Tracheal occlusion pressure: a simple index to monitor respiratory muscle fatigue during acute respiratory failure in patients with chronic obstructive pulmonary disease

Diaphragm ultrasound as a new method to predict extubation outcome in mechanically ventilated patients

AIM

To evaluate role of diaphragmatic thickening and excursion, assessed ultrasonographically, in predicting extubation outcome.

METHODS

Fifty-four patients who successfully passed spontaneous breathing trial (SBT) were enrolled. They were assessed by ultrasound during SBT evaluating diaphragmatic excursion, diaphragmatic thickness (Tdi) at end inspiration, at end expiration and diaphragmatic thickness fraction (DTF%). Simultaneously traditional weaning parameters were recorded. Patients were followed up for 48h after extubation.

RESULTS

Out of 54 included patients, 14 (25.9%) failed extubation. Diaphragmatic excursion, Tdi at end inspiration, at end expiration and DTF% were significantly higher in the successful group compared to those who failed extubation (p<0.05). Cutoff values of diaphragmatic measures associated with successful extubation were ≥10.5mm for diaphragmatic excursion, ≥21mm for Tdi at end inspiration, ≥10.5mm for Tdi at end expiration, ≥34.2% for DTF% giving 87.5%, 77.5%, 80% and 90% sensitivity respectively and 71.5%, 86.6%, 50% and 64.3% specificity respectively. Combining diaphragmatic excursion ≥10.5mm and Tdi at end inspiration ≥21mm decreased sensitivity to 64.9% but increased specificity to 100%. Rapid shallow breathing index (RSBI) <105 had 90% sensitivity but 18.7% specificity.

CONCLUSION

Ultrasound evaluation of diaphragmatic excursion and thickness at end inspiration could be a good predictor of extubation outcome in patients who passed SBT. It is recommended to consider the use of these parameters with RSBI consequently to improve extubation outcome.

Treatment with a corticotrophin releasing factor 2 receptor agonist modulates skeletal muscle mass and force production in aged and chronically ill animals

Background

Muscle weakness is associated with a variety of chronic disorders such as emphysema (EMP) and congestive heart failure (CHF) as well as aging. Therapies to treat muscle weakness associated with chronic disease or aging are lacking. Corticotrophin releasing factor 2 receptor (CRF2R) agonists have been shown to maintain skeletal muscle mass and force production in a variety of acute conditions that lead to skeletal muscle wasting.

Hypothesis

We hypothesize that treating animals with a CRF2R agonist will maintain skeletal muscle mass and force production in animals with chronic disease and in aged animals.

Methods

We utilized animal models of aging, CHF and EMP to evaluate the potential of CRF2R agonist treatment to maintain skeletal muscle mass and force production in aged animals and animals with CHF and EMP.

Results

In aged rats, we demonstrate that treatment with a CRF2R agonist for up to 3 months results in greater extensor digitorum longus (EDL) force production, EDL mass, soleus mass and soleus force production compared to age matched untreated animals. In the hamster EMP model, we demonstrate that treatment with a CRF2R agonist for up to 5 months results in greater EDL force production in EMP hamsters when compared to vehicle treated EMP hamsters and greater EDL mass and force in normal hamsters when compared to vehicle treated normal hamsters. In the rat CHF model, we demonstrate that treatment with a CRF2R agonist for up to 3 months results in greater EDL and soleus muscle mass and force production in CHF rats and normal rats when compared to the corresponding vehicle treated animals.

Conclusions

These data demonstrate that the underlying physiological conditions associated with chronic diseases such as CHF and emphysema in addition to aging do not reduce the potential of CRF2R agonists to maintain skeletal muscle mass and force production.

Hypercapnia Test and Weaning outcome from Mechanical Ventilation in COPD Patients

We evaluated the hypercapnia response test as a weaning outcome predictor from mechanical ventilation in patients with chronic obstructive pulmonary disease (COPD).

We studied 44 COPD intubated patients considered ready for a weaning trial. The hypercapnia test was based on the modified method of re-inhalation of expired air. Through the hypercapnic test we calculated the ratio of the change in minute volume (VE) to the change in PaCO2 (ΔVE/ΔPaCO2), the change in airway occlusion pressure at 0.1 second of inspiration (P0.1) to change in PaCO2 (ΔP0.1/ΔPaCO2), the ratio of the change in P0.1 to baseline PaCO2 (ΔP0.1/PaCO2) and the ratio of the change in VE to baseline PaCO2 (ΔVE/PaCO2).

Nineteen patients were successfully weaned and 25 patients failed. No differences in baseline clinical characteristics were found between the two groups. Weaning failure COPD patients had lower ΔP0.1/ΔPaCO2 (0.19±0.11 and 0.34±0.20 cm H2O/mmHg respectively, P=0.006) and lower ΔVE/ΔPaCO2 (0.21±0.15 and 0.40±0.22 l/min/mmHg respectively, P=0.002) than successfully weaned patients. The area under the receiver operating characteristic curve to discriminate weaning outcome was for the baseline PaCO2 0.81 (95% confidence interval: 0.66 to 0.91), hypercapnic PaCO2 0.76 (0.61 to 0.88), hypercapnic drive response 0.74 (0.59 to 0.86), hypercapnic ventilatory response 0.76 (0.60 to 0.87), ΔP0.1/PaCO2 0.76 (0.60 to 0.87) and for the ΔVE/PaCO2 0.81 (0.67 to 0.91).

COPD patients with weaning failure have a significantly more blunted response to the hypercapnia response test than weaning success patients. This test could be useful to predict weaning failure patients if the combined values of the hypercapnic drive and hypercapnic ventilatory response were below the threshold values.

Effect of intrapulmonary percussive ventilation on expiratory flow limitation in chronic obstructive pulmonary disease patients

PURPOSE

The aims of this prospective study were (1) to select, after weaning and extubation, chronic obstructive pulmonary disease (COPD) patients with expiratory flow limitation (EFL) measured by the negative expiratory pressure method and (2) to assess, in these patients, the short-term (30 minutes) physiologic effect of a session of intrapulmonary percussive ventilation (IPV).

MATERIALS AND METHODS

All COPD patients who were intubated and needed weaning from mechanical ventilation were screened after extubation. The patients were placed in half-sitting position and breathed spontaneously. The EFL and the airway occlusion pressure after 0.1 second (P0.1) were measured at the first hour after extubation. In COPD patients with EFL, an IPV session of 30 minutes was promptly performed by a physiotherapist accustomed to the technique. Expiratory flow limitation, gas exchange, and P0.1 were recorded at the end of the IPV session.

