0734 Does Noise Masking Improve Sleep Consolidation In Patients Weaning From Prolonged Mechanical Ventilation?

Introduction

Sound masking is a noise reduction strategy that adds a mixed-frequency blend of ambient sound to the environment and may improve sleep. Critically ill patients often cite noise as one of the main factors preventing sleep while they are cared for in an intensive care unit (ICU). The effect of sound masking on sleep in patients weaning from prolonged mechanical ventilation is unknown.

Methods

12-hour overnight polysomnography was obtained in eight patients undergoing weaning from prolonged mechanical ventilation. None had hearing impairment, delirium, sedation or agitation. In random order, patients were exposed to sound masking half of the recording time. Noise events were defined a 10dB increase from baseline or any sound peak over 75dB. Arousals or awakenings were attributed to noise if they occurred within 5 seconds of the noise event.

Results

Environmental sound was 61.7± 0.9 dB (mean±SE) during sound masking and 55.9±1.4 dB during no sound masking. During sound masking, there were fewer sound events per hour of sleep when compared to no sound masking (4.1/hr vs 9.3/hr p=0.03). The percentage of sound events leading to a subsequent arousal or awakening with sound masking was less than during no sound masking:11% vs 22% (p=0.04). Arousal index and fragmentation index (arousal and awakenings/hr of sleep) were similar between the two conditions. In a post-study survey, five patients reported improved sleep quality with sound masking while the remaining three reported no difference.

Conclusion

Sound masking decreases sound-induced arousal from sleep in patients being weaned from prolonged mechanical ventilation.

Support

Veterans Administration Research Service

Effect of endotoxin on ventilation and breath variability: role of cyclooxygenase pathway

To evaluate the effects of endotoxemia on respiratory controller function, 12 subjects were randomized to receive endotoxin or saline; six also received ibuprofen, a cyclooxygenase inhibitor, and six received placebo. Administration of endotoxin produced fever, increased respiratory frequency, decreased inspiratory time, and widened alveolar-arterial oxygen tension gradient (all p < or = 0.001); these responses were blocked by ibuprofen. Independent of ibuprofen, endotoxin produced dyspnea, and it increased fractional inspiratory time, minute ventilation, and mean inspiratory flow (all p < or = 0.025). Endotoxin altered the autocorrelative behavior of respiratory frequency by increasing its autocorrelation coefficient at a lag of one breath, the number of breath lags with significant serial correlations, and its correlated fraction (all p < 0.05); these responses were blocked by ibuprofen. Changes in correlated behavior of respiratory frequency were related to changes in arterial carbon dioxide tension (r = 0.86; p < 0.03). Endotoxin decreased the oscillatory fraction of inspiratory time in both the placebo (p < 0.05) and ibuprofen groups (p = 0.06). In conclusion, endotoxin produced increases in respiratory motor output and dyspnea independent of fever and symptoms, and it curtailed the freedom to vary respiratory timing-a response that appears to be mediated by the cyclooxygenase pathway.

Effect of isocapnic hypoxia on variational activity of breathing

In the presence of either hypocapnia or sleep, hypoxia has been shown to induce periodic breathing and increase the total variational activity of breath components. It is not known whether hypoxia induces alterations in breathing variability during wakefulness and in the absence of hypocapnia. To address this issue, we studied nonobtrusively 14 healthy awake subjects before and during the delivery of a hypoxic gas mixture via a plastic hood; the subjects' oxygen saturation decreased from 98 to 79% and end-tidal carbon dioxide tension was kept constant. Compared with air, isocapnic hypoxia increased the gross variability of minute ventilation (V I), tidal volume (VT), inspiratory time (TI), and expiratory time (TE) (all p < 0.004). Isocapnic hypoxia decreased the autocorrelation coefficient at a lag of one breath for TE (p < 0. 008) and V I (p = 0.07), the number of consecutive breath lags having significant autocorrelation coefficients for TE (p = 0.03), and the cycle time of oscillations in V I (p = 0.03). When partitioned, the increase in total variational activity during isocapnic hypoxia was found to result from increases in the random fractions of V I, VT, TI, and TE (all p < 0.05), and the oscillatory fractions of V I, VT, and TE (all p < 0.03). In conclusion, hypoxia induced hidden oscillations in V I, VT, and TE despite wakefulness and an isocapnic state, suggesting that neural responses may have a more important role in the genesis of hypoxia-induced oscillations than previously reported.

Hypercapnic respiratory failure during weaning: neuromuscular capacity versus muscle loads

Patients who fail a weaning trial develop hypercapnia as a result of alveolar hypoventilation, which, in turn, is caused by an imbalance between the respiratory muscle load and capacity. In some patients, especially those with obstructive lung diseases, respiratory muscle performance is impaired as a result of dynamic hyperinflation and paradoxical motion of the rib cage and abdomen. Worsening of pulmonary mechanics causes further embarrassment of the respiratory muscles and can lead to marked alterations of oxygen use by the peripheral tissues. The development of rapid shallow breathing together with worsening of pulmonary mechanics results in inefficient clearance of COcf152cf1 during a failed weaning attempt.

