Improved efficacy of spontaneous breathing with inspiratory pressure support

Mechanical Ventilation: Negative to Positive and Back Again

Critical care and mechanical ventilation have a relatively brief history in medicine. Premises existed through the seventeenth to nineteenth centuries but modern mechanical ventilation started in the twentieth century. Noninvasive ventilation techniques had started both in the intensive care unit and for home ventilation at the end of the 1980s and the 1990s. The need for mechanical ventilation is increasingly influenced worldwide by the spread of respiratory viruses, and the last coronavirus disease 2019 pandemic has seen a massive successful use of noninvasive ventilation.

Comparison between pressure support ventilation and T-piece in spontaneous breathing trials

Background

Recent guidelines recommended conducting spontaneous breathing trial (SBT) with modest inspiratory pressure augmentation rather than T-piece or continuous positive airway pressure. However, it was based on few studies focused on the outcomes of extubation rather than the weaning process, despite the existence of various weaning situations in clinical practice. This study was designed to investigate the effects of SBT with pressure support ventilation (PSV) or T-piece on weaning outcomes.

Methods

All consecutive patients admitted to two medical intensive care units (ICUs) and those requiring mechanical ventilation (MV) for more than 24 h from November 1, 2017 to September 30, 2020 were prospectively registered. T-piece trial was used until March 2019, and then, pressure support of 8 cmH₂O and 0 positive end-expiratory pressure were used for SBT since July 2019, after a 3-month transition period for the revised SBT protocol. The primary outcome of this study was successful weaning defined according to the WIND (Weaning according to a New Definition) definition and were compared between the T-piece group and PSV group. The association between the SBT method and weaning outcome was evaluated with logistic regression analysis.

Results

In this study, 787 eligible patients were divided into the T-piece (n = 473) and PSV (n = 314) groups after excluding patients for a 3-month transition period. Successful weaning was not different between the two groups (85.0% vs. 86.3%; p = 0.607). However, the PSV group had a higher proportion of short weaning (70.1% vs. 59.0%; p = 0.002) and lower proportion of difficult weaning (13.1% vs. 24.1%; p < 0.001) than the T-piece group. The proportion of prolonged weaning was similar between the two groups (16.9% vs. 16.9%; p = 0.990). After excluding patients who underwent tracheostomy before the SBTs, similar results were found. Reintubation rates at 48 h, 72 h, and 7 days following the planned extubation were not different between the PSV and T-piece groups. Moreover, no significant differences in intensive care unit and hospital mortality and length of stay were observed.

Conclusions

In critically ill medical patients, SBT using PSV was not associated with a higher rate of successful weaning compared with SBT using T-piece. However, PSV could shorten the weaning process without increasing the risk of reintubation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12931-022-01942-w.

Model-based estimation of negative inspiratory driving pressure in patients receiving invasive NAVA mechanical ventilation

BACKGROUND

Optimisation of mechanical ventilation (MV) and weaning requires insight into underlying patient breathing effort. Current identifiable models effectively describe lung mechanics, such as elastance (E) and resistance (R) at the bedside in sedated patients, but are less effective when spontaneous breathing is present. This research derives and regularises a single compartment model to identify patient-specific inspiratory effort.

METHODS

Constrained second-order b-spline basis functions (knot width 0.05 s) are used to describe negative inspiratory drive (Pp, cmH₂O) as a function of time. Breath-breath Pp are identified with single E and R values over inspiration and expiration from n = 20 breaths for N = 22 patients on NAVA ventilation. Pp is compared to measured electrical activity of the diaphragm (Eadi) and published results.

RESULTS

Average per-patient root-mean-squared model fit error was (median [interquartile range, IQR]) 0.9 [0.6-1.3] cmH₂O, and average per-patient median Pp was -3.9 [-4.5- -3.0] cmH₂O, with range -7.9 - -1.9 cmH₂O. Per-patient E and R were 16.4 [13.6-21.8] cmH₂O/L and 9.2 [6.4-13.1] cmH₂O.s/L, respectively. Most patients showed an inspiratory volume threshold beyond which Pp started to return to baseline, and Pp at peak Eadi (end-inspiration) was often strongly correlated with peak Eadi (R²=0.25-0.86). Similarly, average transpulmonary pressure was consistent breath-breath in most patients, despite differences in peak Eadi and thus peak airway pressure.

CONCLUSIONS

The model-based inspiratory effort aligns with electrical muscle activity and published studies showing neuro-muscular decoupling as a function of pressure and/or volume. Consistency in coupling/dynamics were patient-specific. Quantification of patient and ventilator work of breathing contributions may aid optimisation of MV modes and weaning.

Continuous assessment of neuro-ventilatory drive during 12 h of pressure support ventilation in critically ill patients

Introduction

Pressure support ventilation (PSV) should allow spontaneous breathing with a “normal” neuro-ventilatory drive. Low neuro-ventilatory drive puts the patient at risk of diaphragmatic atrophy while high neuro-ventilatory drive may causes dyspnea and patient self-inflicted lung injury. We continuously assessed for 12 h the electrical activity of the diaphragm (EAdi), a close surrogate of neuro-ventilatory drive, during PSV. Our aim was to document the EAdi trend and the occurrence of periods of “Low” and/or “High” neuro-ventilatory drive during clinical application of PSV.

Method

In 16 critically ill patients ventilated in the PSV mode for clinical reasons, inspiratory peak EAdi peak (EAdi_PEAK), pressure time product of the trans-diaphragmatic pressure per breath and per minute (PTP_DI/b and PTP_DI/min, respectively), breathing pattern and major asynchronies were continuously monitored for 12 h (from 8 a.m. to 8 p.m.). We identified breaths with “Normal” (EAdi_PEAK 5–15 μV), “Low” (EAdi_PEAK < 5 μV) and “High” (EAdi_PEAK > 15 μV) neuro-ventilatory drive.

Results

Within all the analyzed breaths (177.117), the neuro-ventilatory drive, as expressed by the EAdi_PEAK, was “Low” in 50.116 breath (28%), “Normal” in 88.419 breaths (50%) and “High” in 38.582 breaths (22%). The average times spent in “Low”, “Normal” and “High” class were 1.37, 3.67 and 0.55 h, respectively (p < 0.0001), with wide variations among patients. Eleven patients remained in the “Low” neuro-ventilatory drive class for more than 1 h, median 6.1 [3.9–8.5] h and 6 in the “High” neuro-ventilatory drive class, median 3.4 [2.2–7.8] h. The asynchrony index was significantly higher in the “Low” neuro-ventilatory class, mainly because of a higher number of missed efforts.