RESULTS

Among 35 patients studied after extubation, 25 patients presented an EFL and were included in the study. Intrapulmonary percussive ventilation led to a significant improvement in EFL, respectively, before and 30 minutes after IPV (65.4 +/- 18.2 vs 35.6 +/- 22.8; P < .05). Three patients were not expiratory flow limited after IPV. Intrapulmonary percussive ventilation led to a significant decrease in P0.1 (3.9 +/- 1.6 vs 2.8 +/- 1.1; P < .05). Thirty minutes of IPV led to a significant increase in Pao(2) and pH and a decrease in Paco(2) and respiratory rate (P < .05).

CONCLUSION

In COPD patients, a session of IPV allowed a significant reduction of EFL and of P01 and a significant improvement of gas exchange.

Weaning from Mechanical Ventilation

Approximately 20% of all mechanically ventilated patients fail their first attempt to wean. Prolonged mechanical ventilation increases morbidity, mortality, and costs. No single weaning parameter predicts patient ability to wean. Weaning studies suggest that daily trials of spontaneous breathing for appropriate patients assured by standing protocol and driven by respiratory care practitioners and/or nurses improve the weaning process and patient outcome.

Measurement of neural respiratory drive in patients with COPD

Assessment of neural respiratory drive may be useful in patients with chronic obstructive pulmonary disease (COPD) for diverse clinical and academic reasons. We hypothesised that the oesophageal diaphragm EMG during CO2 rebreathing and treadmill exercise could be used for this purpose. The oesophageal catheter consisted of nine consecutive recording electrode coils, which formed five pairs of electrodes with an inter-electrode distance 3.2 cm within a recording pair. Each coil was 1cm in length and the gap between adjacent coils was 0.5 mm. Maximal isometric contractions at functional residual capacity (FRC) and maximal voluntary inspirations from FRC to total lung capacity (TLC) were performed. All subjects performed CO2 rebreathing until end-tidal CO2 was approximately 9% or they became intolerably breathless. There was a good linear relationship between peak of root mean square (RMS) of the diaphragm EMG and end-tidal CO2 (r = 0.92 +/- 0.06) during CO2 rebreathing. The method was also shown to be feasible during exercise. It is concluded that the diaphragm EMG recorded from an oesophageal electrode is a useful technique to assess neural respiratory drive in patients with COPD.

Closed-Loop Control of Respiratory Drive Using Pressure-Support Ventilation

By using diaphragm electrical activity (multiple-array esophageal electrode) as an index of respiratory drive, and allowing such activity above or below a preset target range to indicate an increased or reduced demand for ventilatory assistance (target drive ventilation), we evaluated whether the level of pressure-support ventilation can be automatically adjusted in response to exercise-induced changes in ventilatory demand. Eleven healthy individuals breathed through a circuit (18 cm H2O/L/second inspiratory resistance at 1 L/second flow; 0.5-1.0 L/second expiratory flow limitation) connected to a modified ventilator. Subjects breathed for 6-minute periods at rest and during 20 and 40 W of bicycle exercise, with and without target drive ventilation (the target was set to 60% of the increase in diaphragm electrical activity observed between rest and 20 W of unassisted exercise). With target drive ventilation during exercise, the level of pressure-support ventilation was automatically increased, reaching 13.3 +/- 4.0 and 20.3 +/- 2.8 cm H2O during 20- and 40-W exercise, respectively, whereas diaphragm electrical activity was reduced to a level within the target range. Both diaphragmatic pressure-time product and end-tidal CO2 were significantly reduced with target drive ventilation at the end of the 20- (p < 0.01) and 40-W (p < 0.001) exercise periods. Minute ventilation was not altered. These results demonstrate that target drive ventilation can automatically adjust pressure-support ventilation, maintaining a constant neural drive and compensating for changes in respiratory demand.

Extubation failure: diagnostic value of occlusion pressure (P0.1) and P0.1-derived parameters

OBJECTIVE

To evaluate the ability of the new, built-in occlusion pressure (P0.1) measurement to predict extubation failure.

DESIGN AND SETTING

Prospective observational multicentre study in the ICU of five general hospitals.

PATIENTS

Hundred thirty patients on mechanical ventilation longer than 48 h when considered ready for weaning.

MEASUREMENTS AND RESULTS

Patients underwent a 30-min spontaneous breathing trial with simultaneous monitoring of occlusion pressure (P0.1) and breathing pattern (f/Vt). Sixteen patients (12%) failed the weaning trial and full ventilatory support was resumed, while 114 tolerated the trial and were extubated. Twenty-one (18%) required reintubation within 48 h. The area under the ROC curve for diagnosing extubation failure was 0.53 for f/Vt, 0.59 for P0.1 and 0.61 for P0.1*f/Vt (p=NS). Accordingly, P0.1*f/Vt more than 100 detected extubation failure with a sensitivity of 0.89, specificity of 0.35, positive predictive value of 0.21 and negative predictive value of 0.94.

CONCLUSION

During a first trial of spontaneous breathing on pressure support ventilation (PSV), bedside P0.1 and P0.1*f/Vt are of little help, if any, for predicting extubation failure.

Extubation failure: magnitude of the problem, impact on outcomes, and prevention

Extubation failure, defined as the need for reinstitution of ventilatory support within 24 to 72 hours of planned endotracheal tube removal, occurs in 2 to 25% of extubated patients. The pathophysiologic causes of extubation failure include an imbalance between respiratory muscle capacity and work of breathing, upper airway obstruction, excess respiratory secretions, inadequate cough, encephalopathy, and cardiac dysfunction. Compared with patients who tolerate extubation, those who require reintubation have a higher incidence of hospital mortality, increased length of ICU and hospital stay, prolonged duration of mechanical ventilation, higher hospital costs, and an increased need for tracheostomy. Given the lack of proven treatments for extubation failure, clinicians must be aware of the factors that predict extubation outcome to improve clinical decision making. Risk factors for extubation failure include being a medical, multidisciplinary, or pediatric patient; age greater than 70 years; a longer duration of mechanical ventilation; continuous intravenous sedation; and anemia. Tests designed to assess for upper airway obstruction, secretion volume, and the effectiveness of cough can help to improve prediction of extubation failure. Rapid reinstitution of ventilatory support in patients who fail extubation may improve outcome.