Assessment of neural inspiratory time in ventilator-supported patients

Neural inspiratory time (TI) is a measurement of fundamental importance in studies of patient-ventilator interaction. The measurement is usually based on recordings of flow, esophageal pressure (Pes), and transdiaphragmatic pressure (Pdi), but the concordance of such estimates of neural TI with a more direct measurement of neural activity has not been systematically evaluated. To address this issue, we studied nine ventilator-supported patients in whom we employed esophageal electrode recordings of the diaphragmatic electromyogram (EMG) as the reference measurement of neural TI. Comparison of the indirect estimates of neural TI duration, based on flow, Pes, and Pdi against the reference measurement, revealed a mean difference (bias) ranging from -54 to 612 ms during spontaneous breathing and from -52 to 714 ms during mechanical ventilation; the respective precisions (standard deviations of the differences) ranged from 79 to 175 ms and from 74 to 221 ms. Because an indirect estimate of neural TI duration could be identical to that of the reference measurement and yet be displaced in time, this lag or lead was quantified as the phase angle of neural TI onset. Flow-based estimates of the onset of neural TI displayed a systematic lag, which may be explained at least in part by concurrent intrinsic positive end-expiratory pressure. In conclusion, the indirect estimates of the onset and duration of neural TI in ventilator-dependent patients displayed poor agreement with the diaphragmatic EMG measurement of neural TI.

Advances in respiratory monitoring during mechanical ventilation

This review provides an update on the various techniques that are available to monitor patients during mechanical ventilation with an emphasis on clinical observations and applications in critically ill patients.

Continuous recordings of mixed venous oxygen saturation during weaning from mechanical ventilation and the ramifications thereof

To define the importance of hemodynamic performance and global tissue oxygenation in determining weaning outcome, we recorded mixed venous oxygen saturation (SvO2) continuously in eight ventilator-supported patients who failed a trial of spontaneous breathing and 11 patients who tolerated a trial and were successfully extubated. Immediately before the weaning trial, SvO2 was not statistically different in the two groups (p = 0.28). On discontinuation of the ventilator, SvO2 fell progressively in the failure group (p < 0.01), whereas it did not change in the success group. During the trial of spontaneous breathing, O2 demand was similar in the two groups, but it differed in the manner with which it was met. The success group demonstrated an increase in cardiac index (p < 0.05) and O2 transport (p < 0.02). The failure group did not increase O2 transport, partly because of elevations in right- and left-ventricular afterload, but, instead, increased O2 extraction ratio (p < 0.02) with a consequent fall in SvO2. In turn, the low SvO2 combined with greater venous admixture (p < 0.0006) led to rapid arterial desaturation (p < 0.006) and a relative decrease in O2 being supplied to the tissues. In conclusion, ventilator-supported patients who failed a trial of spontaneous breathing developed a progressive decrease in SvO2 caused by the combination of a relative decrease in convective O2 transport and an increase in O2 extraction by the tissues.

Cycling of inspiratory and expiratory muscle groups with the ventilator in airflow limitation

Research on patient-ventilator interactions has largely focused on inspiratory events, with little attention paid to expiration. We sought to determine the importance of the timing and magnitude of expiratory muscle activity in causing patient-ventilator dyssynchrony. Our study was done with healthy subjects receiving pressure support in whom we induced airflow limitation with a Starling resistor. The timing and magnitude of expiratory muscle activity were obtained by wire electromyographic recording of the activity of the transversus abdominis muscle, and were compared with the cycling of the ventilator and inspiratory muscle activity as determined from a flow tracing and diaphragmatic electromyogram (EMG), respectively. Induction of airflow limitation produced significant phase differences in the cycling of the subjects' expiratory muscle group and that of the machine. Some inspiratory efforts failed to trigger the ventilator, owing in part to an increase in elastic recoil consequent to the commencement of expiratory efforts before the termination of mechanical inflation. A delay in relaxation of the expiratory muscles did not interfere with the success of subsequent inspiratory efforts to trigger the ventilator. We also investigated the accuracy of two approaches for distinguishing between the contributions of expiratory muscle activity and elastic recoil to intrinsic positive end-expiratory pressure (PEEPi): the expiratory increase in gastric pressure (Pga) correlated better with transversus abdominis electromyographic activity (r = 0.7 to 0.95) than did the early inspiratory decrease in Pga (r = 0.04 to 0.53). In conclusion, the continuation of mechanical inflation into neural expiration was associated with failure of the subsequent inspiratory attempt to trigger the ventilator.

Monitoring patient mechanics during mechanical ventilation

This article provides a review of respiratory mechanics that can be monitored in ventilator-dependent patients during passive and spontaneous breathing. Special focus is placed on resistance, compliance, and work of breathing. A description of methods and techniques, and a summary of clinical observations and applications in critically-ill patients are also included.

