Comparative effects of helium-oxygen and external positive end-expiratory pressure on respiratory mechanics,gas exchange,and ventilation-perfusion relationships in mechanically ventilated patients with chronic obstructive pulmonary disease

Objective To compare the effects of He/O₂ and external PEEP (PEEPe) on intrinsic PEEP (PEEPi), respiratory mechanics, gas exchange, and ventilation/perfusion (_A/) in mechanically ventilated COPD patients.Design and setting Prospective, interventional study in the intensive care unit of a university hospital.Interventions Ten intubated, sedated, paralyzed, mechanically ventilated COPD patients studied in the following conditions: (a) baseline settings made by clinician in charge, air/O₂, ZEEP; (b) He/O₂, ZEEP; (c) air/O₂, ZEEP; (d) air/O₂, PEEPe 80% of PEEPi. Measurements at each condition included _A/ by the multiple inert gas elimination technique (MIGET).Results PEEPi and trapped gas volume were comparably reduced by He/O₂ (4.2±4 vs. 7.7±4 cmH₂O and 98±82 vs. 217±124 ml, respectively) and PEEPe (4.4±1.3 vs. 7.8±3.6 cmH₂O and 120±107 vs. 216±115 ml, respectively). He/O₂ reduced inspiratory and expiratory respiratory system resistance (15.5±4.4 vs. 20.7±6.9 and 19±9 vs. 28.8±15 cmH₂O l^–1s^–1, respectively) and plateau pressure (13±4 vs. 17±6 cmH₂O). PEEPe increased airway pressures, including total PEEP, and elastance. PaO₂/FIO₂ was slightly reduced by He/O₂ (225±83 vs. 245±82) without significant _A/ change.Conclusions He/O₂ and PEEPe comparably reduced PEEPi and trapped gas volume. However, He/O₂ decreased airway resistance and intrathoracic pressures, at a small cost in arterial oxygenation. He/O₂ could offer an attractive option in COPD patients with PEEPi/dynamic hyperinflation.     

Effects of extrinsic positive end-expiratory pressure on mechanically ventilated patients with chronic obstructive pulmonary disease and dynamic hyperinflation

Objective To examine the circulatory and respiratory effects of extrinsic positive end-expiratory pressure (PEEPe) in patients with chronic obstructive pulmonary disease (COPD) and dynamic hyperinflation during controlled mechanical ventilation.Design Different levels of PEEPe were applied randomly in mechanically ventilated patients with COPD and dynamic hyperinflation.Setting Respiratory Intensive Care Unit of a University Hospital.Patients 9 patients with acute respiratory failure and dynamic hyperinflation due to acute exacerbation of COPD.Interventions PEEPe 35%, 58% and 86% of intrinsic PEEP (PEEPi) were applied.Measurements and results Using flow-directed pulmonary artery catheters hemodynamic measurements were obtained, while simultaneously lung volumes, airflows and airway pressures were recorded. In order to estimate alveolar pressures (P_alv), rapid airway occlusions during passive expiration were also performed. At no level of PEEPe were significant changes in cardiac output, gas exchange variables, dead space, airways inflation resistances and respiratory system static end-inspiratory compliance observed. At high level of PEEPe central venous, mean pulmonary arterial and pulmonary capillary wedge pressures were increased significantly. All but one patient were flow-limited during passive expiration. PEEPe 86% of PEEPi caused a significant increase in end-expiratory lung volume and total PEEP. Iso-volume pressure-flow curves showed volume-dependence expiratory flow limitation in 2 patients, while in 8 patients volume-dependence of critical driving pressure (P_alv-mouth pressure) that decreased flows was also observed.Conclusions The effects of PEEPe on iso-volume flow and hence on lung mechanics and hemodynamics, depend on many factors, such as airways resistances, lung volumes and airway characteristics, making the patient response to PEEPe unpredictable.     