The pulmonary physician in critical care. 10: difficult weaning

The study of patients being weaned from mechanical ventilation has offered new insights into the physiology of respiratory failure. Assessment of the balance between respiratory muscle strength, work and central drive is essential if difficulty in weaning occurs, and optimisation of these elements may improve the success of weaning. Psychological support of patients and the creation of units specialising in weaning have also resulted in a higher success rate.

Variations in the measurement of weaning parameters: a survey of respiratory therapists

OBJECTIVES

Respiratory therapists differ in the methods used to obtain weaning parameters. A questionnaire survey was conducted to better characterize those differences.

DESIGN

A questionnaire survey was conducted among respiratory therapists from nine hospitals in the Los Angeles area. The four-page, 32-question instrument was self-administered and anonymous. Responses were tabulated for analysis.

SETTING

Respondents from nine hospitals, three hospitals with residency training programs and six community hospitals without training programs in the Los Angeles area.

PARTICIPANTS

One hundred two respiratory therapists.

RESULTS

There was no universally acknowledged group of weaning parameters, although four parameters were named by > 90%. There was wide variation in methods used to obtaining weaning parameters. Almost all (91%) obtained measurements with the patients breathing their current fraction of inspired oxygen, but there was great variability in the ventilator mode used to collect these parameters (T-tube, continuous positive airway pressure, pressure support), with an equally wide range of pressures added to each mode (0 to 10 cm H(2)O). There was great variation in the time (< 1 to > 15 min) before recording weaning parameters. Measurement of parameters was done either with bedside instruments or read from the ventilator display. The maximal inspiratory pressure had great variation in the duration of airway occlusion (< 1 to 20 s), with the most frequent time frame being 2 to 4 s. Differences were noted between therapists from the same hospital as well as between hospitals.

CONCLUSIONS

There is great variation among respiratory therapists when obtaining weaning parameters. This calls for further standardization of the measurement of weaning parameters.

Respiratory muscle function and drive in chronic obstructive pulmonary disease

Respiratory, and particularly inspiratory, muscle function is altered in COPD. Many of these alterations are secondary to a mechanical disadvantage related to hyperinflation. Other factors, including corticosteroid therapy and nutritional depletion, are also deleterious to muscle function. In addition, the load imposed on the respiratory muscles is increased in COPD. Combined with the altered respiratory muscle function, this increase induces important changes in respiratory muscle drive and recruitment. Moreover, the imbalance between respiratory muscle function and load is an important determinant of dyspnea and hypercapnia. Because much of the lung and airway derangements are irreversible in COPD, the respiratory muscles appear to be an attractive target for therapeutic interventions.

Muscular contractile failure in septic patients: role of the inducible nitric oxide synthase pathway

Skeletal muscle failure is a frequent manifestation of sepsis that affects prognosis and rehabilitation by impairing respiration and ambulation. Animal studies have shown that the inducible NO synthase (NOS2) is expressed in skeletal muscles during sepsis, likely affecting muscular function, by promoting the formation of the strong oxidant peroxynitrite. In contrast, whether human skeletal muscle expresses a functional NOS2 in similar conditions is unknown. We studied NOS2 expression (mRNA and protein) and activity and its role in contractile function in samples from rectus abdominis muscle obtained during surgical procedure in 16 septic patients and in 21 controls. Peroxynitrite formation was detected by immunohistochemical detection of nitrotyrosine residues. The main results of this study are as follows: (1) A significant increase in NOS2 mRNA, protein, and activity was found in muscles from septic patients, the expression of NOS2 protein positively correlating with sepsis severity. (2) Contractile force was significantly lower in septic than in control muscles. This phenomenon was not reverted by muscle incubation ex vivo with the NOS inhibitor L-NMMA, indicating that NO was not involved in force reduction at the time of biopsy. (3) NOS2 expression in skeletal myocytes was strongly co-localized with nitrotyrosine, revealing muscular peroxynitrite generation during the septic process, before the muscle was biopsied. Exposure of control muscles to an amount of peroxynitrite similar to that generated in septic muscles during the septic process resulted in a nonreversible reduction in force generation. These results suggest that NOS2 could be involved in the decreased muscular force of septic patients via the local generation of peroxynitrite.

Physiologic determinants of ventilator dependence in long-term mechanically ventilated patients

To investigate the pathophysiologic mechanisms of ventilator dependence, we took physiologic measurements in 28 patients with COPD and 11 postcardiac surgery (PCS) patients receiving long-term mechanical ventilation during a spontaneous breathing trial, and in 20 stable, spontaneously breathing patients matched for age and disease. After 40 +/- 14 min of spontaneous breathing, 20 of 28 patients with COPD and all 11 PCS patients were judged ventilator-dependent (VD). We found that in the 31 VD patients tidal volume was low (VT: 0.36 +/- 0.12 and 0.31 +/- 0.08 L for COPD and PCS, respectively), neuromuscular drive was high (P(0.1): 5.6 +/- 1. 6 and 3.9 +/- 1.9 cm H(2)O), inspiratory muscle strength was reduced (Pdi(max): 42 +/- 12 and 28 +/- 15 cm H(2)O), and lung mechanics were abnormal, particularly PEEPi (5.9 +/- 3.0 cm H(2)O) and lung resistance (22.2 +/- 9.2 cm H(2)O/L/s) in COPD. The load/capacity balance was altered (Pdi/Pdi(max) and Ppl/Ppl(max) > 0.4) and the effective inspiratory impedance was high (P(0.1)/VT/TI >/= 10 cm H(2)O/L/s). Failure to wean occurred in patients with f/VT > 105 breaths/min/L and 56% of patients with COPD with f/VT < 80 breaths/min/L. Those who failed despite a low f/VT ( < 80 breaths/min/L) either showed ineffective inspiratory efforts, which artificially lowered f/ VT (n = 8), or did not increase breathing frequency (n = 5), but P(0.1) and P(0.1)/VT/TI were as high as in other VD patients. In the 31 VD patients, Pa(CO(2)) increased during the weaning trial (+12.3 +/- 8.0 mm Hg). We conclude that in the presence of a high drive to breathe, the imbalance between increased work load and reduced inspiratory muscle strength causes respiratory distress and CO(2) retention. Noninvasive measurements (breathing pattern, P(0.1), P(0.1)/ VT/TI) may give better insight into weaning failure useful in clinical decision-making, particularly in patients with COPD not showing rapid shallow breathing (56% in this study).