Conclusions

We observed wide variations in EAdi amplitude and unevenly distributed “Low” and “High” neuro ventilatory drive periods during 12 h of PSV in critically ill patients. Further studies are needed to assess the possible clinical implications of our physiological findings.

Oesophageal pressure and respiratory muscle ultrasonographic measurements indicate inspiratory effort during pressure support ventilation

BACKGROUND

Bedside measures of patient effort are essential to properly titrate the level of pressure support ventilation. We investigated whether the tidal swing in oesophageal (ΔPes) and transdiaphragmatic pressure (ΔPdi), and ultrasonographic changes in diaphragm (TFdi) and parasternal intercostal (TFic) thickening are reliable estimates of respiratory effort. The effect of diaphragm dysfunction was also considered.

METHODS

Twenty-one critically ill patients were enrolled: age 73 (14) yr, BMI 27 (7) kg m^-2, and Pao₂/Fio₂ 33.3 (9.2) kPa. A three-level pressure support trial was performed: baseline, 25% (PS-medium), and 50% reduction (PS-low). We recorded the oesophageal and transdiaphragmatic pressure-time products (PTPs), work of breathing (WOB), and diaphragm and intercostal ultrasonography. Diaphragm dysfunction was defined by the Gilbert index.

RESULTS

Pressure support was 9.0 (1.6) cm H₂O at baseline, 6.7 (1.3) (PS-medium), and 4.4 (1.0) (PS-low). ΔPes was significantly associated with the oesophageal PTP (R²=0.868; P<0.001) and the WOB (R²=0.683; P<0.001). ΔPdi was significantly associated with the transdiaphragmatic PTP (R²=0.820; P<0.001). TFdi was only weakly correlated with the oesophageal PTP (R²=0.326; P<0.001), and the correlation improved after excluding patients with diaphragm dysfunction (R²=0.887; P<0.001). TFdi was higher and TFic lower in patients without diaphragm dysfunction: 33.6 (18.2)% vs 13.2 (9.2)% and 2.1 (1.7)% vs 12.7 (9.1)%; P<0.0001.

CONCLUSIONS

ΔPes and ΔPdi are adequate estimates of inspiratory effort. Diaphragm ultrasonography is a reliable indicator of inspiratory effort in the absence of diaphragm dysfunction. Additional measurement of parasternal intercostal thickening may discriminate a low inspiratory effort or a high effort in the presence of a dysfunctional diaphragm.

Impact of prolonged assisted ventilation on diaphragmatic efficiency: NAVA versus PSV

Background

Prolonged controlled mechanical ventilation depresses diaphragmatic efficiency. Assisted modes of ventilation should improve it. We assessed the impact of pressure support ventilation versus neurally adjusted ventilator assist on diaphragmatic efficiency.

Method

Patients previously ventilated with controlled mechanical ventilation for 72 hours or more were randomized to be ventilated for 48 hours with pressure support ventilation (n =12) or neurally adjusted ventilatory assist (n = 13). Neuro-ventilatory efficiency (tidal volume/diaphragmatic electrical activity) and neuro-mechanical efficiency (pressure generated against the occluded airways/diaphragmatic electrical activity) were measured during three spontaneous breathing trials (0, 24 and 48 hours). Breathing pattern, diaphragmatic electrical activity and pressure time product of the diaphragm were assessed every 4 hours.

Results

In patients randomized to neurally adjusted ventilator assist, neuro-ventilatory efficiency increased from 27 ± 19 ml/μV at baseline to 62 ± 30 ml/μV at 48 hours (p <0.0001) and neuro-mechanical efficiency increased from 1 ± 0.6 to 2.6 ± 1.1 cmH₂O/μV (p = 0.033). In patients randomized to pressure support ventilation, these did not change. Electrical activity of the diaphragm, neural inspiratory time, pressure time product of the diaphragm and variability of the breathing pattern were significantly higher in patients ventilated with neurally adjusted ventilatory assist. The asynchrony index was 9.48 [6.38– 21.73] in patients ventilated with pressure support ventilation and 5.39 [3.78– 8.36] in patients ventilated with neurally adjusted ventilatory assist (p = 0.04).

Conclusion

After prolonged controlled mechanical ventilation, neurally adjusted ventilator assist improves diaphragm efficiency whereas pressure support ventilation does not.

Trial registration

ClinicalTrials.gov study registration: NCT0247317, 06/11/2015.

Measurement of esophageal pressure at bedside: pros and cons

PURPOSE OF REVIEW

Esophageal pressure measurement well estimates pleural pressure. The interpretation of absolute values is often debated for various reasons, but the changes in pressure measured are considered very accurate provided that a number of precautions are taken. The information provided by these measurements is unique in nature and has an enormous potential to influence management. It allows to study the exact influence of the chest wall and to determine the real lung distending pressure. It is also the only way to quantify respiratory muscle activity and the work of breathing.

RECENT FINDINGS

The application of esophageal pressure monitoring potentially covers a large field, especially for what concerns mechanical ventilation. This goes from the acute phase of the acute respiratory distress syndrome (ARDS) to weaning and patient-ventilator interactions. During ARDS, recent findings indicate that this measurement may help titrating the level of positive end-expiratory pressure or to determine the well tolerated upper limit of airway pressure.

SUMMARY

Application of esophageal pressure monitoring is limited by technical issues, the need for background physiological knowledge and the fact that very few studies have assessed a direct influence of this measurement on patients' outcome. The technique is underused in everyday practice.

