Levels Of Pressure Support Ventilation Research Articles

Effectiveness of mask and helmet interfaces to deliver noninvasive ventilation in a human model of resistive breathing

The helmet, a transparent latex-free polyvinyl chloride cylinder linked by a metallic ring to a soft collar that seals the helmet around the neck, has been recently proposed as an effective alternative to conventional face mask to deliver pressure support ventilation (PSV) during noninvasive ventilation in patients with acute respiratory failure. We tested the hypothesis that mechanical characteristics of the helmet (large internal volume and high compliance) might impair patient-ventilator interactions compared with standard face mask. Breathing pattern, CO(2) clearance, indexes of inspiratory muscle effort and patient-ventilator asynchrony, and dyspnea were measured at different levels of PSV delivered by face mask and helmet in six healthy volunteers before (load-off) and after (load-on) application of a linear resistor. During load-off, no differences in breathing pattern and inspiratory muscle effort were found. During load-on, the use of helmet to deliver pressure support increased inspiratory muscle effort and patient-ventilator asynchrony, worsened CO(2) clearance, and increased dyspnea compared with standard face mask. Autocycled breaths accounted for 12 and 25% of the total minute ventilation and for 10 and 23% of the total inspiratory muscle effort during mask and helmet PSV, respectively. We conclude that PSV delivered by helmet interface is less effective in unloading inspiratory muscles compared with PSV delivered by standard face mask. Other ventilatory assist modes should be tested to exploit to the most the potential benefits offered by the helmet.

Additional work of breathing and breathing patterns in spontaneously breathing patients during pressure support ventilation, automatic tube compensation and amplified spontaneous pattern breathing

Additional work of breathing and breathing patterns in spontaneously breathing patients during pressure support ventilation, automatic tube compensation and amplified spontaneous pattern breathing

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.

Assessment of Physiologic Variables and Subjective Comfort Under Different Levels of Pressure Support Ventilation

To evaluate the effects of 12 ventilator settings (pressure support ventilation [PSV] plus positive end-expiratory pressure [PEEP], 30 + 0 cm H(2)O; 25 + 5 cm H(2)O; 25 + 0 cm H(2)O; 20 + 5 cm H(2)O; 20 + 0 cm H(2)O; 15 + 5 cm H(2)O; 15 + 0 cm H(2)O; 10 + 5 cm H(2)O; 10 + 0 cm H(2)O; 5 + 5 cm H(2)O; 5 + 0 cm H(2)O; and 0 + 5 cm H(2)O) on physiologic variables; the percentage of ineffective efforts; patient comfort; and whether the diagnosis of COPD may influence results. Prospective, randomized, physiologic study. Three weaning centers. Thirty-six consecutive patients (20 patients with COPD). Patients were randomly submitted to the 12 settings. Breathing pattern, respiratory drive (p0.1), arterial oxygen saturation (Sato(2)), heart rate, percentage of ineffective efforts per minute, patient comfort measured by means of a visual analogue scale (VAS), and BORG scale were recorded under each setting. Under different levels of assistance, breathing pattern, Sato(2), and p0.1 significantly and linearly changed (p < 0.0001) while VAS and BORG scale presented a significant (p = 0.027) U-shaped trend; high or low assistance caused the most discomfort. Under high levels of assistance, a higher (analysis of variance, p = 0.023) frequency of ineffective effort percentage was observed in the subgroup of 26 patients who presented this phenomenon. Breathing pattern significantly (p = 0.013) changed when compared to PSV alone (PSV plus zero end-expiratory pressure [ZEEP]) at the same total inspiratory pressure assistance (PSV plus PEEP). A huge variability among patients in breathing pattern and comfort was found under the setting rated as the most comfortable by patients. The diagnosis of COPD did not influence the overall results. The following conclusions are made: (1) physiologic variables followed a linear trend, while comfort followed a U-shaped trend under different levels of PSV (irrespective of COPD diagnosis); (2) high assistance caused an increase in ineffective efforts; (3) only the breathing pattern significantly changed when total assistance was given as PSV plus PEEP when compared to PSV alone (PSV plus ZEEP); and (4) the extreme levels of PSV are not associated with the best comfort.