Measurement of diaphragm loading during pressure support ventilation

Objective The diaphragmatic pressure-time product (PTPdi) has been used to quantify the loading and unloading of the diaphragm. The validity of the relationship between PTPdi and diaphragm electrical activity (EMGdi) during pressure-support ventilation (PSV) is unclear. We examined this relationship.Design and setting Physiological study in a physiology laboratory.Subjects Six healthy adults.Interventions Spontaneous breathing (SB) and two levels of PSV (6 and 12 cmH₂O), breathing room air and incremental concentrations of carbon dioxide, sufficient to achieve an EMGdi signal of approximately 200% of baseline value.Measurements and results We measured the electrical (EMGdi) and mechanical (PTPdi) activity of the diaphragm using oesophageal electrode and oesophageal and gastric balloon catheters. The relationship between EMGdi and PTPdi during SB was linear in five subjects and curvilinear in one. However, with PSV 12 cmH₂O we observed that the relationship between EMGdi and PTPdi was left shifted; specifically, for any given level of EMGdi the PTPdi was smaller with PSV 12 cmH₂O than during SB. However, when PTPdi was converted to power (the product of pressure and flow) the tendency to left shift was largely reversed.Conclusions We conclude that when assessing of diaphragm unloading during PSV flow measurements are required. Where flow is constant, PTPdi is a valid measure of diaphragm unloading, but if not these data may be used to make an appropriate correction.Brigitte Fauroux was supported by a grant from the Société de Pneumologie de Langue Française.     

Neural versus pneumatic control of pressure support in patients with chronic obstructive pulmonary diseases at different levels of positive end expiratory pressure: a physiological study

IntroductionIntrinsic positive end-expiratory pressure (PEEPi) is a “threshold” load that must be overcome to trigger conventional pneumatically-controlled pressure support (PS_P) in chronic obstructive pulmonary disease (COPD). Application of extrinsic PEEP (PEEPe) reduces trigger delays and mechanical inspiratory efforts. Using the diaphragm electrical activity (EAdi), neurally controlled pressure support (PS_N) could hypothetically eliminate asynchrony and reduce mechanical inspiratory effort, hence substituting the need for PEEPe. The primary objective of this study was to show that PS_N can reduce the need for PEEPe to improve patient-ventilator interaction and to reduce both the “pre-trigger” and “total inspiratory” neural and mechanical efforts in COPD patients with PEEPi. A secondary objective was to evaluate the impact of applying PS_N on breathing pattern.MethodsTwelve intubated and mechanically ventilated COPD patients with PEEPi ≥ 5 cm H₂O underwent comparisons of PS_P and PS_N at different levels of PEEPe (at 0 %, 40 %, 80 %, and 120 % of static PEEPi, for 12 minutes at each level on average), at matching peak airway pressure. We measured flow, airway pressure, esophageal pressure, and EAdi, and analyzed neural and mechanical efforts for triggering and total inspiration. Patient-ventilator interaction was analyzed with the NeuroSync index.ResultsMean airway pressure and PEEPe were comparable for PS_P and PS_N at same target levels. During PS_P, the NeuroSync index was 29 % at zero PEEPe and improved to 21 % at optimal PEEPe (P < 0.05). During PS_N, the NeuroSync index was lower (<7 %, P < 0.05) regardless of PEEPe. Both pre-trigger (P < 0.05) and total inspiratory mechanical efforts (P < 0.05) were consistently higher during PS_P compared to PS_N at same PEEPe. The change in total mechanical efforts between PS_P at PEEPe_0% and PS_N at PEEPe_0% was not different from the change between PS_P at PEEPe_0% and PS_P at PEEPe_80%.ConclusionPS_N abolishes the need for PEEPe in COPD patients, improves patient-ventilator interaction, and reduces the inspiratory mechanical effort to breathe.

Trial registration

Clinicaltrials.gov {"type":"clinical-trial","attrs":{"text":"NCT02114567","term_id":"NCT02114567"}}NCT02114567. Registered 04 November 2013.     

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

Objective To 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).Design Prospective study.Setting General intensive care unit in a university teaching hospital.Patients Seven mechanically ventilated flow-limited COPD patients (PEEPi=9.7±1.3 cmH₂O, mean±SD) with acute respiratory failure.Intervention Hemodynamic and respiratory mechanic data were collected at four different levels of PEEP (0-5-10-15 cmH₂O).Measurements and results Hemodynamic 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.Conclusions In 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.This study was supported in part by a grant from the Consiglio Nazionale delle Ricerche, Italy     