The application of esophageal pressure measurement in patients with respiratory failure

This report summarizes current physiological and technical knowledge on esophageal pressure (Pes) measurements in patients receiving mechanical ventilation. The respiratory changes in Pes are representative of changes in pleural pressure. The difference between airway pressure (Paw) and Pes is a valid estimate of transpulmonary pressure. Pes helps determine what fraction of Paw is applied to overcome lung and chest wall elastance. Pes is usually measured via a catheter with an air-filled thin-walled latex balloon inserted nasally or orally. To validate Pes measurement, a dynamic occlusion test measures the ratio of change in Pes to change in Paw during inspiratory efforts against a closed airway. A ratio close to unity indicates that the system provides a valid measurement. Provided transpulmonary pressure is the lung-distending pressure, and that chest wall elastance may vary among individuals, a physiologically based ventilator strategy should take the transpulmonary pressure into account. For monitoring purposes, clinicians rely mostly on Paw and flow waveforms. However, these measurements may mask profound patient-ventilator asynchrony and do not allow respiratory muscle effort assessment. Pes also permits the measurement of transmural vascular pressures during both passive and active breathing. Pes measurements have enhanced our understanding of the pathophysiology of acute lung injury, patient-ventilator interaction, and weaning failure. The use of Pes for positive end-expiratory pressure titration may help improve oxygenation and compliance. Pes measurements make it feasible to individualize the level of muscle effort during mechanical ventilation and weaning. The time is now right to apply the knowledge obtained with Pes to improve the management of critically ill and ventilator-dependent patients.

Effect of pressure support ventilation during weaning on ventilation and oxygenation indices in healthy horses recovering from general anesthesia

OBJECTIVE

To determine if pressure support ventilation (PSV) weaning from general anesthesia affects ventilation or oxygenation in horses.

STUDY DESIGN

Prospective randomized clinical study.

ANIMALS

Twenty client-owned healthy horses aged 5 ± 2 years, weighing 456 ± 90 kg.

METHODS

In the control group (CG; n = 10) weaning was performed by a gradual decrease in respiratory rate (fR ) and in the PSV group (PSVG; n = 10) by a gradual decrease in fR with PSV. The effect of weaning was considered suboptimal if PaCO2 > 50 mmHg, arterial pH < 7.35 plus PaCO2 > 50 mmHg or PaO2 < 60 mmHg were observed at any time after disconnection from the ventilator until 30 minutes after the horse stood. Threshold values for each index were established and the predictive power of these values was tested.

RESULTS

Pressure support ventilation group (PSVG) had (mean ± SD) pH 7.36 ± 0.02 and PaCO2 41 ± 3 mmHg at weaning and the average lowest PaO2 69 ± 6 mmHg was observed 15 minutes post weaning. The CG had pH 7.32 ± 0.02 and PaCO2 57 ± 6 mmHg at weaning and the average lowest PaO2 48 ± 5 mmHg at 15 minutes post weaning. No accuracy in predicting weaning effect was observed for fR (p = 0.3474), minute volume (p = 0.1153), SaO2 (p = 0.1737) and PaO2 /PAO2 (p = 0.1529). A high accuracy in predicting an optimal effect of weaning was observed for VT > 10 L (p = 0.0001), fR /VT ratio ≤ 0.60 breaths minute(-1) L(-1) (p = 0.0001), VT /bodyweight > 18.5 mL kg(-1) (p = 0.0001) and PaO2 /FiO2 > 298 (p = 0.0002) at weaning. A high accuracy in predicting a suboptimal effect of weaning was observed for VT < 10 L (p = 0.0001), fR /VT ratio ≥ 0.60 breaths minute(-1) L(-1) (p = 0.0001) and Pe'CO2 ≥ 38 mmHg (p = 0.0001) at weaning.

CONCLUSIONS AND CLINICAL RELEVANCE

Pressure support ventilation (PSV) weaning had a better respiratory outcome. A higher VT , VT /body weight, PaO2 /FiO2 ratio and a lower fR /VT ratio and Pe'CO2 were accurate in predicting the effect of weaning in healthy horses recovering from general anesthesia.

Sleep quality in mechanically ventilated patients: comparison between NAVA and PSV modes

BACKGROUND

Mechanical ventilation seems to occupy a major source in alteration in the quality and quantity of sleep among patients in intensive care. Quality of sleep is negatively affected with frequent patient-ventilator asynchronies and more specifically with modes of ventilation. The quality of sleep among ventilated patients seems to be related in part to the alteration between the capacities of the ventilator to meet patient demand. The objective of this study was to compare the impact of two modes of ventilation and patient-ventilator interaction on sleep architecture.

METHODS

Prospective, comparative crossover study in 14 conscious, nonsedated, mechanically ventilated adults, during weaning in a university hospital medical intensive care unit. Patients were successively ventilated in a random ordered cross-over sequence with neurally adjusted ventilatory assist (NAVA) and pressure support ventilation (PSV). Sleep polysomnography was performed during four 4-hour periods, two with each mode in random order.

RESULTS

The tracings of the flow, airway pressure, and electrical activity of the diaphragm were used to diagnose central apneas and ineffective efforts. The main abnormalities were a low percentage of rapid eye movement (REM) sleep, for a median (25th-75th percentiles) of 11.5% (range, 8-20%) of total sleep, and a highly fragmented sleep with 25 arousals and awakenings per hour of sleep. Proportions of REM sleep duration were different in the two ventilatory modes (4.5% (range, 3-11%) in PSV and 16.5% (range, 13-29%) during NAVA (p = 0.001)), as well as the fragmentation index, with 40 ± 20 arousals and awakenings per hour in PSV and 16 ± 9 during NAVA (p = 0.001). There were large differences in ineffective efforts (24 ± 23 per hour of sleep in PSV, and 0 during NAVA) and episodes of central apnea (10.5 ± 11 in PSV vs. 0 during NAVA). Minute ventilation was similar in both modes.

CONCLUSIONS

NAVA improves the quality of sleep over PSV in terms of REM sleep, fragmentation index, and ineffective efforts in a nonsedated adult population.

Weaning by gradual pressure support (PS) reduction without an initial spontaneous breathing trial (SBT) versus PS-supported SBT: a pilot study

BACKGROUND AND AIM

Studies on weaning strategies have yielded conflicting results regarding the superiority of different methods. The aim of this RCT was to compare the efficacy of gradual pressure support (PS) reduction without an initial spontaneous breathing trial (SBT) with PS-supported SBT.

METHODS

Patients mechanically ventilated for >24h were randomized to weaning by gradual reduction of PS without an initial SBT versus once daily SBT (PS 7cm H(2)O). The primary outcomes were the rates of successful weaning trial and time to successful extubation. The secondary outcomes were the ICU and hospital length of stay, hospital mortality and the occurrence of ventilator-associated pneumonia (VAP).