Sparfloxacin vs ofloxacin in the treatment of acute bacterial exacerbations of chronic bronchitis: a multicenter, double-blind, randomized, comparative study. Sparfloxacin Multicenter ABECB Study Group

STUDY OBJECTIVE

Comparison of efficacy and safety of sparfloxacin vs ofloxacin for treatment of acute bacterial exacerbations of chronic bronchitis (ABECB).

DESIGN

Multicenter, double-blind, randomized study.

SETTING

Sixty-eight private offices and outpatient clinics in the United States and Canada.

PATIENTS

Seven hundred ninety-eight adults with ABECB, as confirmed by the acute onset of new (or worsened from the immediate premorbid state) cough and sputum production.

INTERVENTIONS

Randomization 1:1 to sparfloxacin, 400 mg on day 1, then 200 mg once daily, or ofloxacin, 400 mg twice daily, with matching comparator placebos, given concurrently for 10 consecutive days.

RESULTS

The primary efficacy parameter was overall response in the bacteriologically evaluable population. Overall success rates in this population were 85.3% and 89.3% for sparfloxacin and ofloxacin, respectively. The two-sided 95% confidence interval was -9.9, 1.9, indicating that sparfloxacin was statistically equivalent to ofloxacin. The all-treated population analysis was similar to that in the evaluable population. Bacterial eradication rates were similar in both treatment groups for Haemophilus influenzae, Streptococcus pneumoniae, Moraxella catarrhalis, Chlamydia pneumoniae, Haemophilus parainfluenzae, Klebsiella pneumoniae, Enterobacter cloacae, and Staphylococcus aureus. The frequency of adverse events overall was comparable in the two treatment groups. The sparfloxacin group had a lower frequency of digestive and nervous system adverse events, but a higher frequency of photosensitivity reactions than the ofloxacin group.

CONCLUSIONS

Once-daily oral treatment with 200 mg sparfloxacin (after initial 400 mg dose) is as effective as twice-daily treatment with 400 mg ofloxacin in patients with ABECB.

Partitioning of lung and chest-wall mechanics before and after lung-volume-reduction surgery

In the study reported here, we partitioned the mechanics of the respiratory system into lung and chest-wall components, using the rapid occlusion technique in seven patients with severe emphysema before lung-volume-reduction surgery and 3 mo later. Patients showed improvements in 6-min walk (p < 0.01) and dyspnea (p < 0.05). The resistances of the respiratory system and chest wall were not altered by surgery. Ohmic airway resistance did not change, but the component of lung resistance (DeltaRL) due to viscoelastic behavior (stress relaxation) and time-constant inhomogeneities (pendelluft) decreased in six patients (p < 0.03). Dynamic elastance of the lung (Edyn,L) decreased after surgery (p < 0.02), whereas dynamic elastance of the chest wall did not change. The ratio of dynamic intrinsic positive end-expiratory pressure (PEEPi) to static PEEPi, which also reflects viscoelastic properties and time-constant inhomogeneities, increased after surgery (p < 0.05). The decrease in dyspnea was related to the decrease in Edyn,L (r = 0.81, p = 0.03), and tended to be related to the decrease in DeltaRL (r = 0.71, p = 0. 07). In conclusion, lung-volume-reduction surgery decreased dynamic pressure dissipations caused by stress relaxation and time-constant inhomogeneities within lung tissue, and it had no effect on the static mechanical properties of the chest wall.

Effect of resistive loading on variational activity of breathing

To examine the effect of resistive loading on variational activity of breathing, we studied 18 healthy subjects breathing at rest and with inspiratory resistive loads of 3 and 6 cm H2O/L/s, applied randomly for 1 h each. Compared with resting breathing, a resistive load of 3 cm H2O/L/s decreased the total variational activity of expiratory time (TE) and minute ventilation (V I), whereas a load of 6 cm H2O/L/s increased the total variational activity of inspiratory time (TI). Compared with the load of 3 cm H2O/L/s, the load of 6 cm H2O/L/s increased total variational activity of tidal volume (VT), TI, TE, and V I. Partitioning of the total variational activity revealed that these alterations were due to changes in the random uncorrelated fraction. Compared with rest, both the resistive loads of 3 and 6 cm H2O/L/s increased the number of breath lags displaying significant serial correlations ("short-term memory") of TI. Compared with rest, the load of 3 cm H2O/L/s increased the autocorrelation coefficient at a lag of one breath for VT and the load of 6 cm H2O/L/s increased the correlated fraction of variational activity of VT. Thus, three measures of correlated behavior-autocorrelation coefficient at a lag of 1 breath, "short-term memory," and the correlated fraction of total variational activity- increased with loading. In conclusion, resistive loading changed total variational activity according to the size of the load: the random fraction decreased with the smaller load but increased with the larger load; in contrast, correlated behavior increased with both loads. The different behaviors of random and correlated variability with loading may reflect different physiologic influences on respiratory control.