Linear and nonlinear analysis of pressure and flow during mechanical ventilation

Objective: Linear modeling as a method of exploring respiratory mechanics during mechanical ventilation, was compared to nonlinear modeling for flow dependence of resistance in three distinct groups of patients, those with: (a) normal respiratory function (NRF), (b) chronic obstructive pulmonary disease (COPD), or (c) adult respiratory distress syndrome (ARDS). Design and patients: Airways opening pressure (Pao), flow (V′), and volume (V) signals were recorded in 32 ICU mechanically ventilated patients, under sedation and muscle relaxation (10 NRF, 11 COPD, 11 ARDS). All patients were ventilated with controlled mandatory ventilation mode at three levels of end-expiratory pressure (PEEPe): 0, 5, and 10 hPa. Data were analyzed according to: (a) Pao = PE + Ers V + Rrs V′ and (b) Pao = PE + Ers V + k ₁V′ + k ₂|V′|V′, where Ers and Rrs represent the intubated respiratory system (RS) elastance and resistance, k ₁ and k ₂ the linear and the nonlinear RS resistive coefficients, and PE the end-expiratory pressure. The model's goodness of fit to the data was evaluated by the root mean square difference of predicted minus measured Pao values. Results: NRF data fit both models well at all PEEPe levels. ARDS and particularly COPD data fit the nonlinear model better. Values of k ₂ were often negative in COPD and ARDS groups, and they increased in parallel with PEEPe. A gradual increase in PEEPe resulted in better fit of ARDS and COPD data to both models. Conclusions: The model of V′ dependence of resistance is more suitable for the ARDS and particularly the COPD groups. PEEP tends to diminish the V′ dependence of respiratory resistance during the respiratory cycle, particularly in the COPD group, probably through an indirect effect of the increased lung volume. Received: 14 June 1999 Final revision received: 19 November 1999 Accepted: 11 April 2000     

Respiratory compliance and resistance in mechanically ventilated patients with acute respiratory failure

The purpose of this study was noninvasive assessment of respiratory compliance and resistance in mechanically ventilated patients with acute respiratory failure (ARF). To this end, flow, change in lung volume, and airway pressure were measured at the proximal tip of the endotracheal tubes in twenty nine critically ill unselected patients. Eleven had acute exacerbation of chronic obstructive pulmonary disease (COPD), 8 had adult respiratory distress syndrome (ARDS) and 10 had ARF of various etiologies. Static compliance (Cst,rs), intrinsic PEEP (PEEPi), as well as minimum and maximum resistance (Rrs,min and Rrs,max, respectively) were obtained with end-inspiratory and end-expiratory airway occlusions. We found that: (1) PEEPi was present in all patients with COPD (up to 11.4 cmH₂O) and it was not uncommon in patients with ARF without history of chronic airway disease (up to 4.1 cmH₂O). (2) Without correction for PEEPi average Cst,rs was not significantly different between ARDS and COPD patients, whereas the average corrected compliance was significantly lower in ARDS patients. (3) Substantial frequency-dependence of resistance was exhibited not only by COPD patients, but also by ARDS patients.A preliminary report was presented at the joint meeting SEP-SEPCR, Paris, September 3rd, 1986.Currently on sabbatical leave from the University of Cincinnaty     

Influence of respiratory rate on gas trapping during low volume ventilation of patients with acute lung injury

OBJECTIVE: Reduction in tidal volume (Vt) associated with increase in respiratory rate to limit hypercapnia is now proposed in patients with acute lung injury (ALI). The aim of this study was to test whether a high respiratory rate induces significant intrinsic positive end-expiratory pressure (PEEPi) in these patients. DESIGN: Prospective crossover study. SETTING: A medical intensive care unit. INTERVENTIONS AND MEASUREMENTS: Ten consecutive patients fulfilling criteria for severe ALI were ventilated with a 6 ml/kg Vt, a total PEEP level at 13+/-3 cmH(2)O and a plateau pressure kept at 23+/-4 cmH(2)O. The respiratory rate was randomly set below 20 breaths/min (17+/-3 breaths/min) and increased to 30 breaths/min (30+/-3 breaths/min) to compensate for hypercapnia. External PEEP was adjusted to keep the total PEEP and the plateau pressure constant. PEEPi was computed as the difference between total PEEP and external PEEP. The lung volume retained by PEEPi was then measured. RESULTS: Increase in respiratory rate resulted in significantly higher PEEPi (1.3+/-0.4 versus 3.9+/-1.1 cmH(2)O, p<0.01) and trapped volume (70+/-43 versus 244+/-127 ml, p<0.01). External PEEP needed to be reduced from 11.9+/-3.4 to 9.7+/-2.9 cmH(2)O ( p<0.01). PaO(2) was not affected but the alveolar-arterial oxygen tension difference slightly worsened with the high respiratory rate (p<0.05). CONCLUSIONS: An increase in respiratory rate used to avoid Vt reduction-induced hypercapnia may induce substantial gas trapping and PEEPi in patients with ALI.     