Estimating Respiratory Pattern Variability by Symbolic Dynamics

The traditional techniques of data analysis are often not sufficient to characterize the complex dynamics of respiration. In this study the respiratory pattern variability was analyzed using symbolic dynamics. A group of 20 patients on weaning trials from mechanical ventilation were studied at two different pressure support ventilation levels. Breath duration (T(TOT)) time series and the relation T(I)/T(TOT), that contains the influence of inspiratory time (T(I)), were considered. Length-3 words and 3 different symbols were proposed. The incidence of the overlapping tau and the parameter alpha were analyzed. From the breath duration time series, the distribution of words with probability of occurrence higher than 6% was concentrated on one word for low respiratory variability, whereas high variability was characterized by 4 words, presenting a statistically significant difference (p </= 0.0005). The probability occurrence of words "110" and "111" was also significantly different (p</= 0.0005) when comparing both variabilities. The analysis carried out obtained discriminant functions able to correctly classify all the testing set series. These results permit the consideration of symbolic dynamics as a promising methodology to study the respiratory pattern variability.

Measurement of diaphragm loading during pressure support ventilation.

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. Physiological study in a physiology laboratory. Six healthy adults. 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. 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. 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.

Effect of Different Levels of Pressure Support and Proportional Assist Ventilation on Breathing Pattern, Work of Breathing and Gas Exchange in Mechanically Ventilated Hypercapnic COPD Patients with Acute Respiratory Failure

Background: Proportional assist ventilation (PAV) has been shown to maintain better patient-ventilator synchrony than pressure support ventilation (PSV); however, its clinical advantage regarding invasive ventilation of COPD patients has not been clarified. Objectives: To compare the effect of PAV and PSV on respiratory parameters of hypercapnic COPD patients with acute respiratory failure (ARF). Methods: Nine intubated hypercapnic COPD patients were placed on the PAV or PSV mode in random sequence. For each mode, four levels (L₁–L₄) of support were applied. At each level, blood gases, flow, tidal volume (V_T), airway pressure (Paw), esophageal pressure (Pes) (n = 7), patient respiratory rate (fp), ventilator rate (fv), missing efforts (ME = fp – fv) were measured. Results: We found increases in ME with increasing levels of PSV but not with PAV. PO₂and V_T increased whereas PCO₂decreased significantly with increasing levels of PSV (p < 0.05). With PAV, PCO₂decreased and V_Tincreased significantly only at L₄ whereas PO₂increased from L₁ to L₄. Runaways were observed at L₃and L₄of PAV. The pressure-time product (PTP) was determined for effective and missing breaths. The mean total PTP per minute (of effective plus missing breaths) was 160 ± 57 cm H₂O/s·min in PSV and 194 ± 60 cm H₂O/s·min in PAV. Conclusion: We conclude that in COPD patients with hypercapnic ARF, with increasing support, PSV causes the appearance of ME whereas PAV develops runaway phenomena, due to the different patient-ventilator interaction; however, these do not limit the improvement of blood gases with the application of both methods.

Automatic pressure support reduction is effective in weaning postoperative patients in the intensive care unit

Automatic pressure support reduction based on a targeted respiratory frequency or MRV is disposable in the TAENA ventilator for an automatic reduction of pressure support during weaning of patients in the intensive care unit (ICU). We studied 23 patients (63.52 years) in the postoperative period (14 cardiac, two thoracic and seven abdominal surgeries) in a prospective, randomized protocol comparing automatic pressure support weaning with the traditional manual reduction of pressure support to 5–7 cmH2O in our ICU. After arriving in the ICU after cardiac, thoracic or abdominal surgery, the patients were randomly assigned to traditional weaning consisting of manual reduction of pressure support (the pressure support was decreased every 30 min, keeping the RR/TV(L) < 80 until 5–7 cmH2O pressure support ventilation) or to the automatic pressure support reduction (MRV) with a respiratory frequency target of 20/min (the TAENA ventilator automatically decreased the pressure support ventilation level by 1 cmH2O every four respiratory cycles if the patient's RR was less than 16/min). Twelve patients were assigned to manual weaning whereas 11 patients were assigned to the automatic pressure support reduction weaning. The weaning mean time for the manual group was 3.18 hours while the weaning mean time for the automatic pressure support reduction group was 2.24 hours. There was no reintubation in both groups.