Physiologic response of ventilator-dependent patients with chronic obstructive pulmonary disease to proportional assist ventilation and continuous positive airway pressure

To investigate the physiologic effects of proportional assist ventilation (PAV) in difficult-to-wean, mechanically ventilated patients with advanced COPD, we measured in eight ICU patients the breathing pattern, neuromuscular drive (P0.1), lung mechanics, and inspiratory muscle effort (PTPdi and PTPpl) during both spontaneous breathing (SB) and ventilatory support with PAV, CPAP, and CPAP + PAV (in random sequence). PAV (volume assist [VA] and flow assist [FA]) was set as follows: dynamic lung elastance and inspiratory pulmonary resistance were measured during SB; then VA and FA were set to counterbalance the elastic and resistive loads exceeding the normal values, respectively, the inspiratory muscles bearing a normal elastic and resistive workload. CPAP was set close to dynamic intrinsic PEEP (8.3 +/- 3.4 cm H2O). We found significant reductions in P0.1 and PTPdi during both CPAP (-45 and -37%, respectively) and PAV (-50 and -48%, respectively). However, only the combination of PAV and CPAP brought P0.1 (1.69 +/- 0.97 cm H2O) and PTPdi (100 +/- 68 cm H2O. s) within normal values, and ameliorated the breathing pattern compared with SB (tidal volume: 0.69 +/- 0.33 versus 0.33 +/- 0.14 L; breathing frequency, 14.6 +/- 4.6 versus 21.0 +/- 6.5 breaths/min, respectively), without generating ineffective inspiratory efforts. We conclude that in difficult-to-wean COPD patients, (1) PAV improves ventilation and reduces both P0.1 and inspiratory muscle effort; (2) the combination of PAV and CPAP can unload the inspiratory muscles to values close to those found in normal subjects.

Changes in occlusion pressure (P0.1) and breathing pattern during pressure support ventilation

BACKGROUND

The purpose of this study was to investigate changes in breathing pattern, neuromuscular drive (P0.1), and activity of the sternocleidomastoid muscles (SCM) during a gradual reduction in pressure support ventilation (PSV) in patients being weaned off controlled mechanical ventilation.

METHODS

Eight non-COPD patients recovering from acute respiratory failure were included in this prospective interventional study. All patients were unable to tolerate discontinuation from mechanical ventilation. Each patient was evaluated during a period of spontaneous breathing and during PSV. Four successive levels of PSV were assessed in the following order: 20 cm H2O (PS20), 15 cm H2O (PS15), 10 cm H2O (PS10), and 5 cm H2O (PS5).

RESULTS

When pressure support was reduced from PS20 to PS10 the respiratory rate (f) and the rapid shallow breathing index (f/VT) significantly increased and tidal volume (VT) significantly decreased. These parameters did not vary when pressure support was reduced from PS10 to PS5. Conversely, P0.1 varied negligibly between PS20 and PS15 but increased significantly at low PSV levels. P0.1 values were always greater than 2.9 cm H2O (4.1 (1.1) cm H2O) when SCM activity was present. When contraction of the SCM muscles reappeared the P0.1 was the only parameter that changed significantly.

CONCLUSIONS

In postoperative septic patients the value of P0.1 seems to be more useful than breathing pattern parameters for setting the optimal level of pressure assistance during PSV.

Changes in the work of breathing induced by tracheotomy in ventilator-dependent patients

Tracheotomy is widely performed on ventilator-dependent patients, but its effects on respiratory mechanics have not been studied. We measured the work of breathing (WOB) in eight patients before and after tracheotomy during breathing at three identical levels of pressure support (PS): baseline level (PS-B), PS + 5 cm H2O (PS+5), and PS - 5 cm H2O (PS-5). After the procedure, we also compared the resistive work induced by the patients' endotracheal tubes (ETTs) and by a new tracheotomy cannula in an in vitro bench study. A significant reduction in the WOB was observed after tracheotomy for PS-B (from 0.9 +/- 0.4 to 0.4 +/- 0.2 J/L, p < 0.05), and for PS-5 (1.4 +/- 0.6 to 0.6 +/- 0.3 J/L, p < 0.05), with a near-significant reduction for PS+5 (0.5 +/- 0.5 to 0.2 +/- 0.1 J/L, p = 0.05). A significant reduction was also observed in the pressure-time index of the respiratory muscles (181 +/- 92 to 80 +/- 56 cm H2O. s/min for PS-B, p < 0.05). Resistive and elastic work computed from transpulmonary pressure measurements decreased significantly at PS-B and PS-5. A significant reduction in occlusion pressure and intrinsic positive end-expiratory pressure (PEEP) was also observed for all conditions, with no significant change in breathing pattern. Three patients had ineffective breathing efforts before tracheotomy, and all had improved synchrony with the ventilator after the procedure. In vitro measurements made with ETTs removed from the patients, with new ETTs, and with the tracheotomy cannula showed that the cannula reduced the resistive work induced by the artificial airway. Part of these results was explained by a slight, subtle reduction of the inner diameter of used ETTs. We conclude that tracheotomy can substantially reduce the mechanical workload of ventilator-dependent patients.

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

PURPOSE

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).

MATERIALS AND METHODS

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.

RESULTS

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.

CONCLUSIONS

Optimal PS level established with diEMG 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.

Respiratory function in children during recovery from neuromuscular blockade

Residual neuromuscular blockade is a major risk factor for respiratory insufficiency. We examined the relationship between neuromuscular and respiratory function in 18 ASA I or II children aged 2-4 years. Lung function was measured by pneumotachography and transpulmonary pressure, neuromuscular transmission by first twitch response ratio (T1:T1) and train-of-four ratio (TOFR), before and at specific points in recovery from vecuronium paralysis. The tidal volume was directly related to maximal inspiratory pressure at occlusion (PIOCC), P < 0.001, whereas the minute ventilation (VE) was related to the respiratory drive (P0.1), P < 0.001. The best predictors of minute ventilation were the P0.1 (r = 0.57), and the TOFR (r = 0.62). PIOCC and P0.1 correlated closely (r = 0.889, P = 0.002) but TOFR and T1:T1 did not correlate with either. Our results show that the occlusion pressure measurements, P0.1 and PIOCC, were good predictors of both VE.kg-1 and respiratory work.

Changes in breathing pattern and respiratory muscle performance parameters during difficult weaning

OBJECTIVE

This study examined, using noninvasive means, the changes in breathing pattern and inspiratory muscle pressure-time indices during difficult progressive withdrawal of pressure-support ventilation.

DESIGN

A prospective analysis of the temporal evolution of several respiratory variables in difficult-to-wean patients.