Automated patient-ventilator interaction analysis during neurally adjusted non-invasive ventilation and pressure support ventilation in chronic obstructive pulmonary disease

Introduction

Delivering synchronous assist during non-invasive ventilation (NIV) is challenging with flow- or pressure-controlled ventilators, especially in patients with chronic obstructive pulmonary disease (COPD). Neurally adjusted ventilatory assist (NAVA) uses diaphragm electrical activity (EAdi) to control the ventilator. We evaluated patient-ventilator interaction in patients with COPD during NIV with pressure support ventilation (PSV) and NAVA using a recently introduced automated analysis.

Methods

Twelve COPD patients underwent three 30-minute trials: 1) PSV with dedicated NIV ventilator (NIV-PSV_Vision), 2) PSV with intensive care unit (ICU) ventilator (NIV-PSV_Servo-I), and 3) with NIV-NAVA. EAdi, flow, and airway pressure were recorded. Patient-ventilator interaction was evaluated by comparing airway pressure and EAdi waveforms with automated computer algorithms. The NeuroSync index was calculated as the percentage of timing errors between airway pressure and EAdi.

Results

The NeuroSync index was higher (larger error) for NIV-PSV_Vision (24 (IQR 15 to 30) %) and NIV-PSV_Servo-I (21 (IQR 15 to 26) %) compared to NIV-NAVA (5 (IQR 4 to 7) %; P <0.001). Wasted efforts, trigger delays and cycling-off errors were less with NAVA (P <0.05 for all). The NeuroSync index and the number of wasted efforts were strongly correlated (r² = 0.84), with a drastic increase in wasted efforts after timing errors reach 20%.

Conclusions

In COPD patients, non-invasive NAVA improves patient-ventilator interaction compared to PSV, delivered either by a dedicated or ICU ventilator. The automated analysis of patient-ventilator interaction allowed for an objective detection of patient-ventilator interaction during NIV. In addition, we found that progressive mismatch between neural effort and pneumatic timing is associated with wasted efforts.     

神经调节辅助通气对AECOPD患者呼吸功的影响

目的探讨不同外源性呼气末正压（extrinsic positive end expiratory pressure,PEEPe）条件下,神经调节辅助通气（neurally adjusted ventilatory assist,NAVA）对慢性阻塞性肺疾病急性加重（acute exacerbation of chronicobstructive pulmonary disease,AECOPD）患者呼吸功及触发功的影响。 方法以2012年5月至2013年5月入住东南大学附属中大医院ICU、静态内源性呼气末正压（intrinsic positive end-expiratory pressure,PEEPi）（PEEPi_stat）≥5 cm H2O（1 cm H2O=0.098 kPa）的AECOPD患者为研究对象。本研究方案已通过东南大学附属中大医院伦理委员会批准（批准号：2010ZDLL018.0）,并与患者签署了知情同意书。将控制通气下PEEPe由0升至40%PEEPi_stat,总呼气末正压（total-PEEP）不增加的患者作为呼气流速受限（expiratory flow limitation,EFL）组,增加的患者为呼气阻力（expiratory resistance,Re）增高组。共纳入AECOPD患者12例,其中EFL组6例,Re组6例。患者分组后调节镇静深度至Ramsay3分,在PEEPe设定为0、40%、80%、120%PEEPi_stat条件下,随机进行支持力度相同压力支持通气（pressure support ventilation,PSV）及NAVA通气。通过NAVA压力限定实现NAVA与PSV支持水平的等效性。监测食道内压（esophageal pressure,Pes）、膈肌电活动（electrical activity diaphragm,EAdi）,采集流速、压力波形并计算呼吸功（PTPes_ins）和触发功（PTPes_tri）。测量参数在通气模式和PEEPe水平之间的比较采用两因素的重复测量方差分析。在NAVA或PSV模式下,不同PEEPe水平之间的多重比较采用SNK检验。 结果2组患者年龄与急性生理与慢性健康评分II（acute physidogy and chronic health evaluation,APACHE II）等一般情况无显著差异。① NAVA与PSV支持水平的等效性：NAVA通气时可以获得与PSV通气类似的方波压力-时间曲线,且与PSV相比NAVA通气时呼吸频率、吸气时间、气道峰值压、平均气道压均无显著差异（t=0.720,0.817,0.621,1.579,均P>0.05）。② NAVA对呼吸功影响：在相同PEEPe水平下NAVA通气时呼吸功明显低于PSV通气（t=3.816,3.117,2.758,2.572,均P<0.05）。PEEPe由0逐渐增至120%PEEPi_stat时,在NAVA及PSV模式下,EFL组患者呼吸功均显著下降（t=4.629,4.431,4.165,5.082,均P<0.05）;RE组患者呼吸功无显著变化（F=8.12,7.64,均P>0.05）。③ NAVA对触发功的影响：相同PEEPe水平下,NAVA通气触发功明显低于PSV通气（t=4.624,4.431,4.165,5.082,均P<0.05）。PEEPe由0逐渐增至120%PEEPi_stat时,NAVA模式下EFL组及RE组患者触发功均无显著变化（F=5.71,5.93,均P>0.05）;PSV模式下,EFL组患者触发功显著下降（F=16.21,P<0.05）;RE组患者随着PEEPe增加触发功无明显变化（F=6.12,P>0.05）。 结论与PSV相比,NAVA通气显著降低AECOPD患者的呼吸功及触发功。NAVA通气时触发功不受PEEPe的影响,PSV通气时增加PEEPe可降低呼气流速受限患者的触发功。     