Non invasive mechanical ventilation delivered by a new 'helmet' versus a standard face mask

We compared in six healthy subjects non invasive mechanical ventilation delivered by a new 'helmet' (large [L] and small [S] size) and by a standard face mask (FM). We tested three different ventilatory setting: a continuous flow CPAP (CPAPCF), a CPAP delivered by a ventilator (CPAPVent) and a pressure support ventilation (PSV). The level of PEEP and PSV were 5 cmH2O while FIO2 was 21%. Tidal volume VT, respiratory rate (RR), work of breathing (WOB L), pressure time product (PTP) were measured. Our results suggest that during CPAP the three systems are equals while during PSV the FM is better. Table

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.

Relationship between work of breathing provided by a ventilator and patients' inspiratory drive during pressure support ventilation; effects of inspiratory rise time.

Inspiratory drive and work of breathing provided by a ventilator (WOBv) during pressure support ventilation (PSV) were examined in 15 patients. At PSV 10 and 15 cm H2O during CPAP 5 cm H2O, patients with low P0.1 (<4.2 cm H2O, n=9) showed WOBv 0.57 and 0.92 J/l, those with high P0.1 (>4.2 cm H2O, n=6) showed 0.31 and 0.62 J/l respectively. WOBv was smaller and pressure-time product of oesophageal pressure (PTP) was significantly larger in high P0.1 patients. Peak inspiratory flow for low P0.1 patients increased as PSV level increased but high P0.1 patients showed no significant change. In a lung model, effects of inspiratory rise time (IRT) and PSV were studied at high and low inspiratory drives by using ventilators with (Servo 300) and without (Mallinckrodt 7200a) adjustable IRT. With 7200a, PSV 10 cm H2O during low drive was compared with PSV 10 and 15 cm H20 during high drive. In Servo 300, PSV 10 cm H2O (IRT 0.6 and 0.0 sec) during low drive was compared with PSV 10 cm H20 (IRT 0.6 and 0.0 sec) and PSV 15 cm H2O (IRT 0.6 sec) during high drive. Raising PSV and shortening IRT both increased peak inspiratory flow. Initial inspiratory flow increased in inverse proportion to IRT, but higher PSV had a little effect. WOBv with high drive was less than with low drive. Higher PSV preserved WOBv by increasing tidal volume. Shortening IRT recruited WOBv by increasing initial inspiratory flow without changing airway pressure and tidal volume. Compared with higher PSV, shorter IRT reduced PTP more. In conclusion, WOBv decreased as inspiratory drive increased due to inability to increase inspiratory flow. Increasing initial inspiratory flow was more effective than raising PSV to preserve inspiratory assistance of PSV at high inspiratory drive.

Effects of different positive end expiratory pressure and pressure support ventilation levels on breathing pattern in acute respiratory failure

Effects of different positive end expiratory pressure and pressure support ventilation levels on breathing pattern in acute respiratory failure

Pressure support ventilation in patients with acute lung injury.