Effect of lung volume reduction surgery on neuromechanical coupling of the diaphragm

The mechanisms for symptomatic improvement following lung volume reduction surgery for emphysema are poorly understood. We hypothesized that enhanced neuromechanical coupling of the diaphragm is an important factor in this improvement. We studied seven patients with diffuse emphysema before and 3 mo after surgery. Patients showed improvements in 6-min walking distance (p = 0.002) and dyspnea (p = 0.04). The pressure output of the respiratory muscles, quantified as pressure-time product per minute (PTP/min), decreased after surgery (p = 0.03), as did PaCO2 (p = 0.02). Maximal transdiaphragmatic pressures (Pdi(max)) increased from 80.3 +/- 9.5 (SE) to 110.8 +/- 9.3 cm H2O after surgery (p = 0.03), and the twitch transdiaphragmatic pressure response to phrenic nerve stimulation (Pdi(tw)) increased from 17.2 +/- 2.4 to 25.9 +/- 3.0 cm H2O (p = 0.02); these increases were greater than could be accounted for by a decrease in lung volume. The contribution of the diaphragm to tidal breathing, assessed by relative changes in gastric and transdiaphragmatic pressures, increased after surgery (p = 0.008). Net diaphragmatic neuromechanical coupling, quantified as the quotient of tidal volume (normalized to total lung capacity) to tidal change in Pdi (normalized to Pdi(max)), improved after surgery (p = 0.03) and was related to the increase in 6-min walking distance (r = 0.86, p = 0.03) and decrease in dyspnea (r = 0.76, p = 0.08). In conclusion, lung volume reduction surgery effects an improvement in diaphragmatic function, greater than can be accounted for by a decrease in operating lung volume, and enhances diaphragmatic neuromechanical coupling.

Respiratory muscle dysfunction in mechanically-ventilated patients

The interaction between a patient and a ventilator is the major determinant of the amount of respiratory muscle rest achieved by the machine. We are beginning to acquire a better understanding of the mechanisms that underlie this complex interaction, but this information has yet to be integrated into the routine clinical management of ventilator-supported patients. To achieve that goal, we need better techniques of detecting and monitoring patient-ventilation asynchrony, and the development of simple algorithms that can minimize its occurrence. Finally, research is needed to determine the occurrence and importance of respiratory muscle fatigue during failed weaning attempts so as to better guide the timing and pace of the weaning process in problematic patients.

Effects of aerosolized surfactant in patients with stable chronic bronchitis: a prospective randomized controlled trial

CONTEXT

Chronic bronchitis, estimated to affect more than 13 million adults in the United States, is characterized in part by retention of airway secretions, but no approved or effective therapy for airway mucus retention in patients with chronic bronchitis has been established. Surfactant reduces sputum adhesiveness, which contributes to difficulty in clearing secretions, but surfactant has not been tested in patients with chronic bronchitis.

OBJECTIVE

To examine the effects of exogenous surfactant on sputum clearance and pulmonary function in patients with stable chronic bronchitis.

DESIGN

A prospective, multicenter, randomized, double-blind, parallel-group, placebo-controlled comparison of the effects of 2 weeks of treatment with 3 doses of aerosolized surfactant (palmitoylphosphadidylcholine [DPPC]) or saline (placebo).

SETTING

Four US teaching hospitals.

PARTICIPANTS

A total of 87 adult patients with the diagnosis of stable chronic bronchitis.

MAIN OUTCOME MEASURES

Pulmonary function, respiratory symptoms, and sputum properties before treatment (day 0), after 2 weeks of treatment (day 14), and 7 days after stopping treatment (day 21).

RESULTS

A total of 66 patients were randomized to surfactant treatment and 21 to saline treatment. Patient demographic characteristics between groups were similar at baseline. In patients who received a DPPC dose of 607.5 mg/d for 2 weeks, prebronchodilator forced expiratory volume in 1 second (FEV1) increased from 1.22 L (SEM, 0.08 L) at day 0 to 1.33 L (SEM, 0.09 L) at day 21 (P=.05), an improvement of 11.4%; postbronchodilator FEV1 improved 10.4% by days 14 and 21 (P=.02); and the ratio of residual volume to total lung capacity, a measure of thoracic gas trapping, decreased 6.2% by day 21 (P=.009). In the surfactant groups, there was a dose-dependent increase in the ability of sputum to be transported by cilia in vitro.

CONCLUSION

Aerosolized surfactant improved pulmonary function and resulted in a dose-related improvement in sputum transport by cilia in patients with stable chronic bronchitis.

Effect of hyperoxic hypercapnia on variational activity of breathing

Dysrhythmias of breathing occur in several clinical disorders, but their mechanistic basis is obscure. To understand their pathophysiology, factors responsible for the variability of breathing need to be defined. We studied the effect of hyperoxic hypercapnia (CO2) on the variational activity of breathing in 14 volunteers before and after delivering CO2 nonobstrusively via a plastic hood. Compared with air, CO2 increased the gross variability of minute ventilation (VI) and tidal volume (VT), and decreased that of inspiratory time (TI) and expiratory time (TE) (all p < 0.03). CO2 increased the autocorrelation coefficient at a lag of one breath for VI (p < 0.05), the number of consecutive breath lags having significant autocorrelation coefficients for VI and VT (both p < 0.01), and the cycle time of oscillations in VI (p = 0.03) and VT (p = 0.04). Uncorrelated random behavior constituted > or = 80% of the variance of each breath component, correlated behavior represented 9 to 20%, and oscillatory behavior represented < 1% during both air and CO2. CO2 increased the correlated behavior of volume components, which was accompanied by development of low-frequency oscillations with a cycle time consistent with central chemoreceptor activation.