RESULTS

Of the 120 patients (61 males, median age 35 years), 58 were assigned to PS and 62 to the SBT group. The median (IQR) duration of ventilation prior to weaning was 80.2 (50.5-175.6)h. The baseline characteristics were similar in the two groups except the PaO(2)/FiO(2) ratio, which was significantly higher in SBT group. The rates of successful weaning trial (89.7% versus 69.4%) were significantly higher in the PS group. The median duration of weaning (66h versus 81.5h, P=0.05) and the median duration of ICU stay (8 days versus 9.4 days, P=0.027) were lower in the PS group. There was no difference in hospital stay, mortality rates or occurrence of VAP in the two arms. On multivariate analysis, the duration of ventilation prior to weaning, baseline SOFA score and the weaning method were predictors of successful extubation.

CONCLUSIONS

Gradual reduction of PS without an initial SBT was found to be associated with better outcomes compared to once daily PS-supported SBT.

Estimation of inspiratory muscle pressure in critically ill patients

BACKGROUND

Recently, a new technology has been introduced aiming to monitor and improve patient ventilator interaction (PVI monitor). With the PVI monitor, a signal representing an estimation of the patient's total inspiratory muscle pressure (Pmus(PVI)) is calculated from the equation of motion, utilizing estimated values of resistance and elastance of the respiratory system.

OBJECTIVE

The aim of the study was to prospectively examine the accuracy of Pmus(PVI) to quantify inspiratory muscle pressure.

METHODS AND INTERVENTIONS

Eleven critically ill patients mechanically ventilated on proportional assist ventilation with load-adjustable gain factors were studied at three levels of assist (30, 50 and 70%). Airway, esophageal, gastric and transdiaphragmatic (Pdi) pressures, volume and flow were measured breath by breath, whereas the total inspiratory muscle pressure (Pmus) was calculated using the Campbell diagram.

RESULTS

For a given assist, Pmus(PVI) throughout inspiration did not differ from the corresponding values calculated using the Pdi and Pmus signals. Inspiratory and expiratory time did not differ among the various methods of calculation. Inspiratory muscle pressure decreased with increasing assist, and the magnitude of this decrease did not differ among the various methods of pressure calculation.

CONCLUSIONS

A signal generated from flow, volume and airway pressure may be used to provide breath-by-breath quantitative information of inspiratory muscle pressure.

The impact of spontaneous ventilation on distribution of lung aeration in patients with acute respiratory distress syndrome: airway pressure release ventilation versus pressure support ventilation

BACKGROUND

In this study, we sought to determine which mode, airway pressure release ventilation (APRV) or pressure support ventilation (PSV), decreases atelectasis more in patients with acute lung injury/acute respiratory distress syndrome (ARDS).

METHODS

This was a retrospective study in the intensive care unit. Between 2006 and 2007, we identified 18 patients with acute lung injury/ARDS who received either APRV or PSV and had a helical computed tomography scan twice in 3 days.

RESULTS

Computed tomography data from the APRV and PSV groups (n = 9 each) were analyzed for 3-dimensional reconstruction and volumetry. Aerated lung regions (normally aerated, poorly aerated, nonaerated, and hyperinflated) were identified by their densities in Hounsfield units. The Pao(2)/Fio(2) ratio and alveolar-arteriolar oxygen gradient after ventilation were improved in both groups (P = 0.008); however, the improvements in the APRV group exceeded those in the PSV group when delivered with equal mean airway pressure (P = 0.018 and 0.015, respectively). Atelectasis decreased significantly from 41% (range, 17%-68%) to 19% (range, 6%-40%) (P = 0.008) and normally aerated volume increased significantly from 29% (range, 13%-41%) to 43% (range, 25%-56%) (P = 0.008) in the APRV group, whereas lung volume did not change in the PSV group.

CONCLUSIONS

Spontaneous ventilation during APRV improves lung aeration by decreasing atelectasis. PSV for gas exchange is effective but not sufficient to improve lung aeration. These results indicate that APRV is more efficient than PSV as a mode of primary ventilatory support to decrease atelectasis in patients with ARDS.

Pressure support ventilation attenuates ventilator-induced protein modifications in the diaphragm

Introduction

Controlled mechanical ventilation (CMV) induces profound modifications of diaphragm protein metabolism, including muscle atrophy and severe ventilator-induced diaphragmatic dysfunction. Diaphragmatic modifications could be decreased by spontaneous breathing. We hypothesized that mechanical ventilation in pressure support ventilation (PSV), which preserves diaphragm muscle activity, would limit diaphragmatic protein catabolism.

Methods

Forty-two adult Sprague-Dawley rats were included in this prospective randomized animal study. After intraperitoneal anesthesia, animals were randomly assigned to the control group or to receive 6 or 18 hours of CMV or PSV. After sacrifice and incubation with ¹⁴C-phenylalanine, in vitro proteolysis and protein synthesis were measured on the costal region of the diaphragm. We also measured myofibrillar protein carbonyl levels and the activity of 20S proteasome and tripeptidylpeptidase II.

Results

Compared with control animals, diaphragmatic protein catabolism was significantly increased after 18 hours of CMV (33%, P = 0.0001) but not after 6 hours. CMV also decreased protein synthesis by 50% (P = 0.0012) after 6 hours and by 65% (P < 0.0001) after 18 hours of mechanical ventilation. Both 20S proteasome activity levels were increased by CMV. Compared with CMV, 6 and 18 hours of PSV showed no significant increase in proteolysis. PSV did not significantly increase protein synthesis versus controls. Both CMV and PSV increased protein carbonyl levels after 18 hours of mechanical ventilation from +63% (P < 0.001) and +82% (P < 0.0005), respectively.

Conclusions

PSV is efficient at reducing mechanical ventilation-induced proteolysis and inhibition of protein synthesis without modifications in the level of oxidative injury compared with continuous mechanical ventilation. PSV could be an interesting alternative to limit ventilator-induced diaphragmatic dysfunction.

Meta-analysis under the spotlight: focused on a meta-analysis of ventilator weaning

OBJECTIVE

Because the results of a meta-analysis are used to formulate the highest level recommendation in clinical practice guidelines, clinicians should be mindful of problems inherent in this technique. Rather than reviewing meta-analysis in abstract, general terms, we believe readers can gain a more concrete understanding of the problems through a detailed examination of one meta-analysis. The meta-analysis on which we focus is that conducted by an American College of Chest Physicians/American Association for Respiratory Care/American College of Critical Care Medicine Task Force on ventilator weaning.

