC has beneficial effects in patients with mild-to-moderate ALI in terms of ventilatory support(VS),FIO2 requirement-,evolution of the lung injury score(LIS),development of severe lung injury(ARDS)and mortality rate,we prospectively enrolled 61 adult patients with ALI to receive either NAC 40 mg/kg/day or Placebo(PL)during 3 days.Respiratory dysfunction was assessed daily considering the need of VS,the F102 necessary to achieve a Pa02 of 70 to 80 mmHg and the evolution of 3 components of the LIS (chest X-ray,Pa02-FIO2 ratio and respiratory system compliance).Data were collected at baseline (day 0),on the first 3 days after admission to the ICU and on discharge.NAC and PL groups(32 vs 29 patients)were comparable at entry in terms of SAPS and values of the LIS.At day 0, 69% of the patients were ventilated in the NAC group versus 76% in the PL group;at day 3, 83% of the NAC treated patients did not require any further VS, versus 52% in the PL group(p=0.01).Pa02/FIO2 improved significantly(p=0.05)from day 0 to day 3 only in the NAC group.The LIS showed a signifi cant improvement(p=0.003)in the NAC treated group within the first 10 days of treatment;no change was observed in the PL group.3 patients in each group progressed to ARDS.The one-month mortality rate was 22% for the NAC and 35% for the PL group In conclusion,early treatment with NAC seems to affect favourably pulmonary gas exchange and decrease the need for prolonged VS in patients with mild-to-moderate ALI.

Respiratory mechanics I

The ARDS is a clinical entity which from a mechanical point of view is characterized not only by a low respiratory compliance but also by a high respiratory resistance (R). In five ARDS patients we analyzed the changes in total R of the respiratory system (Rmax) and airway plus endotracheal tube R (Rmin) induced by changing the airflow (V) rate at constant tidal volume (VT) and total PEEP (PEEP plus autoPEEP). Patients were ventilated in volume controlled mode (VC) and VC with inverse ratio (VCIRV), both with constant insufflation flow. Signals of V, and airway pressure (Paw) were digitized (sampling frequency 200Hz) and acquired by an IBM 55SX computer in order to perform the following calculations: Rmax, Rmin, DR (Rmax-Rmin), static compliance (C). VT was obtained by digital integration of V. Rmin was corrected for the closing time of the inspiratory valve. Statistical analysis was performed with a two-way ANOVA. Results: V PEEPt Ti/Tt VT L/e cmH20 8 reL VC .76±.06 9.5±.4 34+.5 590±42 VCIRV .30+.02 8.4+.5 73+2 610±47 P= <.001 .15 <.001 .5

Hemodynamic effects of PEEP

PP was introduced because of bilateral alveolar consolidation, severe hypoxemia despite adequate level of PEEP (12±2). PP and SP were alternatively used by 4 hours periods with constant ventilatory settings. All patients were hemodynamically monitored (Swan-Ganz catheter). Patients were mean aged 59±19 with a SAPS at 12±3 and under mechanical ventilation since 3,5±4,3 days for pneumonias. 2 evolving as ARDS (Murray score of 3,5 each). All patients improved during the first period: PaO2/FiO2=92±37 before PP (min.=56, max.=128), PaO21FiO2-153±77 at the end of PP (min.=82, max.=230). When the 4 patients were placed back on SP, there was a fall of Pa02/FiO2 resolutive when they returned to PP (208±65 to 77+±21, extreme variations 120 to 61 and 275 to 95). This PP dependency lasted 5,2±0,9 days including 67±14 h under PP before stabilisation of PaO2/FiO2 values whatever the posture was. No severe complication (haemodynamics, extubation) occurred during the procedure. The 2 patients with ARDS died.