SETTING

A university hospital intensive care unit.

PATIENTS

A heterogeneous group of 17 patients receiving prolonged mechanical ventilation.

INTERVENTIONS

Daily measurements of breathing pattern and respiratory muscle performance parameters in difficult-to-wean patients.

MEASUREMENTS AND MAIN RESULTS

We examined breathing pattern variables, rapid shallow breathing (respiratory rate/tidal volume), tracheal occlusion pressure, maximal inspiratory pressure (P(I)max), and the tension-time index of the inspiratory muscles (TTmus = P(I)/P(I)max x Ti/Ttot) (where Ti/Ttot is inspiratory fraction of the cycle). All measurements were repeated at 24-hr intervals throughout the difficult weaning period. The patients were extubated on satisfying ten of 12 classical weaning criteria. Eleven patients were successfully weaned from mechanical ventilation while six patients were not. Weaning failure was associated with the following: a) longer periods of mechanical ventilation before weaning; b) high values of tracheal occlusion pressure, respiratory rate, minute ventilation, and effective impedance maintained throughout the difficult weaning period; and c) persistent high PaCO2 and intrinsic positive end-expiratory pressure values. As the weaning failure patients' inspiratory muscles confronted an increasing inspiratory load, values of the tension-time index of the inspiratory muscles entered or remained in the fatigue zone. In contrast, weaning success patients normalized their breathing pattern and decreased their tracheal occlusion pressure, effective impedance, and tension-time index values.

CONCLUSIONS

Breathing pattern alterations and respiratory muscle performance impairments lead to ventilator dependency after prolonged mechanical ventilation. The measurement of variables such as the noninvasive tracheal occlusion pressure, inspiratory power of breathing, and tension-time index of the inspiratory muscles facilitate the management of difficult-to-wean patients.

Respiratory drive and pulmonary mechanics during haemodialysis with ultrafiltration in ventilated patients

The improvements of respiratory drive and pulmonary mechanics which follow haemodialysis with ultrafiltration in mechanically ventilated renal failure patients seem predictable but have not been studied before. In this study, 14 renal failure patients with stable haemodynamics mechanically ventilated with pressure support ventilation (PSV) were enrolled. Respiratory drive (represented as P0.1), pulmonary mechanics, breathing pattern, arterial blood gas and haemodynamics were measured according to the time schedule: pre-dialysis (Time 0), and at 60, 120, 180, 240 minutes thereafter. Following the removal of excess lung water during haemodialysis, auto-PEEP and patient's work of breathing (WOBp) decreased gradually. P0.1 lessened progressively along with the improvement in pulmonary mechanics. The changes in auto-PEEP and WOBp correlated closely to the pre- and post-dialysis decline of P0.1 (delta P0.1). There was a negative, moderately significant correlation between the amount of fluid ultrafiltrated during dialysis (delta UF) and the delta P0.1 (R = -0.54). The breathing pattern remained stable during dialysis. No hypoventilation or hypoxaemia occurred despite the development of metabolic alkalosis induced by bicarbonate dialysate. We have shown that respiratory drive decreases gradually during bicarbonate haemodialysis. The improvements of pulmonary mechanics, rather than the rapid alkalization of body fluids, responds to the decrease of P0.1 in renal failure patients ventilated with PSV.

Lung reduction surgery in severe COPD decreases central drive and ventilatory response to CO2

BACKGROUND AND OBJECTIVES

Lung volume reduction surgery (LVRS) improves ventilatory function in selected patients with severe COPD. The reasons for the observed benefits include the following: increased elastic recoil, improved airflow, and lesser dynamic hyperinflation and decreased lung volumes. We reasoned that these changes could also alter respiratory drive.

METHODS

Respiratory central drive was prospectively assessed using the mouth occlusion pressure (P0.1), and the P0.1 response to increasing CO2 (P0.1/PETCO2 [end-tidal CO2 pressure]), in eight sequential patients before and 3 to 5 months after LVRS. Results were compared with those from 13 control subjects.

RESULTS

LVRS decreased total lung capacity from 7.44+/-1.8 L to 5.92+/-1.3 L (p<0.05) and residual volume from 4.97+/-1.5 L to 3.56+/-1.1 L (p<0.05). It also significantly improved FEV1 from 0.85+/-0.26 L to 0.99+/-0.26 L (p<0.05). Baseline P0.1 (3.4+/-1.8 vs 1.4+/-0.4 cm H2O, p<0.01) and P0.1/PETCO2 (0.24+/-0.07 vs 0.11+/-0.04 cm H2O/mm Hg, p<0.05) were higher in patients than in control subjects. After LVRS, P0.1 decreased from 3.4+/-1.8 to 1.3+/-0.75 cm H2O (p<0.01) and P0.1/PETCO2 from 0.24+/-0.07 to 0.16+/-0.06 cm H2O/mm Hg (p<0.05). These postoperative values were similar to those of control subjects. There were no correlations between changes in the factors known to influence central drive (PaO2, PaCO2, age, weight, height, FVC, and FEV1) and changes in P0.1.

CONCLUSIONS

We conclude that decreased ventilatory drive should be added to the list of benefits of LVRS, and may help explain the symptomatic improvement reported by many patients after this surgery.

Dynamic ventilatory characteristics during weaning in postoperative critically ill patients

Postoperative patients occasionally require more than 48 h of mechanical ventilation. This study examined whether there were distinct differences in dynamic respiratory variables between patients who successfully weaned from mechanical ventilation and those who failed. Forty general and thoracic surgery patients underwent a standardized weaning sequence: 25 min of synchronous intermittent mandatory ventilation (SIMV) at 8 bpm plus 5 cm H2O pressure support ventilation (PSV), then SIMV at 4 bpm plus 5 cm H2O PSV, followed by continuous positive airway pressure (CPAP) plus 5 cm H2O PSV and, finally, CPAP without PSV. Twenty-eight patients successfully weaned and 12 failed. During SIMV at 4 bpm plus 5 cm H2O PSV, the spontaneous respiratory rate to spontaneous tidal volume ratio (sRR/sV(T)) and total and spontaneous respiratory rates were higher (P < 0.01) in the failure group. sRR/sV(T) values (threshold 65 bpm/L, sensitivity 1.00, specificity 0.82) and sRR values (threshold 12 bpm, sensitivity 0.95, specificity 0.84) were distinctive. During CPAP plus 5 cm H2O of PSV, respiratory rate, minute ventilation, patient work of breathing, and P0.1 were higher (P < 0.01) in those who failed. P0.1 (threshold 4.5 cm H2O, sensitivity 1.00, specificity 1.00), patient work of breathing (threshold 1.3 J/L, sensitivity 0.92, and specificity 0.98), and the sRR/sV(T) ratio (threshold 65 bpm/L, sensitivity 0.90, specificity 0.80) were distinctive. Most unique was the analysis of spontaneous breaths during low SIMV rates. This appears to permit an early determination of whether weaning would succeed.