DATA SOURCE

Two authors extracted data from all studies included in the Task Force's meta-analysis. DATA SYNTHESIS AND OVERVIEW: The major obstacle to reliable internal validity and, thus, reliable external validity (generalizability) in biological research is systematic error, not random error. If systematic errors are present, averaging (as with a meta-analysis) does not decrease them--instead, it reinforces them, producing artifact. The Task Force's meta-analysis commits several examples of the three main types of systematic error: selection bias (test-referral bias, spectrum bias), misclassification bias (categorizing reintubation as weaning failure, etc.), and confounding (pressure support treated as unassisted breathing). Several additional interpretative errors are present.

CONCLUSIONS

An increase in study size, as achieved through the pooling of data in a meta-analysis, is mistakenly thought to increase external validity. On the contrary, combining heterogeneous studies poses considerable risk of systematic error, which impairs internal validity and, thus, external validity. The strength of recommendations in clinical practice guidelines is based on a misperception of the relative importance of systematic vs. random error in science.

Improved synchrony and respiratory unloading by neurally adjusted ventilatory assist (NAVA) in lung-injured rabbits

With increasing pressure support ventilation (PSV), a form of pneumatically triggered ventilation, there can be an increase in wasted inspiratory efforts (neural inspiratory efforts that fail to trigger the ventilator). With neurally adjusted ventilatory assist (NAVA), a mode of ventilation controlled by the electrical activity of the diaphragm (EAdi), synchrony should be maintained at high levels of assist. The aim of this study was to evaluate the response to increasing levels of PSV and NAVA on synchrony and diaphragm unloading in lung-injured rabbits. Animals were ventilated on PSV or NAVA in random order, each at three levels. We measured neural and ventilator respiratory rates, EAdi, transdiaphragmatic pressure (Pdi), and tidal volume (VT). At low PSV, 95% of neural efforts were triggered, compared with high PSV, where only 66% of the neural efforts were triggered. During NAVA, all neural efforts were triggered, regardless of level. Increasing NAVA levels reduced EAdi and Pdi-time products by 48% (p < 0.05) and 66% (p < 0.05). In contrast, increasing PSV did not reduce the diaphragm electrical activity-time product and increased the transdiaphragmatic pressure-time product (p < 0.05) due to the increased wasted efforts. We conclude that synchrony with the ventilator is an important determinant for diaphragm unloading.

Remembrance of weaning past: the seminal papers

The approach to ventilator weaning has changed considerably over the past 30 years. Change has resulted from research in three areas: pathophysiology, weaning-predictor testing, and weaning techniques. Physiology research illuminated the mechanisms of weaning failure. It also uncovered markers of weaning success. Through more reliable prediction, patients whose weaning would have been tedious in the 1970s are now weaned more rapidly. The weaning story offers several lessons in metascience: importance of creativity, the asking of heretical questions, serendipity, mental-set psychology, cross-fertilization, and the hazards of precocity. Weaning research also illustrates how Kuhnian normal (me-too) science dominates any field. Making the next quantum leap in weaning will depend on spending less time on normal science and more on the raising (and testing) of maverick ideas.

Respiratory muscle testing: a valuable tool for children with neuromuscular disorders

RATIONALE

Data on respiratory muscle performance in children with neuromuscular disorders are limited.

OBJECTIVES

The aim of this study was to assess respiratory muscle strength by volitional and nonvolitional tests and to compare these tests with forced vital capacity.

METHODS

Inspiratory muscle strength was assessed by measuring transdiaphragmatic and esophageal pressures generated during volitional and nonvolitional maneuvers, whereas expiratory muscle strength was assessed by measuring the gastric pressure generated during a cough maneuver. Lung volumes were assessed by measuring forced vital capacity.

MEASUREMENTS AND MAIN RESULTS

Forty-one patients with Duchenne muscular dystrophy (n = 20), spinal amyotrophy (n = 8), and congenital myopathy (n = 13) were included, aged 2 to 18 yr. All the patients were able to perform the sniff and the cough maneuver. Sniff transdiaphragmatic pressure decreased with age in Duchenne patients, whereas it increased with age in patients with spinal amyotrophy and congenital myopathy. Magnetic stimulation of the phrenic nerves was obtained in all patients. Twenty-five (61%) patients were able to perform forced vital capacity. In the three groups of patients, a positive correlation was observed between volitional, assessed by the sniff maneuver, and nonvolitional respiratory muscle tests, assessed by the magnetic stimulation of the phrenic nerves. Also, forced vital capacity correlated with sniff transdiaphragmatic pressure and cough gastric pressure.

CONCLUSIONS

Volitional respiratory muscle tests correlated with nonvolitional tests and with forced vital capacity. Simple volitional respiratory muscle tests constitute a valuable tool for the assessment of respiratory muscle strength in young patients with neuromuscular disorders.

Physiologic comparison between conventional mechanical ventilation and transtracheal open ventilation in acute traumatic quadriplegic patients*

OBJECTIVE

To evaluate the efficacy of mechanical ventilation administered through a small-bore, uncuffed tracheotomy tube, so-called transtracheal open ventilation (TOV), in comparison with conventional mechanical ventilation via a cuffed tracheal tube (endotracheal invasive ventilation, EIV).

DESIGN

Physiologic study.

SETTING

Intensive care unit of a referral trauma center.

PATIENTS

Ten acute quadriplegic patients.

INTERVENTIONS

In acute quadriplegic patients receiving EIV, TOV was subsequently applied via an uncuffed, small-bore tube (internal diameter of 4 or 5 mm).

MEASUREMENTS AND MAIN RESULTS

Compared with EIV, arterial blood gases were not significantly different after 1 hr of TOV (Pao2/Fio2, 222.8 +/- 60.9 vs. 218.5 +/- 60.3; Paco2, 37.8 +/- 7.1 torr [5.04 +/- 0.95 kPa] vs. 35.5 +/- 6.8 torr [4.73 +/- 0.91 kPa], for EIV and TOV, respectively). Respiratory rate (19.5 +/- 4.7 vs. 19.6 +/- 5 breaths/min) and inspiratory effort (pressure-time product of esophageal pressure during a 1-min period, 125.9 +/- 48.4 vs. 112.8 +/- 36.4 cm H2O.sec.min) were also no different between the two modes. After 24 hrs of TOV, compared with EIV and TOV after 1 hr, respiratory rate and arterial blood gases remained stable, and the pressure-time product of esophageal pressure during a 1-min period was slightly, but significantly, reduced (83.5 +/- 16.6 cm H2O.sec.min, p < .05).