The effect of back-up rate during non-invasive ventilation in young patients with cystic fibrosis

Objective The aim of the study was to evaluate the effect of the back-up rate on respiratory effort during non-invasive mechanical ventilation.Design An in vitro study evaluated the inspiratory trigger in seven domiciliary ventilators. Then, a prospective, randomized, crossover trial compared the effect on respiratory effort of three different back-up rates during pressure support (PS) and assist-control/volume-targeted (AC/VT) ventilation.Setting A research unit and a tertiary referral pediatric center.Patients Ten patients with cystic fibrosis (CF).Interventions During the in vivo study, the back-up rate was progressively increased to the maximum that patients could tolerate (Fmax) and respiratory effort, as judged by pressure/time product of the diaphragm (PTPdi/min), was compared between the two ventilatory modes.Results Differences were observed between trigger pressure, trigger time delay, trigger pressure/time product and the slope between flow and pressure in the seven ventilators. PS and AC/VT ventilation were associated with a decrease in respiratory effort (PTPdi/min was 518±172, 271±119 and 291±138 cmH₂O . s^–1 . min^-1, for spontaneous breathing, PS and AC/VT ventilation, respectively, p=0.05). During the two modes, increasing the back-up rate to Fmax resulted in a greater reduction in PTPdi/min (p=0.001), which was more pronounced during AC/VT ventilation, due to the automatic adjustment of the inspiratory/expiratory time ratio.Conclusions Increasing the back-up rate during PS and AC/VT ventilation decreases respiratory effort in young patients with CF, but this effect was more marked with AC/VT ventilation.Electronic Supplementary Material Supplementary material is available in the online version of this article at This study was supported with grants from Vaincre la Mucoviscidose (VLM), the Comité National contre les Maladies Respiratoires (CNMR) and the Société de Pneumologie de Langue Française (SPLF).     

Physiological comparison of three spontaneous breathing trials in difficult-to-wean patients

Purpose

To compare cardiovascular and respiratory responses to different spontaneous breathing trials (SBT) in difficult-to-wean patients using T-piece and pressure support ventilation (PSV) with or without positive end-expiratory pressure (PEEP).

Methods

Prospective physiological study. Fourteen patients who were monitored with a Swan-Ganz catheter and had failed a previous T-piece trial were studied. Three SBTs were performed in random order in all patients: PSV with PEEP (PSV-PEEP), PSV without PEEP (PSV-ZEEP), and T-piece. PSV level was 7 cmH₂O, and PEEP was 5 cmH₂O. Inspiratory muscle effort was calculated, and hemodynamic parameters were measured using standard methods.

Results [median (and interquartile range)]

Most patients succeeded in the PSV-PEEP (11/14) and PSV-ZEEP (8/14) trials, but all failed the T-piece trial. Patient effort was significantly higher during T-piece than during PSV with or without PEEP [esophageal pressure-time product was 292 (238–512), 128 (58–299), and 148 (100–465) cmH₂O·s/min, respectively, p < 0.05]. Left ventricular heart failure was observed in 11 of the 14 patients during the T-piece trial. Pulmonary artery occlusion pressure and respiratory rate were significantly higher during T-piece than with PSV-PEEP [21 (18–24) mmHg versus 17 (14–22) mmHg, p < 0.05 and 27 (21–35) breaths/min versus 19 (16–29) breaths/min, p < 0.05 respectively]. Tidal volume was significantly lower during the T-piece trial.

Conclusion

In this selected population of difficult-to-wean patients, PSV and PSV plus PEEP markedly modified the breathing pattern, inspiratory muscle effort, and cardiovascular response as compared to the T-piece. Caregivers should be aware of these differences in SBT as they may play an important role in weaning decision-making.     