Comparison of assisted ventilator modes on triggering, patient effort, and dyspnea

In 11 ventilator-dependent patients, we undertook a head-to-head comparison of patient-ventilator interaction during four ventilator modes: assist-control ventilation (ACV), intermittent mandatory ventilation (IMV), pressure support (PS), and a combination of IMV and PS. Progressive increases in IMV rate and PS level each decreased inspiratory pressure-time product (PTP) (p < 0.0001). These reductions in PTP were greater with PS than with IMV at lower but proportional levels of maximal assistance (p < 0.005). When PS 10 cm H2O was added to a given level of IMV, greater reductions in PTP were achieved not only during intervening (PS) breaths (p < 0.001), but also during mandatory (volume-assisted) breaths (p < 0.0005); this additional unloading during mandatory breaths was proportional to the decrease in respiratory drive (dP/dt) during intervening breaths (r = 0.67, p < 0.0001). Maximal unloading occurred with ACV, achieving more than a fivefold decrease in PTP compared with unassisted breathing. Decreases in PTP were confined to the post-trigger phase, and PTP of the post-trigger phase correlated with dP/dt (r = 0.78, p < 0.0001). Effort during the trigger phase remained constant despite marked changes in drive and intrinsic positive end-expiratory pressure (PEEPi). Ineffective triggering occurred with all modes, and wasted PTP increased with increasing levels of assistance as a result of the accompanying decrease in drive and increase in volume. Breaths preceding nontriggering efforts had shorter respiratory cycle times (p < 0.0005) and expiratory times (p < 0.0001) and higher PEEPi (p < 0.0001), indicating that neural-mechanical asynchrony resulted from inspiratory activity commencing prematurely before elastic recoil pressure had fallen to a level that could be overcome by a patient's muscular effort. Thus, increases in the level of ventilator assistance produced progressive decreases in inspiratory muscle effort and dyspnea,which were accompanied by increases in the rate of ineffective triggering.

Effect of elastic loading on variational activity of breathing

To examine the effect of elastic loading on variational activity of breathing, we studied 11 healthy subjects breathing at rest and with inspiratory elastic loads of 9 and 18 cm H2O/L, applied randomly for 1 h each. Compared with rest, a load of 18 cm H2O/L decreased gross variability, quantitated as standard deviation, of tidal volume (VT) and expiratory time (TE) (p < 0.01 in both instances) but increased that of inspiratory time (TI) (p < 0.03). The autocorrelation coefficients at a lag of 1 breath for each breath component were not altered by elastic loading, although the number of breath lags with significant serial correlations for TE tended to increase with a load of 18 cm H2O/L (p = 0.08). A load of 18 cm H2O/L decreased only the fraction of variational activity of VT and TE due to uncorrelated, random behavior (white noise), while it increased that fraction for TI (p < 0.05 in each instance); the correlated and oscillatory fractions did not change. Uncorrelated random behavior constituted > 87% of the variance of each breath component, correlated behavior represented 3 to 11%, and oscillatory behavior represented < 1.5% during both rest and loaded breathing. Elastic loading changed the gross variability of each primary breath component by altering the random fraction of variational activity; it had no significant effect on the structured, correlated fraction. We speculate that the observed changes in variational activity may reflect an attempt by the controller to compensate for the increased load while simultaneously minimizing load-induced dyspnea.

Pathophysiologic basis of acute respiratory distress in patients who fail a trial of weaning from mechanical ventilation

To determine the mechanisms of acute respiratory distress and failure in patients with chronic obstructive pulmonary disease (COPD), we studied 17 ventilator-supported patients who failed a trial of spontaneous breathing and 14 patients who tolerated such a trial and were successfully extubated. Immediately before the weaning trials, maximal inspiratory pressure was not statistically different between the two groups (p = 0.48). On discontinuation of the ventilator, the failure group immediately developed rapid shallow breathing, and higher values of dynamic lung elastance (EdynL) (p < 0.01) and intrinsic positive end-expiratory pressure (PEEPi, p < 0.03) than did the success group. Between the onset and end of the trial, the failure group developed further increases in EdynL (p < 0.0001) and PEEPi (p < 0.0001), and increases in inspiratory resistance (p < 0.009) and inspiratory pressure-time product (PTP) (p < 0.0001). Partitioning of PTP at the end of the trial revealed a 111% increase in the PEEPi component, a 33% increase in the non-PEEPi elastic component, and a 42% increase in the resistive component (all p < 0.0001). Despite the increase in PTP, 13 of the failure patients developed an increase in PaCO2. The product of PTP and PaCO2, an index of inefficient CO2 clearance, was more than twice as high in the failure group than in the success group at the end of the trial (p < 0.0005). Thus, development of acute respiratory distress during a failed weaning attempt was due to worsening of pulmonary mechanics, which in conjunction with rapid shallow breathing led to inefficient clearance of CO2.