CONCLUSIONS

In acute quadriplegic patients receiving mechanical ventilation, TOV was as effective as EIV in providing ventilatory support.

L’assistance ventilatoire non invasive à domicile chez l’enfant

INTRODUCTION

Non-invasive positive pressure ventilation (NPPV) represents a particularly interesting technique of ventilatory support in paediatrics. Indeed, a significant number of pathologies that may be responsible for chronic respiratory insufficiency in childhood, such as neuromuscular diseases, obstruction of the upper airways, disorders of chest wall and/or the lungs, and disorders of ventilatory control may all lead to alveolar hypoventilation that can be improved by ventilatory support.

BACKGROUND

Few physiological studies have been performed on NPPV in children. The most appropriate modes and settings for each pathology have not been clearly defined, and the criteria for commencing NPPV are based essentially on consensus guidelines for the management of neuromuscular disorders.

VIEWPOINT

All the health care professionals managing these children should combine their efforts to evaluate more precisely the medium and long-term physiological effects of NPPV on the respiratory muscles, the development of the respiratory system, inspiratory activity, the indications for starting treatment and, above all, the benefits in terms of psycho-neurological development and quality of life.

CONCLUSIONS

A better evaluation of the medium and long-term physiological and psychological benefits together with technical improvements in ventilators and associated equipment should allow a rapid expansion in the use of domiciliary NPPV in children.

Sedation with sufentanil in patients receiving pressure support ventilation has no effects on respiration: a pilot study

PURPOSE

To evaluate the effects of sedation with sufentanil on respiratory drive, respiratory pattern, and gas exchange of critically ill patients during pressure support ventilation.

METHODS

In this prospective observational cohort study, we observed 12 adult patients receiving partial ventilatory support for acute respiratory failure. Each subject received a continuous infusion of sufentanil at 0.2 to 0.3 micro g x kg(-1) x hr(-1) to obtain a modified Ramsay sedation score between 2 and 3. In basal conditions and at variable distance from the beginning of the sufentanil infusion (10', 30', 60', 120', 24 hr) we evaluated gas exchange, hemodynamic variables, respiratory rate (RR), tidal volume (TV), respiratory pattern, respiratory drive (P0.1) and inspiratory impedance of the respiratory system [P0.1/TV/inspiratory time (Ti)].

RESULTS

The continuous iv administration of 0.2 to 0.3 micro g x kg(-1) x hr(-1) of sufentanil resulted in the desired level of sedation. No significant heart rate, heart rhythm and blood pressure changes were observed. Sufentanil infusion did not affect TV, minute volume, Ti/inspiratory duty cycle, RR, P0.1, P0.1/TV/Ti and gas exchange did not change significantly over the study period.

CONCLUSION

A continuous infusion of sufentanil induces "awake" sedation with no detectable effects on respiratory variables in critically ill patients during partial ventilatory support.

Airway pressure release ventilation as a primary ventilatory mode in acute respiratory distress syndrome

BACKGROUND

Airway pressure release ventilation (APRV) is a ventilatory mode, which allows unsupported spontaneous breathing at any phase of the ventilatory cycle. Airway pressure release ventilation as compared with pressure support (PS), another partial ventilatory mode, has been shown to improve gas exchange and cardiac output. We hypothesized whether the use of APRV with maintained unsupported spontaneous breathing as an initial mode of ventilatory support promotes faster recovery from respiratory failure in patients with acute respiratory distress syndrome (ARDS) than PS combined with synchronized intermittent ventilation (SIMV-group).

METHODS

In a randomized trial 58 patients were randomized to receive either APRV or SIMV after a predefined stabilization period. Both groups shared common physiological targets, and uniform principles of general care were followed.

RESULTS

Inspiratory pressure was significantly lower in the APRV-group (25.9 +/- 0.6 vs. 28.6 +/- 0.7 cmH2O) within the first week of the study (P = 0.007). PEEP-levels and physiological variables (PaO2/FiO2-ratio, PaCO2, pH, minute ventilation, mean arterial pressure, cardiac output) were comparable between the groups. At day 28, the number of ventilator-free days was similar (13.4 +/- 1.7 in the APRV-group and 12.2 +/- 1.5 in the SIMV-group), as was the mortality (17% and 18%, respectively).

CONCLUSION

We conclude that when used as a primary ventilatory mode in patients with ARDS, APRV did not differ from SIMV with PS in clinically relevant outcome.

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 cmH(2)O. 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.

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 cmH(2)O), 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 cmH(2)O 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 cmH(2)O 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.

Positive pressure mechanical ventilation

There have been numerous advances in the application of positive pressure mechanical ventilation in the last two decades. As knowledge of pulmonary physiology expands, the application of modes and parameters to maximize the efficacy and minimize the complications of ventilatory support continues to advance. As the use of noninvasive ventilation becomes more widespread, its usefulness in certain clinical entities such as COPD exacerbations and acute cardiogenic pulmonary edema will become more prominent. The role of specific modes and parameters of these devices likely will be further refined to maximize outcomes.

Oscillations and noise: inherent instability of pressure support ventilation?

Pressure support ventilation (PSV) is almost universally employed in the management of actively breathing ventilated patients with acute respiratory failure. In this partial support mode of ventilation, a fixed pressure is applied to the airway opening, and flow delivery is monitored by the ventilator. Inspiration is terminated when measured inspiratory flow falls below a set fraction of the peak flow rate (flow cutoff); the ventilator then cycles to a lower pressure and expiration commences. We used linear and nonlinear mathematical models to investigate the dynamic behavior of pressure support ventilation and confirmed the predicted behavior using a test lung. Our mathematical and laboratory analyses indicate that pressure support ventilation in the setting of airflow obstruction can be accompanied by marked variations in tidal volume and end-expiratory alveolar pressure, even when subject effort is unvarying. Unstable behavior was observed in the simplest plausible linear mathematical model and is an inherent consequence of the underlying dynamics of this mode of ventilation. The mechanism underlying the observed instability is "feed forward" behavior mediated by oscillatory elevation in end-expiratory pressure. In both mathematical and mechanical models, unstable behavior occurred at impedance values and ventilator settings that are clinically realistic.