Monitoring during mechanical ventilation

Approximately half of the patients admitted to an ICU are admitted for the purposes of monitoring rather than interventional therapy. In the last decade, significant technologic advances have enhanced monitoring capacities, and the understanding of the pathophysiology of respiratory failure has improved pari passu, allowing clinicians to employ monitors in a more intelligent manner. This article deals with new developments in arterial blood gas monitoring, pulse oximetry, capnometry, and monitoring of neuromuscular function and pulmonary mechanics, emphasizing issues most relevant to mechanical ventilation.

Closed-loop control of airway occlusion pressure at 0.1 second (P0.1) applied to pressure-support ventilation: algorithm and application in intubated patients

OBJECTIVE

Airway occlusion pressure at 0.1 sec (P0.1) is an index of respiratory center output. During pressure-support ventilation, P0.1 correlates with the mechanical output of the inspiratory muscles and has an inverse relationship with the amount of pressure-support ventilation. Based on these observations, we designed a closed-loop control which, by automatically adjusting pressure-support ventilation, stabilizes P0.1, and hence patient inspiratory activity, at a desired target. The purpose of the study was to demonstrate the feasibility of the method, rather than its efficacy or even its influence on patient outcome.

DESIGN

Prospective, randomized trial.

SETTING

A general intensive care unit of a university hospital in Italy.

PATIENTS

Eight stable patients intubated and ventilated with pressure-support ventilation for acute respiratory failure.

INTERVENTIONS

Patients were transiently connected to a computer-controlled ventilator on which the algorithm for closed-loop control was implemented. The closed-loop control was based on breath by breath measurement of P0.1, and on comparison with a target set by the user. When actual P0.1 proved to be higher than the target value, the P0.1 controller automatically increased pressure-support ventilation, and decreased it when P0.1 proved to be lower than the target value. For safety, a volume controller was also implemented. Four P0.1 targets (1.5, 2.5, 3.5, and 4.5 cm H2O) were applied at random for 15 mins each.

MEASUREMENTS AND MAIN RESULTS

The closed-loop algorithm was able to control P0.1, with a difference from the set targets of 0.59 +/- 0.27 (SD) cm H2O.

CONCLUSIONS

The study shows that P0.1 can be automatically controlled by pressure-support ventilation adjustments with a computer. Inspiratory activity can thus be stabilized at a level prescribed by the physician.

Mechanism of relief of tachypnea during pressure support ventilation

Pressure support ventilation (PSV) provides a range of ventilatory support from partial respiratory muscle unloading, where inspiratory work is shared between the patient and the mechanical ventilator, to total respiratory muscle unloading, where inspiratory work is performed solely by the ventilator. This study is designed to determine if minimizing work fully accounts for relief of tachypnea during PSV. We examined respiratory parameters over a range of PSV that includes the crossover from partial to total respiratory muscle unloading. Eight studies were obtained on seven intubated patients in respiratory failure. Ventilation, occlusion pressure (P0.1), and patient inspiratory work (WOBinsp) were measured while PSV was varied. In all patients, WOBinsp decreased as PSV increased. The level of PSV where WOBinsp was minimized was identified; this marked the crossover from partial to total respiratory muscle unloading. Frequency decreased with increasing PSV but remained elevated (range, 22 to 38 breaths/min) at the crossover. Frequency was normalized only at PSV levels 131 to 193% of the levels of pressure at the crossover. Tidal volume (VT) changed little during partial support and averaged 5.9 mL/kg at the crossover. VT increased only on PSV providing total unloading. Six of seven patients exhibited increasing static compliance with increasing VT suggesting alveolar recruitment. P0.1 tracked WOBinsp over the entire range of PSV (r = 0.95, p < 0.001). The normalization of frequency observed above the crossover coincided with increasing VT rather than decreasing work. These observations suggest that reflexes resulting from increased VT and/or alveolar recruitment may have contributed to the normalization of frequency.

Needle EMG of the human diaphragm: power spectral analysis in normal subjects

Needle EMG of the diaphragm was performed in 43 diaphragms in 23 healthy volunteers. The mean +/- standard deviation for the median frequency (MF) of the power spectrum was 233.3 +/- 58.1 Hz. The MF increased with age and showed a negative correlation with the forced vital capacity (FVC), but there was no correlation with other anthropometric measures or the results of phrenic nerve conduction study. The higher MF in older subjects may be due to early recruitment of larger units. The negative correlation between MF and FVC is likely secondary to a lower level of contraction required for normal breathing in subjects with high FVC. The integrated EMG (iEMG) of each inspiration strongly correlated with the tidal volume and the duration of inspiration. Power spectral analysis of diaphragmatic EMG is feasible and reliable. It can be used to assess respiratory muscle fatigue and may help in the diagnosis of neuromuscular disorders affecting the diaphragm.

Occlusion pressure and its ratio to maximum inspiratory pressure are useful predictors for successful extubation following T-piece weaning trial

STUDY OBJECTIVE

In most weaning studies, failure group patients are reventilated prior to extubation, thus compromising the evaluation of the applied weaning indices' predictive values. This study determines the usefulness of both standard and recent indices in predicting successful extubation following prolonged mechanical ventilation.

DESIGN AND METHODS

Following a successful 20-min T-piece trial, ten traditional weaning criteria, as well as airway occlusion pressure (PO.1), maximal inspiratory pressure (MIP), PO.1/MIP ratio, and shallow breathing (F/VT) were determined in unselected patients undergoing prolonged mechanical ventilation. Having satisfied 8 of 10 classic weaning criteria, 67 patients were extubated after an additional 40 min of successful spontaneous T-piece breathing, and included in the study. After extubation, the tracheal tube resistive pressure (RP) values were measured.