Passive mechanics of lung and chest wall in patients who failed or succeeded in trials of weaning

In an accompanying article (Jubran, et al., Am. J. Respir. Crit. Care Med. 155:906-915), we report that patients with chronic obstructive pulmonary disease (COPD) who failed a trial of weaning from mechanical ventilation developed worsening of pulmonary mechanics compared with patients who tolerated the trial and were extubated. We wondered whether the greater derangements in pulmonary mechanics in the weaning failure patients are evident ever before undertaking the weaning trial. We measured mechanics of the respiratory system, lung, and chest wall during passive ventilation at usual ventilator settings in 12 patients who went on to fail a weaning trial and in 12 patients who were successfully weaned. No differences in the resistances of the respiratory system, lung, and chest wall were observed between the two groups or when the resistances were separated into the components derived from ohmic resistance and viscoelastic behavior/time-constant inhomogeneities. Likewise, the groups did not differ in terms of static elastance and dynamic intrinsic positive end-expiratory pressure (PEEPi) of the respiratory system and the respective lung and chest wall components or in terms of dynamic elastances of the respiratory system and chest wall. The failure group had a higher dynamic elastance of the lung than the success group (p < 0.01), but the individual values showed considerable overlap among the patients in the two groups so limiting its usefulness in signaling a patient's ability to sustain spontaneous ventilation. Thus, mechanics of the respiratory system and its lung and chest wall components during passive ventilation did not satisfactorily discriminate between patients who failed a weaning trial and those successfully weaned, and, thus, are unlikely to be useful in signaling a patient's ability to tolerate the discontinuation of mechanical ventilation.

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.

Dose-response to bronchodilator delivered by metered-dose inhaler in ventilator-supported patients

In nonintubated patients, metered-dose inhalers (MDIs) are accepted as the most convenient, efficient, and cost effective method of administering inhaled bronchodilators. Recent studies have demonstrated the efficacy of MDIs in ventilator-supported patients; however, the optimal dose of a bronchodilator from a MDI is unknown. We determined the response to increasing doses of albuterol administered by a MDI and cylindrical spacer to 12 mechanically ventilated patients with chronic obstructive pulmonary disease (COPD). Four, eight, and 16 puffs of albuterol were given at 15-min intervals. Rapid airway occlusion were performed before and at 5-min intervals after albuterol for 80 min. Respiratory mechanics were also measured for 60 min in another group of seven patients with COPD who received four puffs of albuterol. Significant decrease in airways resistance occurred after administration of albuterol (p < 0.001). The decrease in airway resistance with four puffs of albuterol was comparable to that observed with cumulative doses of 12 puffs (p = 0.12) and 28 puffs (p = 0.25). Heart rate increased significantly (p < 0.01) after a cumulative dose of 28 puffs. The decrease in airway resistance was sustained for 60 min in the group that received only four puffs of albuterol (p < 0.003). In conclusion, four puffs of albuterol given by a MDI and spacer provided the best combination of bronchodilator effect and safety in stable mechanically ventilated patients with COPD.

Interrelationship of breath components in neighboring breaths of normal eupneic subjects

To determine the fraction of variational activity that is correlated on a breath-to-breath basis from uncorrelated random fluctuations, we performed autocorrelation analysis in 33 normal subjects during resting breathing. A calibrated inductive plethysmograph was used to nonobtrusively record 700 breaths in each subject. The group mean autocorrelation coefficients at a lag of 1 breath for each of the three primary breath components, tidal volume (VT), inspiratory time (TI), and expiratory time (TE), were significantly different from zero (p < 0.001). The autocorrelation coefficients for VT, 0.295 +/- 0.148 (SD), and TE, 0.259 +/- 0.121, were greater than that for TI, 0.201 +/- 0.135 (p < 0.001 and p < 0.01, respectively). The autocorrelation coefficients for each breath component remained significant for approximately 3 consecutive breaths (p < 0.001), indicating the presence of "short-term memory." Cross-correlation analysis revealed significant interrelationships (p < 0.001) for all component irrespective of which component was leading or following, with the exception of the pairing of VT in the leading breath and TI in the subsequent breath. In conclusion, in resting healthy subjects breath components display considerable breath-to-breath variability that is not completely random in nature, but which, instead, has a significant fraction of structured correlated variational activity.

Using physiologic end points to assess innovations in mechanical ventilation

It is our view that new ventilatory methods should be withheld from clinical practice until there has been adequate evaluation of their effect on physiologic variables and the link to long-term outcomes has been established. In the past, premature and over-enthusiastic acceptance of ventilatory strategies may have resulted in patient discomfort and even harm, and this can be minimized by a more careful evaluation of the physiologic effects of such innovations before their acceptance into clinical practice.