Noninvasive proportional assist ventilation for acute respiratory insufficiency. Comparison with pressure support ventilation

Noninvasive positive pressure ventilation (NPPV) is usually applied using pressure support ventilation (PSV). Proportional assist ventilation (PAV) is a newer mode that delivers assisted ventilation in proportion to patient effort. We hypothesized that PAV for NPPV would support gas exchange and avoid intubation as well as PSV and be more comfortable and tolerable for patients. Adult patients with acute respiratory insufficiency were randomized to receive NPPV with PAV delivered using the Respironics Vision ventilator or PSV using a Puritan-Bennett 7200ae critical care ventilator. Each mode was adjusted to relieve dyspnea and improve gas exchange until patients met weaning or intubation criteria, died, or refused to continue. Twenty-one and 23 patients were entered into the PAV and PSV groups, respectively, and had similar diagnoses and baseline characteristics, although pH was slightly lower in the PAV group (7.30 versus 7.35, p = 0.02). Mortality and intubation rates were similar, but refusal rate was lower, reduction in respiratory rate was more rapid, and there were fewer complications in the PAV group. We conclude that use of the PAV mode is feasible for noninvasive therapy of acute respiratory insufficiency. Compared with PSV delivered with the Puritan-Bennett 7200ae, PAV is associated with more rapid improvements in some physiologic variables and is better tolerated.

In vivo physiologic comparison of two ventilators used for domiciliary ventilation in children with cystic fibrosis

OBJECTIVE

Home noninvasive mechanical ventilation (NIMV) is used with increasing frequency for the treatment of patients with respiratory failure caused by cystic fibrosis, yet the optimal mode of ventilation in such children is unknown. We compared the physiologic short-term effects of two ventilators with different modes (one pressure support and the other assist control/volume-targeted [AC/VT]) commonly used for domiciliary ventilation.

DESIGN

Prospective, randomized, crossover comparison of two ventilators with different modes.

SETTING

Tertiary pediatric university hospital.

PATIENTS

Eight children with cystic fibrosis (age, 11-17 yrs) and chronic respiratory failure (pH 7.4 +/- 0.0; PaO2, 57.5 +/- 7.5 torr; PaCO2, 46.1 +/- 2.5 torr), naive to NIMV.

INTERVENTIONS

Two 20-min runs of pressure support and AC/VT ventilation were performed in random order, each run being preceded and followed by 20 mins of spontaneous breathing.

MEASUREMENTS

Flow and airway pressure and esophageal and gastric pressures were measured to calculate esophageal (PTPes) and diaphragmatic pressure-time product (PTPdi) and the work of breathing.

RESULTS

The two NIMV sessions significantly improved blood gas variables and increased tidal volume with no change in respiratory rate. Indexes of respiratory effort decreased significantly during the two modes of NIMV compared with spontaneous breathing, with PTPdi/min decreasing from 497.8 +/- 115.4 cm H2O x sec x min(-1) during spontaneous breathing to 127.8 +/- 98.3 cm H2O x sec x min(-1) and 184.3 +/- 79.8 cm H2O x sec x min(-1), during AC/VT and pressure support, respectively (p <.0001), and the work of breathing decreasing from 1.83 +/- 0.12 J.L-1 during spontaneous breathing to 0.48 +/- 0.32 J.L-1 and 0.75 +/- 0.30 J.L-1, during AC/VT and pressure support, respectively (p <.0001). In addition, the effect of AC/VT ventilation was significantly superior to pressure support judged by PTPes and the work of breathing, but this result was explained by three patients who adapted extremely well to the AC/VT ventilation, with the disappearance of ventilator triggering, in effect adopting a controlled mode. There was a correlation between the improvement in PTPdi/min or the work of breathing and patient's subjective impression of comfort during the AC/VT ventilation.

CONCLUSIONS

In awake, stable children with cystic fibrosis, both AC/VT and pressure support unloaded the respiratory muscles. The disappearance of ventilator triggering occurred in a subgroup of patients during AC/VT ventilation, and this explained the good tolerance and the superiority of this mode in the present study.

Electrical activity of the diaphragm during pressure support ventilation in acute respiratory failure

We compared crural diaphragm electrical activity (EAdi) with transdiaphragmatic pressure (Pdi) during varying levels of pressure support ventilation (PS) in 13 intubated patients. With changing PS, we found no evidence for changes in neuromechanical coupling of the diaphragm. From lowest to highest PS (2 cm H(2)O +/- 4 to 20 cm H(2)O +/- 7), tidal volume increased from 430 ml +/- 180 to 527 ml +/- 180 (p < 0.001). The inspiratory volume calculated during the period when EAdi increased to its peak did not change from 276 +/- 147 to 277 +/- 162 ml, p = 0.976. Respiratory rate decreased from 23.9 (+/- 7) to 21.3 (+/- 7) breaths/min (p = 0.015). EAdi and Pdi decreased proportionally by adding PS (r = 0.84 and r = 0.90, for mean and peak values, respectively). Mean and peak EAdi decreased (p < 0.001) by 33 +/- 21% (mean +/- SD) and 37 +/- 23% with the addition of 10 cm H(2)O of PS, similar to the decrease in the mean and peak Pdi (p < 0.001) observed (34 +/- 36 and 35 +/- 23%). We also found that ventilator assist continued during the diaphragm deactivation period, a phenomenon that was further exaggerated at higher PS levels. We conclude that EAdi is a valid measurement of neural drive to the diaphragm in acute respiratory failure.

Ventilator-patient dyssynchrony induced by change in ventilation mode

Patient-ventilator interactions may be coordinated (synchronous) or uncoordinated (dyssynchronous). Ventilator-patient dyssynchrony increases the work of breathing by imposing a respiratory muscle workload. Respiratory centre output responds to feedback from respiratory muscle loading. Mismatching of respiratory centre output and mechanical assistance results in dyssynchrony. We describe a case of severe patient-ventilator dyssynchrony and hypothesize that dyssynchrony was induced by a change in mode of ventilation from pressure-cycled to volume-cycled ventilation, due to both ventilator settings and by the patient's own respiratory centre adaptation to mechanical ventilation. The causes, management and clinical implications of dyssynchrony are discussed.