RESULTS

Twelve (18%) patients failed extubation. The failure group's average age was significantly greater (69.43 vs 48.43 years). The PO.1, PO.1/MIP, and F/VT values of the success (3.62 +/- 1.35 cm H2O, 0.05 +/- 0.04, and 50 +/- 23 b.min-1.L-1) and failure (7.38 +/- 2.67 cm H2O, 0.14 +/- 0.04, and 69 +/- 25 b.min-1.L-1) groups were significantly different (p < 0.005). The diagnostic accuracies of these indices were, respectively, 88%, 98%, and 73%. The spirometric, gas exchange, and tracheal tube RP values of the two groups showed no significant differences.

CONCLUSION

PO.1 and PO.1/MIP ratio provide the best means of predicting extubation success, and they are not influenced by tracheal tube resistance.

P0.1 is a useful parameter in setting the level of pressure support ventilation

OBJECTIVE

The purpose of this study was to investigate whether changes in breathing pattern, neuromuscular drive (P0.1), and the work involved in breathing might help to set the individual appropriate level of pressure support ventilation (PSV) in patients with acute respiratory failure (ARF) requiring ventilatory assistance.

DESIGN

A prospective, interventional study.

SETTING

An 8-bed multidisciplinary intensive care unit (ICU).

PATIENTS

Ten patients with ARF due to adult respiratory distress syndrome (ARDS), sepsis or airway infection were included in the study. Chronic obstructive pulmonary disease (COPD) patients with acute exacerbation were excluded. None of these patients was in the weaning process.

INTERVENTIONS

We found a level of pressure support able to generate a condition of near-relaxation in each patient, as evidenced by work of breathing (WOB) values close to 0 J/1. This level was called PS 100 and baseline physiological measurements, namely, breathing pattern, P0.1 and WOB were obtained. Pressure support was then reduced to 85%, 70%, and 50% of the initial value and the same set of measurements was obtained.

MEASUREMENTS AND RESULTS

Flow (V) was measured by a flow sensor (Varflex) positioned between the Y-piece of the breathing circuit and the endotracheal tube. Tidal volume was obtained by numerical integration of the flow signal. Airway pressure (P(aw)) was sampled through a catheter attached to the flow sensor. Esophageal pressure (Pes) was measured with a nasogastric tube incorporating an esophageal balloon. The esophageal balloon and flow and pressure sensors were connected to a portable monitor (CP 100 Bicore) that provided real-time display of flow, volume, P(aw) and Pes tracings and loops of Pes/V, P(aw)/V and V/V relationships. The breathing pattern was analyzed from the flow signal. Patient work of breathing (WOB) was calculated by integration of the area of the Pes/V loop. Respiratory drive (P0.1) was measured at the esophageal pressure change during the first 100 ms of a breath, by the quasi-occlusion technique. When pressure support was reduced, we found that the respiration rate significantly increased from PS 100 to PS 85, but varied negligibly with lower pressure support levels. Tidal volume behaved in a similar way, decreasing significantly from PS 100 to PS 85, but hardly changing at PS 70 and PS 50. In contrast, WOB and P0.1 increased progressively with decreasing pressure support levels. The changes in WOB were significant at each stage in the trial, whereas P0.1 increased significantly from PS 100 at other stages. Linear regression analysis revealed a highly positive, significant correlation between WOB and P0.1 at decreasing PSV levels (r = 0.87), whereas the correlation between WOB and ventilatory frequency was less significant (r = 0.53). No other correlation was found.

CONCLUSIONS

During pressure support ventilation, P0.1 may be a more sensitive parameter than the assessment of breathing pattern in setting the optimal level of pressure support in individual patients. Although P0.1 was measured with an esophageal balloon in the present study, non-invasive techniques can also be used.

A new method for P0.1 measurement using standard respiratory equipment

The airway occlusion pressure, P0.1, is an index for the neuro-muscular activation of the respiratory system. It has been shown to be a very useful indicator for the ability of patients receiving ventilatory support to be weaned from mechanical ventilation. Since the standard measurement technique for P0.1 determination is technically complex, it is not widely available for clinical purposes. For that reason a P0.1 measurement technique was developed as an integrated function in a standard respirator (Evita, Dräger, Lübeck, Germany). This technique is easy to use and does not need any further equipment. We validated this new technique by comparing it to standard P0.1 measurements in a mechanical lung model as well as in ventilated patients. In the lung model we found a correlation between the Evita measurement and standard measurements of r = 0.99. In 6 ventilated patients the correlation was r = 0.78. Since the Evita P0.1 and the standard measurement had to be performed during two different breaths, this little poorer correlation in patients may be due to a significant breath-by-breath variability in P0.1. Comparing the Evita P0.1 and the standard measurement within one breath resulted in a clearly better correlation (r = 0.89). We conclude that this new measurement technique provides an easy and accurate P0.1 measurement using standard respiratory equipment when tested in a lung model. In patient measurements the method is less precise, which is probably due to the variable waveforms of the inspiratory driving pressure seen in patients, for example when intrinsic PEEP is present.(ABSTRACT TRUNCATED AT 250 WORDS)

Elevated static compliance of the total respiratory system: early predictor of weaning unsuccess in severed COPD patients mechanically ventilated

OBJECTIVE

To assess in a group of COPD patients mechanically ventilated for an episode of acute respiratory failure the respiratory mechanics with a simple and non invasive method at the bedside in order to evaluate if these parameters may be predictive of weaning failure or success.

DESIGN

A prospective study.

SETTING

Intensive care and intermediate intensive care units.

PATIENTS

23 COPD patients ventilated for acute respiratory failure and studied within 24 hours from intubation.

METHODS

Using end-expiratory and end-inspiratory airway occlusion technique, we measured PEEPi, static compliance of the respiratory system (Crs, st) maximum respiratory resistance (Rrsmax) and minimum respiratory resistance (Rrsmin).

MEASUREMENTS AND RESULTS

The weaned group (A) and the not weaned group (B) were not different regarding to static PEEPi (group A 8.5 +/- 4.0 vs group B 8.9 +/- 2.6 cmH2O), TO Rrsmax (22.4 +/- 5.3 versus 22.2 +/- 9.0 cmH2O/1/s) and to Rrsmin (17.6 +/- 5.5 versus 17.9 +/- 8.0 cmH2O/1/s), while a significant difference (p < 0.001) has been found in Cst, rs (62.7 +/- 17.% versus 111.6 +/- 18.0 ml/cm H2O). The threshold value of 88.5 ml/cmH2O was identified by discriminant analysis and provided the best separation between the two groups, with a sensitivity of 0.85 and a specificity of 0.87.

CONCLUSION

Cst, rs measured non invasively in the first 24 h from intubation, provided a good separation between the patients who were successfully weaned and those who failed.