To assess the success rate of pressure support ventilation (PSV) in acute lung injury patients undergoing continuous positive pressure ventilation (CPPV), to study physiologic changes after the transition from CPPV to PSV, and to investigate differences between patients who succeed and patients who fail PSV according to predetermined criteria. Observational study. General intensive care unit in a teaching hospital. We studied 48 patients having acute lung injury, as defined by a PaO2/F(IO2) <300 mm Hg and the presence of bilateral infiltrates on chest radiograph, and ventilated with CPPV. We included patients with PaO2 >80 mm Hg, at positive end-expiratory pressure of <15 cm H2O and with F(IO2) up to 1.0. After enrollment, PSV was instituted and patients were strictly monitored during the following 48 hrs. Subjects who met any of the predefined PSV failure criteria during this period were returned to CPPV (Group F). PSV was continued in the remaining patients (Group S). Gas exchange, respiratory mechanics, and hemodynamics measurements were collected before switching from CPPV to PSV and were repeated at 24 hrs after beginning PSV, or immediately before return to CPPV in Group F patients. The physiologic deadspace volume to tidal volume ratio (V(D)/V(T)) was obtained by the Enghoff's equation from the measurement of the mixed expired CO2 fraction. PSV resulted in a significant PaCO2 decrease (49.2+/-10.9 mm Hg to 44.4+/-7.2 mm Hg) and significant increases in minute volume (V(E))(9.0+/-2.3 L/min to 12.0+/-4.0 L/min) and arterial blood pH (7.405+/-0.054 to 7.435+/-0.064), with stable oxygenation and hemodynamics. In patients who were hypercapnic (PaCO2 >50 mm Hg) during CPPV, the V(E) increase was higher than in normocapnic patients. In the latter patients, PaCO2 and pH did not change significantly going from CPPV to PSV. A total of 38 patients (79%) were allocated to Group S and the remaining 10 patients were included in Group F. In Group S, positive endexpiratory pressure of 9.4+/-2.9 cm H2O (range, 3-14 cm H2O) and a PSV level of 14.9+/-3.8 cm H2O (range, 9-22 cm H2O) were applied. In Group F, positive end-expiratory pressure of 8.9+/-3.1 cm H2O (range, 5-15 cm H2O) and a PSV level of 21.6+/-4.6 cm H2O (range, 16-31 cm H2O) were adopted. Compared with Group S, Group F had a longer duration of intubation (20.2+/-19.2 days vs. 9.2+/-13.5 days), a lower static compliance of the respiratory system (30.4+/-16.5 mL/cm H2O vs. 41.7+/-15.0 mL/cm H2O), and a higher V(D)/V(T) (0.70+/-0.09 vs. 0.52+/-0.10), but similar oxygenation and positive end-expiratory pressure. V(E) was higher in Group F during both CPPV and PSV. In a relatively high proportion of the investigated patients, PSV was successful. The institution of PSV led to no major changes in oxygenation or in hemodynamics. PSV was associated with increases in V(E) and respiratory frequency. In patients who had been hypercapnic during CPPV, PaCO2 decreased despite a compensated pH. Compared with PSV success patients, patients who failed PSV appeared to be sicker, as shown by the higher duration of respiratory support, increased ventilatory needs, and decreased respiratory system compliance, despite similar arterial oxygenation and positive end-expiratory pressure.

Optimal level of pressure support ventilation for recovery from diaphragmatic fatigue in rabbits

To determine whether there is an optimal level of pressure support ventilation (PSV) for recovery from acute diaphragmatic fatigue. Prospective laboratory trial. Experimental laboratory. Twenty healthy adult New Zealand White rabbits. Diaphragmatic fatigue was induced with 50 Hz of phrenic nerve stimulation for 30 mins. Recovery was compared between inspiratory load + PSV of 0 cm H2O (L0), inspiratory load + PSV of 60 cm H2O (L60), inspiratory load + PSV of 80 cm H2O (L80), and PSV of 0 cm H2O without inspiratory load (SB) for 90 mins immediately after the end of the fatigue-inducing procedure. To add inspiratory load during the recovery phase, three pressure threshold valves, each having an opening pressure of -20 cm H2O, were used. After the fatigue-inducing procedure, diaphragmatic electromyogram and transdiaphragmatic pressure remained at baseline in both SB and L60, decreased in L80, and increased in L0. Recovery was assessed by abdominal cavity pressure (Pabd) generated by high-frequency (100 Hz) and low-frequency (20 Hz) stimulation. Pabd at 100 Hz recovered to baseline in L60 and SB but not in L0 and L80 (69.1%, 81.3%, 100.3%, and 100.7% of the baseline at 90 mins for L0, L80, L60, and SB, respectively). Pabd at 20 Hz did not differ among ventilatory settings. There is an optimal range of PSV assist level to improve recovery from diaphragmatic fatigue. Recovery was hampered not only by inadequate PSV but also by excessive PSV.

Prevention of rabbit acute lung injury by surfactant, inhaled nitric oxide, and pressure support ventilation.