Noninvasive ventilation in the ICU / La ventilation non invasive aux soins intensifs

CONCLUSION

La ventilation non invasive est probablement un des progrès majeurs des dix dernières années dans le traitement de l'insuffisance respiratoire aiguë aux soins intensifs. Son efficacité a été démontrée dans plusieurs indications. Dans l'insuffisance respiratoire aiguë hypercapnique, la VNI devrait constituer le traitement de premier choix. En évitant l'intubation endotrachéale, la VNI permet de réduire de façon importante la morbidité infectieuse et non infectieuse associée à la ventilation mécanique, de même que la mortalité dans certains groupes de patients. Comme il demeure toujours possible d'intuber le patient en cas d'échec, la VNI peut être tentée dans plusieurs situations cliniques. Finalement, les succès de la VNI dépendent de l'intérêt et de l'expérience de l'équipe soignante.

Prediction of post-extubation work of breathing

OBJECTIVE

To evaluate which mode of preextubation ventilatory support most closely approximates the work of breathing performed by spontaneously breathing patients after extubation.

DESIGN

Prospective observational design.

SETTING

Medical, surgical, and coronary intensive care units in a university hospital.

PATIENTS

A total of 22 intubated subjects were recruited when weaned and ready for extubation.

INTERVENTIONS

Subjects were ventilated with continuous positive airway pressure at 5 cm H2O, spontaneous ventilation through an endotracheal tube (T piece), and pressure support ventilation at 5 cm H2O in randomized order for 15 mins each. At the end of each interval, we measured pulmonary mechanics including work of breathing reported as work per liter of ventilation, respiratory rate, tidal volume, negative change in esophageal pressure, pressure time product, and the airway occlusion pressure 100 msec after the onset of inspiratory flow, by using a microprocessor-based monitor. Subsequently, subjects were extubated, and measurements of pulmonary mechanics were repeated 15 and 60 mins after extubation.

MEASUREMENTS AND MAIN RESULTS

There were no statistical differences between work per liter of ventilation measured during continuous positive airway pressure, T piece, or pressure support ventilation (1.17+/-0.67 joule/L, 1.11+/-0.57 joule/L, and 0.97+/-0.57 joule/L, respectively). However, work per liter of ventilation during all three preextubation modes was significantly lower than work measured 15 and 60 mins after extubation (p < .05). Tidal volume during pressure support ventilation and continuous positive airway pressure (0.46+/-0.11 L and 0.44+/-0.11 L, respectively) were significantly greater than tidal volume during both T-piece breathing and spontaneous breathing 15 mins after extubation (p < .05). Negative change in esophageal pressure, the airway occlusion pressure 100 msec after the onset of inspiratory flow, and pressure time product were significantly higher after extubation than during any of the three preextubation modes (p < .05).

CONCLUSIONS

Work per liter of ventilation, negative change in esophageal pressure, the airway occlusion pressure 100 msec after the onset of inspiratory flow, and pressure time product all significantly increase postextubation. Tidal volume during continuous positive airway pressure or pressure support ventilation overestimates postextubation tidal volume.

Clinical evaluation of a computer-controlled pressure support mode

We have designed a computerized system providing closed-loop control of the level of pressure support ventilation (PSV). The system sets itself at the lowest level of PSV that maintains respiratory rate (RR), tidal volume (VT), and end-tidal CO(2) pressure (PET(CO(2))) within predetermined ranges defining acceptable ventilation (i.e., 12 < RR < 28 cycles/min, VT > 300 ml [> 250 if weight < 55 kg], and PET(CO(2)) < 55 mm Hg [< 65 mm Hg if chronic CO(2) retention]). Ten patients received computer-controlled (automatic) PSV and physician-controlled (standard) PSV, in random order, during 24 h for each mode. An estimation of occlusion pressure (P(0.1)) was recorded continuously. The average time spent with acceptable ventilation as previously defined was 66 +/- 24% of the total ventilation time with standard PSV versus 93 +/- 8% with automatic PSV (p < 0.05), whereas the level of PSV was similar during the two periods (17 +/- 4 cm H(2)O versus 19 +/- 6 cm H(2)O). The time spent with an estimated P(0.1) above 4 cm H(2)O was 34 +/- 35% of the standard PSV time versus only 11 +/- 17% of the automatic PSV time (p < 0.01). Automatic PSV increased the time spent within desired ventilation parameter ranges and apparently reduced periods of excessive workload.

Assisted pressure control ventilation via a mini-tracheostomy tube for postoperative respiratory management of lung cancer patients

Assisted pressure control ventilation (PCV) via a min-tracheostomy tube (MTT) was conducted to improve gas exchange and reduce the work of breathing of lung cancer patients after surgery. Thirty-two patients with lung cancer underwent lobectomy and were managed postoperatively by assisted PCV via an MTT. On the basis of a simulation study using a lung model for clinical use, we set the inspiratory pressure to 20 cmH2O and inspiratory time to 1.0 sec to produce a 450-ml supported volume via the MTT per breath. The blood gases and respiratory rate of each patient were measured under three sets of conditions: PCV via an MTT transtracheal oxygenation (TTO) via an MTT and a Venturi face mask with the same FiO2. After PCV via an MTT overnight, the blood gases in the room air were measured 2.5 h after withdrawing PCV. In order to determine the effect of PCV via an MTT on gas exchange after PCV withdrawal, 32 other age and sex-matched lung cancer patients, who had undergone lobectomy and oxygenation via a face mask alone after surgery, were used as historical controls. The simulation study showed that the ventilated volume provided by assisted PCV via an MTT was about half that provided via a conventional endotracheal tube, even in the presence of air leakage. The clinical application showed that the ventilated volume obtained with the PCV via an MTT was significantly higher than that with spontaneous breathing (P<0.001). PCV via an MTT increased the PaO2 and reduced both the PaCO2 and respiratory rate significantly in comparison with TTO via an MTT and a face mask (P<0.001). After PCV withdrawal the morning after surgery, the PaO2 of the PCV group was significantly higher than that of the historical controls (P<0.001). No postoperative pulmonary complications were observed in either the PCV or the control groups, however. In addition, no complications or morbidity were seen related to either MTT insertion or PCV via an MTT. Assisted PVC via an MTT increased the tidal volume, improved the gas exchange, reduced the respiratory rate by providing adequate ventilatory support and increased the PaO2, even after withdrawal following lung surgery. Even though we did not observe any benefit of clinical outcome with PCV via an MTT in the present study, this procedure appears to be a potentially useful respiratory management modality for patients with high risk of postoperative pulmonary complications.