Effect of low-level PEEP on inspiratory work of breathing in intubated patients,both with healthy lungs and with COPD

Objective Evaluation of low-level PEEP (5 cm H₂O) and the two different CPAP trigger modes in the Bennett 7200a ventilator (demand-valve and flow-by trigger modes) on inspiratory work of breathing (W_i) during the weaning phase.Design Prospective controlled study.Setting The intensive care unit of a university hospital.Patients Six intubated patients with normal lung function (NL), ventilated because of non-pulmonary trauma or post-operative stay in the ICU, and six patients recovering from acute respiratory failure due to exacerbation of chronic obstructive pulmonary disease (COPD), breathing either FB-CPAP or DV-CPAP with the Bennett 7200a ventilator.Interventions The patients studied were breathing with zero end-expiratory pressure (ZEEP), as well as CPAP of 5 cm H₂O (PEEP), with the following respiratory modes: the demand-valve trigger mode, pressure support of 5 cm H₂O, and the flow-by trigger mode (base flow of 20 l/min and flow trigger of 2 l/min). Furthermore, W_i during T-piece breathing was evaluated.Measurements and results W_i was determined using a modified Campbell's diagram. Total inspiratory work (W_i), work against flow-resistive resistance (W_ires), work against elastic resistance (W_iel), work imposed by the ventilator system (W_imp), dynamic intrinsic positive end-expiratory pressure (PEEP_idyn), airway pressure decrease during beginning inspiration (P_aw) and spirometric parameters were measured. In the NL group, only minor, clinically irrelevant changes in the measured variables were detected. In the COPD group, in contrast, PEEP reduced W_i and its components W_ires and W_iel significantly compared to the corresponding ZEEP settings. This was due mainly to a significant decrease in PEEP_idyn when external PEEP was applied. Flow-by imposed less W_i on the COPD patients during PEEP than did demand-valve CPAP. Differences in W_imp between the flow-by and demand-valve trigger models were significant for both groups. However, in relation to W_i these differences were small.Conclusion We conclude that the application of low-level external PEEP benefits COPD patients because it reduces inspiratory work, mainly by lowering the inspiratory threshold represented by PEEP_idyn. Differences between the trigger modes of the ventilator used in this study were small and can be compensated for by the application of a small amount of pressure support.Supported by Deutsche Forschungsgemeinschaft SFB 330 Organprotektion Project B17     

Functional residual capacity as a noninvasive indicator of optimal positive end-expiratory pressure

We hypothesized that functional residual capacity (FRC) could be used as a noninvasive indicator of optimal positive end-expiratory pressure (PEEP), the level of PEEP that results in venous admixture below 15% with an inspired oxygen fraction less than 0.5. We compared several variables for PEEP optimization—oxygen transport, total respiratory system compliance, FRC-based compliance, mixed venous oxygen saturation, end-tidal to arterial carbon dioxide tension difference, and arterial oxygen saturation—by producing four different PEEP levels, 0, 5, 10, and 15 cm H₂O, in 24 mongrel dogs in which pulmonary injury was produced. The data were regressed versus PEEP by using analysis of variance for regression. Venous admixture (F_1,23=149.3;P<0.0001), end-tidal to arterial carbon dioxide tension difference (F_1,23=64.9;P<0.0001), and oxygen transport (F_1,23=95.1;P<0.0001) decreased linearly with PEEP. FRC (F_1,23=248.1;P<0.0001) and arterial oxygen saturation (F_1,23=66.9;P<0.0001) increased linearly with PEEP. Total respiratory system compliance (F_1,23=66.6;P<0.0001) and mixed venous oxygen saturation (F_1,23=12.2;P<0.002) had a quadratic relationship with respect to PEEP with a peak at 5 cm H₂O. FRC-based compliance did not have a significant relationship to PEEP. The maximum values of total respiratory system compliance, FRC-based compliance, mixed venous oxygen saturation, and oxygen transport did not occur at PEEP levels that corresponded to a venous admixture below 15% (optimal PEEP). In this canine oleic acid lung injury model, maximizing these variables would be a poor technique for PEEP titration. FRC and arterial oxygen saturation had a strong relationship to PEEP and venous admixture, and these two would be good noninvasive variables for use in PEEP titration.This work was supported by a grant from Siemens-Elema Ventilator Division, Solna, Sweden.     