Improvement of pulmonary perfusion and blood oxygenation and prevention of acute lung injury (ALI) may rely on ventilation strategy. We hypothesized that application of a combined surfactant, inhaled nitric oxide (iNO), and pressure support ventilation (PSV) should more effectively protect the lungs from injury. Anesthetized and intubated adult rabbits weighing 2.8 +/- 0.3 kg were allowed to breathe room air while receiving oleic acid intravenously (60 microl/kg). Within 90 min this caused a reduction of Pa(O(2)) from 94 +/- 7 to 48 +/- 3 mm Hg and dynamic lung compliance (Cdyn) from 1.59 +/- 0.22 to 0.85 +/- 0.10 ml/cm H(2)O/kg (both p < 0.01), and increase of intrapulmonary shunting (Q S/Q T) from 9.4 +/- 1.2 to 27 +/- 5% (p < 0.05). PSV was subsequently applied with 3 cm H(2)O of continuous positive airway pressure and FI(O(2)) of 0.3, and the animals were randomly allocated to four groups, receiving: (1) PSV only (Control, n = 10); (2) iNO at 20 ppm (NO, n = 9); (3) surfactant phospholipids at 100 mg/kg (Surf, n = 8); and (4) surfactant at 100 mg/kg and iNO at 20 ppm (SNO, n = 8). PSV level was varied to maintain a tidal volume of 8 to 10 ml/kg for another 12 h or until early animal death. Five animals in the SNO, three each in the NO and Surf group, and one in the Control group survived 12 h (SNO versus Control, p < 0.05). The NO, Surf, and SNO groups had significantly improved mean Pa(O(2)) (> 70 mm Hg, p < 0.05), and reduced Q S/Q T (15, 19, and 17%, respectively, p < 0.05) at 6 and 12 h, but not in the Control group. The SNO group had the highest values of Cdyn at 12 h, alveolar aeration and disaturated phosphatidylcholine-to-total protein ratio in bronchoalveolar lavage fluid, and the lowest wet-to-dry lung weight ratio and lung injury score (p < 0.05). The results indicate that early alleviation of ALI by surfactant, iNO, and PSV is due to synergistic effects, and only PSV in this model had limited effects.

Tracheal Pressure Ventilator Control

Tracheal pressure ventilator control (TPVC) is a ventilator mode that relies on tracheal pressure at the carinal end of the endotracheal tube for triggering the ventilator ;;on,'' controlling pressure, and cycling the ventilator ;;off.'' TPVC automatically nullifies imposed resistive work of the breathing apparatus (endotracheal tube plus ventilator) by providing automatic and variable levels of pressure assist. TPVC improves ventilator responsiveness for a spontaneously breathing patient by providing significantly higher peak inspiratory flow rates much closer to that demanded by a patient. TPVC also provides higher assist pressures and flow rates earlier in the breath and thus better-match ventilator-supplied flow to patient-demanded flow than an equivalent level of pressure support ventilation. Matching patient demand for flow to ventilator supply of flow, early in the breath, promotes patient-ventilator synchrony and minimizes work of breathing. We recommend moving the pressure-triggering and control site to the carinal end of the endotracheal tube to provide TPVC.

Automatic control of pressure support mechanical ventilation using fuzzy logic.

There is currently no universally accepted approach to weaning patients from mechanical ventilation, but there is clearly a feeling within the medical community that it may be possible to formulate the weaning process algorithmically in some manner. Fuzzy logic seems suited this task because of the way it so naturally represents the subjective human notions employed in much of medical decision-making. The purpose of the present study was to develop a fuzzy logic algorithm for controlling pressure support ventilation in patients in the intensive care unit, utilizing measurements of heart rate, tidal volume, breathing frequency, and arterial oxygen saturation. In this report we describe the fuzzy logic algorithm, and demonstrate its use retrospectively in 13 patients with severe chronic obstructive pulmonary disease, by comparing the decisions made by the algorithm with what actually transpired. The fuzzy logic recommendations agreed with the status quo to within 2 cm H(2)O an average of 76% of the time, and to within 4 cm H(2)O an average of 88% of the time (although in most of these instances no medical decisions were taken as to whether or not to change the level of ventilatory support). We also compared the predictions of our algorithm with those cases in which changes in pressure support level were actually made by an attending physician, and found that the physicians tended to reduce the support level somewhat more aggressively than the algorithm did. We conclude that our fuzzy algorithm has the potential to control the level of pressure support ventilation from ongoing measurements of a patient's vital signs.