Variability of patient-ventilator interaction with pressure support ventilation in patients with chronic obstructive pulmonary disease

In 12 patients with chronic obstructive pulmonary disease (COPD) receiving pressure support ventilation (PSV), we studied the variability of respiratory muscle unloading and defined its physiologic determinants using a modified pressure-time product (PTP). Inspiratory PTP/min decreased as PSV was increased (p < 0.001), but there was considerable interindividual variation: coefficients of variations of up to 96%. On multiple linear regression analysis, 73 to 83% of the variability in inspiratory PTP was explained by inspiratory resistance, minute ventilation, and intrinsic positive end-expiratory pressure. Taking an inspiratory PTP/min of < 125 cm H2O.sec/min to represent a desirable level of inspiratory effort during PSV, a respiratory frequency of < or = 30 breaths/min was more accurate than a tidal volume > 0.6 L in predicting this threshold (p < 0.001). At PSV of 20 cm H2O, expiratory effort, quantitated by an expiratory PTP, was clearly evident in five patients before the cessation of inspiratory flow, signifying that the patient was "fighting" the ventilator; of note, these five patients had a frequency of < or = 30 breaths/min. In conclusion, patient-ventilator interactions in patients with COPD are complex, and events in expiration need to be considered in addition to those of inspiration.

Bronchodilator delivery by metered-dose inhaler in ventilator-supported patients

The optimal dose and technique for administration of bronchodilators with a metered-dose inhaler (MDI) in mechanically ventilated patients have not been established. We studied the efficacy and safety of 10 puffs (90 micrograms/puff) of albuterol administered by an MDI in seven mechanically ventilated patients with chronic obstructive pulmonary disease (COPD). Rapid airway occlusions at constant flow inflation were performed before and at 5-min intervals after administration of albuterol for 60 min. Significant decreases in maximum (Rrsmax; p < 0.01) and minimum inspiratory resistance (Rrsmin; p < 0.01) were present at 5 min and persisted for 60 min after administration of albuterol (p < 0.01 for both parameters). Rrsmax indicates maximal inspiratory resistance while Rrsmin represents the ohmic flow resistance. Intrinsic positive end-expiratory pressure decreased significantly (p < 0.05) 15 min after albuterol administration. Heart rate, blood pressure, and arterial oxygenation did not show significant change after albuterol. In summary, 10 puffs of albuterol given by an MDI and spacer produced significant bronchodilation in ventilator-supported patients with COPD, without producing side effects. In conclusion, higher doses of albuterol given by an MDI and spacer could be used routinely in mechanically ventilated patients with COPD.

Pathophysiology of failure to wean from mechanical ventilation

Weaning patients from mechanical ventilation constitutes a major portion of the workload in an intensive care unit, as over 40% of total ventilator time is consumed by the weaning process. Several pathophysiological mechanisms may be responsible for weaning failure, but the precise role of each is incompletely understood. Patients who fail a weaning trial commonly develop hypercapnia, which appears to be due to decreased tidal volume rather than a primary decrease in respiratory drive. Respiratory muscle performance is impaired as a result of dynamic hyperinflation and paradoxic motion of the rib cage and abdomen. Worsening of pulmonary mechanics will cause further embarrassment of the respiratory muscles. However, the clinical importance of respiratory muscle fatigue remains unclear. Afferent stimuli arising in the lung parenchyma, respiratory muscles, or as a consequence of impaired gas exchange will be transmitted to the respiratory control centers and result in severe dyspnea in patients who fail a weaning trial.

Use of flow-volume curves in detecting secretions in ventilator-dependent patients

A noninvasive means of detecting airway secretions in ventilator-dependent patients is desirable because endotracheal suctioning can result in life-threatening complications. In a patient who had copious secretions, we observed a sawtooth pattern on his flow-volume curve that disappeared after suctioning. Accordingly, we systematically examined the usefulness of a sawtooth pattern on flow-volume curves in detecting secretions in ventilator-dependent patients and compared its accuracy with clinical examination. Flow-volume curves were recorded in 50 ventilator-dependent patients over 1 min of spontaneous breathing. In 15 of these patients, clinical examination was performed by three clinicians to determine its accuracy in detection of secretions. Endotracheal suctioning was then performed to determine the presence or absence of secretions. Subsequently, the flow-volume curves of all 50 patients were played back on a video screen, and three observers, who were unaware of the results of suctioning, made a decision regarding the presence or absence of a sawtooth pattern. The sensitivity of the sawtooth pattern in detecting secretions ranged from 0.76 to 0.86, and specificity ranged from 0.86 to 0.90. The likelihood ratio of a positive test ranged from 5.55 to 7.97, whereas the likelihood ratio of a negative test ranged from 0.16 to 0.27. Interobserver agreement, assessed by the kappa statistic, was excellent: 0.76, 0.76, and 0.84. In the subgroup of patients evaluated by both clinical examination and flow-volume curve analysis, clinical examination was less accurate in 11 of the 15 patients. In conclusion, detection of a sawtooth pattern strongly suggests the presence of secretions, and the absence of this pattern suggests that secretions are unlikely to be present.