Positive end-expiratory airway pressure does not aggravate ventilator-induced diaphragmatic dysfunction in rabbits

Introduction

Immobilization of hindlimb muscles in a shortened position results in an accelerated rate of inactivity-induced muscle atrophy and contractile dysfunction. Similarly, prolonged controlled mechanical ventilation (CMV) results in diaphragm inactivity and induces diaphragm muscle atrophy and contractile dysfunction. Further, the application of positive end-expiratory airway pressure (PEEP) during mechanical ventilation would result in shortened diaphragm muscle fibers throughout the respiratory cycle. Therefore, we tested the hypothesis that, compared to CMV without PEEP, the combination of PEEP and CMV would accelerate CMV-induced diaphragm muscle atrophy and contractile dysfunction. To test this hypothesis, we combined PEEP with CMV or with assist-control mechanical ventilation (AMV) and determined the effects on diaphragm muscle atrophy and contractile properties.

Methods

The PEEP level (8 cmH₂O) that did not induce lung overdistension or compromise circulation was determined. In vivo segmental length changes of diaphragm muscle fiber were then measured using sonomicrometry. Sedated rabbits were randomized into seven groups: surgical controls and those receiving CMV, AMV or continuous positive airway pressure (CPAP) with or without PEEP for 2 days. We measured in vitro diaphragmatic force, diaphragm muscle morphometry, myosin heavy-chain (MyHC) protein isoforms, caspase 3, insulin-like growth factor 1 (IGF-1), muscle atrophy F-box (MAFbx) and muscle ring finger protein 1 (MuRF1) mRNA.

Results

PEEP shortened end-expiratory diaphragm muscle length by 15%, 14% and 12% with CMV, AMV and CPAP, respectively. Combined PEEP and CMV reduced tidal excursion of segmental diaphragm muscle length; consequently, tidal volume (VT) decreased. VT was maintained with combined PEEP and AMV. CMV alone decreased maximum tetanic force (Po) production by 35% versus control (P < 0.01). Combined PEEP and CMV did not decrease Po further. Po was preserved with AMV, with or without PEEP. Diaphragm muscle atrophy did not occur in any fiber types. Diaphragm MyHC shifted to the fast isoform in the combined PEEP and CMV group. In both the CMV and combined PEEP and CMV groups compared to controls, IGF-1 mRNAs were suppressed, whereas Caspase-3, MAFbx and MuRF1 mRNA expression were elevated.

Conclusions

Two days of diaphragm muscle fiber shortening with PEEP did not exacerbate CMV-induced diaphragm muscle dysfunction.     

Ventilation distribution measured with EIT at varying levels of pressure support and Neurally Adjusted Ventilatory Assist in patients with ALI

Purpose

The purpose of this study was to compare the effect of varying levels of assist during pressure support (PSV) and Neurally Adjusted Ventilatory Assist (NAVA) on the aeration of the dependent and non-dependent lung regions by means of Electrical Impedance Tomography (EIT).

Methods

We studied ten mechanically ventilated patients with Acute Lung Injury (ALI). Positive-End Expiratory Pressure (PEEP) and PSV levels were both 10 cm H₂O during the initial PSV step. Thereafter, we changed the inspiratory pressure to 15 and 5 cm H₂O during PSV. The electrical activity of the diaphragm (EAdi) during pressure support ten was used to define the initial NAVA gain (100 %). Thereafter, we changed NAVA gain to 150 and 50 %, respectively. After each step the assist level was switched back to PSV 10 cm H₂O or NAVA 100 % to get a new baseline. The EIT registration was performed continuously.

Results

Tidal impedance variation significantly decreased during descending PSV levels within patients, whereas not during NAVA. The dorsal-to-ventral impedance distribution, expressed according to the center of gravity index, was lower during PSV compared to NAVA. Ventilation contribution of the dependent lung region was equally in balance with the non-dependent lung region during PSV 5 cm H₂O, NAVA 50 and 100 %.

Conclusion

Neurally Adjusted Ventilatory Assist ventilation had a beneficial effect on the ventilation of the dependent lung region and showed less over-assistance compared to PSV in patients with ALI.     

Effects of different mechanical ventilation strategies on the mucociliary system

Objective

To evaluate the effects of different mechanical ventilation (MV) strategies on the mucociliary system.

Design and setting

Experimental study.

Subjects

Twenty-seven male New Zealand rabbits.

Interventions

After anesthesia, animals were tracheotomized and ventilated with standard ventilation [tidal volume (Vt) 8?ml/kg, positive end expiratory pressure (PEEP) 5?cmH₂O, flow 3?L/min, FiO₂ 0.4] for 30?min. Next, animals were randomized into three groups and ventilated for 3?h with low volume (LV): Vt 8?ml/kg, PEEP 5?cmH₂O, flow 3?L/min (n?=?6); high volume (HV): Vt 16?ml/kg, PEEP 5?cmH₂O, flow 5?L/min (n?=?7); or high pressure (HP): Ppeak 30?cmH₂O, PEEP 12?cmH₂O (n?=?8). Six animals (controls) were ventilated for 10?min with standard ventilation. Vital signals, blood lactate, and respiratory system mechanics were verified. Tracheal tissue was collected before and after MV.