Medical effectiveness of esophageal balloon pressure manometry in weaning patients from mechanical ventilation

OBJECTIVE

To determine the efficacy of a new respiratory monitor, which uses esophageal balloons, in aiding clinicians attempting to wean patients from mechanical ventilation.

DESIGN

Prospective study of patients who were deemed ready to be weaned after having required mechanical ventilation for a minimum of 3 days. Each of the patients served as his or her own control.

SETTING

University medical intensive care unit.

PATIENTS

The series consisted of 23 consecutive patients who were ready to wean from mechanical ventilation.

INTERVENTIONS

Before the onset of the study, two weaning strategies were developed. One strategy involved using clinically available weaning parameters. The other strategy involved using esophageal balloon data that was recorded via a new respiratory monitor. Each of the weaning strategies resulted in the development of a scoring system that could be rigidly adhered to and which determined, without bias, to what extent the patient could be weaned each day. Rigid criteria were also developed to determine whether the weaning trial was successful or not. The two strategies were then compared to determine the ability of the strategy to shorten ventilatory time.

MEASUREMENTS AND MAIN RESULTS

Each patient was evaluated daily by the two weaning protocols. At each weaning step, the two protocols were compared with respect to degree of aggressiveness and tolerance of the weaning maneuver by the patient. A protocol was judged superior if it resulted in more aggressive weaning without increased patient intolerance. The clinicians evaluating the patient with the clinical protocol could accelerate or retard the number of weaning steps by one step, based on the patient's clinical state and the clinician's experience. There was no such freedom in the esophageal protocol. The major finding was that in 40.5% of the instances, the protocol involving the esophageal balloon resulted in more aggressive weaning without patient intolerance. In 11.6% of the cases, the clinical protocol was more aggressive. Both protocols predicted the same number of weaning steps 39.8% of the time. In all these instances, the patient tolerated the weaning suggested. The use of data from the esophageal protocol resulted in weaning the patients 1.68 days faster than the use of data from the clinical protocol.

CONCLUSIONS

The respiratory monitor, using esophageal balloon technology, is effective in that it can provide the clinician with data that can result in more aggressive weaning from mechanical ventilation without an increase in patient intolerance. The duration of mechanical ventilation can be shortened when these data are applied via a rigidly controlled weaning strategy.

Maximum relaxation rate of the diaphragm during weaning from mechanical ventilation

BACKGROUND

The maximum relaxation rate (MRR; percentage fall in pressure/10 ms) of oesophageal (POES) and transdiaphragmatic (PDI) pressure slows under conditions of loaded breathing, and has been measured previously in normal subjects. MRR has not been measured in intubated patients weaning from mechanical ventilation.

METHODS

Five postoperative patients who were expected to wean and nine patients who had previously failed were studied. POES and PDI MRR, peak oesophageal pressure during spontaneous breathing, maximum oesophageal pressure, and the inspiratory duty cycle were measured at rest during mechanical ventilation, in the first two minutes of spontaneous breathing, and after reventilation in those patients who failed, or before extubation in those patients who succeeded.

RESULTS

At rest POES MRR in intubated patients had a range of 5.6-11 and PDI MRR 6.9-10.0, with a coefficient of variation of 9.9% and 7.3% respectively. POES and PDI MRR were similar before and after extubation in five postoperative patients, and POES MRR was reflected by endotracheal MRR measured at the airway. In five patients who failed to wean POES and PDI MRR slowed by 47% and 44%, and fully recovered after 10 minutes reventilation. In four patients who were successfully weaned MRR was unchanged during spontaneous breathing. At the time when MRR decreased, the respiratory muscles were heavily loaded in relation to their strength.

CONCLUSIONS

Weaning failure occurs when the applied load exceeds the capacity of the respiratory muscles, and this is associated with a slowing of respiratory muscle MRR.

Airway occlusion pressure and breathing pattern as predictors of weaning outcome

Airway occlusion pressure (P0.1) and the ratio of breathing frequency (f) to tidal volume (VT) (f/Vt) are good predictors of weaning outcome. However, the specificity of f/VT in predicting weaning success is relatively low. We postulated that the product of P0.1 and f/VT (P0.1*f/VT) would better predict weaning outcome than either variable alone. In 45 male patients, we prospectively evaluated P0.1*f/VT, P0.1, and f/VT in predicting weaning outcome. The threshold values of each variable were determined from published data. The sensitivity, specificity, and positive and negative predictive values in detecting weaning success, and the area under the receiver operating characteristic (ROC) curves were calculated. Ten (22%) of the 45 patients failed weaning. P0.1*f/VT yielded the highest specificity and positive and negative predictive values. P0.1*f/VT, P0.1, and f/VT were all highly sensitive (0.97); but they were less specific, 0.60 for P0.1*f/Vt and 0.40 for P0.1 and f/VT. The areas under the ROC curves for P0.1*f/VT, P0.1, and f/VT were not significantly different. We conclude that P0.1*f/VT has equivalent sensitivity as P0.1 and f/VT. P0.1 slightly improves the specificity of f/VT in predicting weaning success.

Respiratory muscles and ventilatory failure: 1993 perspective

Some conditions that predispose to ventilatory failure increase the work of breathing (chronic obstructive pulmonary disease [COPD], obesity, kyphoscoliosis), whereas others cause severe respiratory muscle weakness. Specific reasons for muscle weakness include critical illness (electrolyte imbalance, acidemia, shock, sepsis), chronic illness (poor nutrition, cachexia), and neuromuscular diseases. Inspiratory muscle weakness from mechanical disadvantage to the diaphragm is characteristic of asthma and COPD. The increased work of breathing combined with muscle weakness increases the pressure needed to inspire a breath and decreases maximal inspiratory pressure. When this pressure exceeds 0.4, dyspnea and inspiratory muscle fatigue ensue. One way to lower this pressure and avert fatigue is to lower the tidal volume. Ventilatory drive is high, not low, in ventilatory failure. Concomitant shortening of inspiration and breath duration cause the small tidal volume and increased respiratory rate. Gas exchange is compromised by ventilation/perfusion imbalance, and the ratio of dead space to tidal volume is also increased by rapid, shallow breathing. Reduction in tidal volume minimizes dyspnea, but the small tidal volume is inadequate for gas exchange. Acute treatment of respiratory muscle failure involves respiratory muscle rest through mechanical ventilation and removal of noxious influences (infection, metabolic disarray), whereas chronic treatment involves rebuilding the contractile apparatus by nutritional repletion and training.