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

BACKGROUNDThe 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...

Use of the rapid/shallow breathing index as an indicator of patient work of breathing during pressure support ventilation

Background . Measuring patient work of breathing (WOB pt) has been suggested to provide safe, aggressive weaning from mechanical ventilation. We compared WOB pt and pressure-time-product (PTP) to routine weaning parameters [breath rate (f), tidal volume (V T), frequency/tidal volume ratio (f/V T)] at different levels of pressure support ventilation (PSV). Methods . Fifteen patients in the surgical intensive care unit requiring prolonged weaning (more than 3 days) were entered in the study. A balloon-tipped esophageal catheter was placed and position confirmed by inspection of pressure and flow waveforms. Each patient was randomly assigned to breathe with 5, 10, 15, and 20 cm H 2O of PSV. After 30 minutes, 40 breaths were recorded and analyzed. Measurement of WOB pt, PTP, f, V T and f/V T were made using the Bicore CP-100 monitor. Mean values for each parameter were calculated. PTP and WOB pt were plotted against f/V T to determine correlation coefficient. Results . PTP, WOB pt, and f/V T decreased in a stepwise fashion as PSV was increased. The f/V T correlated most closely with WOB pt (r = 0.983) and PTP (r = 0.972). Monitoring f alone also correlated with WOB pt (r = 0.894) and PTP (r = 0.881). All patients were weaned from the ventilator (mean duration, 22 ± 5.9 days). Nine patients required tracheostomy before final liberation from the ventilator. Conclusions . Direct measurement of WOB pt is invasive, expensive, and may be confusing to clinicians. Monitoring f/V T may be useful when changing PSV during weaning.

Patient-ventilator interaction and inspiratory effort during pressure support ventilation in patients with different pathologies.

The aim of this study was to evaluate whether pressure support ventilation (PSV) requires different diaphragmatic efforts and patient-ventilator matching, according to the underlying disease. Four groups of patients requiring PSV were studied: Group A, recovering from an episode of acute respiratory failure due to adult respiratory distress syndrome (ARDS); Group B, with postsurgical complications; and two subsets of chronic obstructive pulmonary disease (COPD) patients, with "normal" static compliance of the respiratory system (Cst,rs) (Group C) or elevated Cst,rs (Group D). Ventilatory pattern, transdiaphragmatic pressure (Pdi), the pressure-time product of the diaphragm (PTPdi), static (PEEPi,stat) and dynamic intrinsic positive end-expiratory pressure (PEEPi,dyn), Cst,rs and resistance of the total respiratory system (Rrs) were recorded. The matching between patient and ventilator was analysed, recording the number of "ineffective efforts" (inspiratory efforts not efficient enough to trigger a new ventilator cycle, despite a positive deflection in Pdi). A satisfactory blood gas equilibrium arterial oxygen saturation (Sa,O2 > 93%, with a pH > 7.32) was obtained in the various groups with different levels of PSV. Minute ventilation was found to be significantly higher in Groups A and B, due to the longer expiratory time (tE) in the COPD groups. Group A (2 out of 7), Group B (3 out of 7), Group C (3 out of 5) patients showed sporadic "ineffective efforts". All Group D patients manifested continuous mismatching with the ventilator, so that the pressure-time product of the diaphragm per minute (PTPdi/min), reflecting the metabolic work of the diaphragm, was not different in the four groups. Tidal volume and the spontaneous inspiratory efforts were similar in the four groups, but the number of breaths delivered by the ventilator was significantly higher in Groups A and B. The application of different levels of pressure support ventilation in patients with acute respiratory failure due to different pathologies, led them to breathe with comparable pressure time product of the diaphragm. The majority of the patients showed mismatching with the ventilator, although this effect was more pronounced in the groups with chronic obstructive pulmonary disease.