Measurements

Lung and tracheal tissue sections were stained to analyze inflammation and mucosubstances by the point-counting method. Electron microscopy verified tracheal cell ultrastructure. In situ tracheal ciliary beating frequency (CBF), determined using a videoscopic technique, and tracheal mucociliary transport (TMCT), assessed by stereoscopic microscope, were evaluated before and after MV.

Results

Respiratory compliance decreased in the HP group. The HV and HP groups showed higher lactate levels after MV. Macroscopy showed areas of atelectasis and congestion on HV and HP lungs. Lung inflammatory infiltrate increased in all ventilated groups. Compared to the control, ventilated animals also showed a reduction of total and acid mucus on tracheal epithelium. Under electron microscopy, injury was observed in the ciliated cells of the HP group. CBF decreased significantly after MV only in the HP group. TMCT did not change significantly in the ventilated groups.

Conclusions

Different MV strategies induce not only distal lung alterations but also morphological and physiological tracheal alterations leading to mucociliary system dysfunction.     

Optimal pressure support level for beginning weaning in patients with COPD: Measurement of diaphragmatic activity with step-by-step decreasing pressure support level

The study objective was to determine an “optimal” individual pressure support (PS) level for beginning weaning with PS ventilation in patients with chronic obstructive pulmonary disease (COPD). Eleven COPD patients intubated and ventilated for acute respiratory failure and judged ready for weaning were studied. The technique consisted of lowering the PS level from a point that was characteristic for each patient and measurable under controlled mechanical ventilation, after setting the ventilator as recommended for COPD patients judged ready for weaning, that is, peak inflation pressure (PIP). This determination was based mainly on exploring the diaphragm with an electromyographic technique by defining the optimal PS level as the lowest PS level associated with no EMG evidence of diaphragmatic stress. Diaphragmatic electromyographic activity (diEMG) was recorded by a bipolar esophageal electrode (Disa-Denmark), and the high-frequency electrical component/low-frequency ratio (H/L) was calculated. The reference H/L was determined during a few spontaneous ventilatory cycles. Muscle stress was defined as a greater than 20% reduction in H/L compared with the reference value. Optimal PS levels ranged from 4 to 24 cm H₂O with a mean of 14 ± 6 cm H₂O. Two patients with optimal PS level at 4 cm H₂O did not require weaning and were quickly extubated. For the nine other patients, optimal PS levels were found to be 70% of PIP; in none was it necessary during weaning to use PS levels higher than individual optimal PS levels. Optimal PS level established with di-EMG monitoring seems to be a useful index for beginning weaning in the PS ventilation mode in COPD patients. The hypothesis of beginning weaning with a PS level equal to 70% of PIP needs to be tested.     

Pressure support ventilation (PSV) reverses hyperinflation induced isorhythmic A-V dissociation

We report a case of hyperinflation induced isorythmic atrio-ventricular dissociation with circulatory failure in a patient with chronic obstructive pulmonary disease. The arrythmia was successfully treated by applying pressure support ventilation (PSV: 20 cmH₂O) which, by decreasing the respiratory rate and increasing the expiratory time reduced the level of auto-PEEP. In order to explain this result the Authors recorded, in the same patient, the level of auto-PEEP and FRC obtained with Intermittent Positive Pressure Ventilation (IPPV), Intermittent Mandatory Ventilation (IMV) and PSV at the same gas exchange values. PSV showed a dramatic reduction of both these parameters. (Auto-PEEP: IPPV 12 cmH₂O, IMV 17 cmH₂O, PSV 7 cmH₂O).     

Optimal endexpiratory airway pressure for ventilated patients

In patients ventilated for acute respiratory failure PEEP was changed either by gradual increase and decrease (5 cm H₂O/min) or in steps of 5 cm H₂O. The effects on gas exchange, pulmonary mechanics and pulmonary and systemic circulation were studied. Total compliance did not change uniformly and cardiac index decreased so much due to PEEP that the increase in P_aO₂ could not prevent the decrease of arterial oxygen transport. No variable was found helpful to predict the best PEEP in a clinical situation.With the support of the Swiss National Fund for the advancement of Scientific Research, Berne. Application No. 3.831-0.79