Comparative cost-effectiveness analysis of theophylline and ipratropium bromide in chronic obstructive pulmonary disease. A three-center study

The charts of 311 patients receiving theophylline (T) and 289 patients receiving ipratropium bromide (IB) for COPD were reviewed to determine the total costs and cost-effectiveness of these 2 agents in 3 different health-care settings. A direct cost-accounting method assessed cost, and a Markov decision-analysis model calculated cost-effectiveness. Costs to treat toxic effects were greater for T versus IB. The types and incidences of toxic effects, by drug, were similar among the three centers. Overall costs for T were $121.40 per patient per therapy-month versus $84.56 per patient per therapy-month for IB, as determined by the cost-accounting method. The marginal cost was $366 for T over IB when extrapolated over 1 year using the Markov model. The Markov model also predicted that patients receiving IB had a greater number of complication-free therapy-months (measurement of effectiveness) than patients receiving T. We conclude that treatment with IB was less costly and more cost-effective than T.

Response time, autonomic mediation, and reversibility of hyperoxic bradycardia in conscious dogs

Normobaric hyperoxia decreases heart rate (HR) in humans and animals. This study explored the mechanisms of hyperoxic bradycardia by examining its response time, autonomic neural mediation, and reversibility in conscious dogs. Five trained mongrel dogs breathed from a mask as the inspired gas was alternated between air and O2 for multiple cycles, and continuous time series records of HR and oxyhemoglobin saturation were recorded on a digital computer and analyzed by the technique of ensemble averaging. Hyperoxia decreased HR by 9% (P < 0.001), but only gradually, requiring 5 min to reach steady state. This delay was much longer than the time required for hyperoxic respiratory depression (10-20 s), a response known to be mediated by chemoreceptor reflexes. The bradycardia was sustained for > or = 30 min. On return to normoxia, HR gradually returned toward, but failed to reach, the baseline HR, suggesting incomplete reversibility of the response. However, in control experiments without hyperoxic challenge, HR showed a slow continuous downward trend that was sufficient to account for the apparent incomplete reversibility of hyperoxic bradycardia. Hyperoxic bradycardia was unaffected by beta-adrenergic blockade but was completely prevented by muscarinic cholinergic blockade. We conclude that 1) hyperoxia-induced bradycardia in conscious dogs is mediated by efferents of the vagus nerve; 2) its afferent pathway remains unknown, but its long response time suggests mechanisms other than chemoreceptor reflexes or other known neural reflexes; and 3) it is completely reversible.

The effect of hyperinflation on rib cage-abdominal motion

Abnormalities of rib cage-abdominal motion are common in patients with chronic obstructive pulmonary disease (COPD), but the basis of the abnormal motion has not been completely determined. Although airway obstruction has been shown to be a major factor in causing abnormal chest wall motion, the effect of hyperinflation (which has numerous adverse effects on respiratory muscle function) has not been systematically examined. We induced graded levels of hyperinflation in six healthy volunteers using continuous positive airway pressure (CPAP) levels of 10, 20, and 30 cm H2O. Chest wall motion was measured by a calibrated inductive plethysmograph. Rib cage-abdominal asynchrony and paradox were quantitated by the Konno-Mead method of analysis. CPAP levels of 10, 20, and 20 cm H2O produced increases in end-expiratory lung volume of 0.98 +/- 0.14 (SE), 1.90 +/- 0.31, and 2.42 +/- 0.37 L, respectively (p < 0.0001). This corresponded to an increase in the ratio of functional residual capacity to predicted total lung capacity from 0.38 +/- 0.08 at baseline to 0.74 +/- 0.14 at 30 cm H2O CPAP-comparable to that seen in patients with COPD. Hyperinflation induced an increase in inspiratory abdominal paradox, 1.0 +/- 0.7% at baseline versus 3.6 +/- 1.7% at 30 cm H2O (p < 0.05), but this is unlikely to be clinically significant. A significant increase in asynchrony or rib cage paradox did not develop with hyperinflation. In conclusion, the primary factor contributing to abnormal chest wall motion in patients with COPD is likely to be increased airway resistance, and hyperinflation makes only a minor contribution.

Experimental polyacrylamide-induced acute injury in rat lung

We recently reported the first case of accidental aspiration of polyacrylamide occurring in a 26-year-old man. The patient developed severe airway obstruction and parenchymal lung damage and died. Autopsy revealed numerous polyacrylamide particles in his lungs, as well as extensive bronchiolar and alveolar damage. Gas chromatographic and mass spectrometric assessment of the lung tissue failed to reveal polyacrylamide activity, although assessment of the suspending solvent of the polyacrylamide showed a pattern characteristic of an aliphatic hydrocarbon mixture with a prominent dodecane peak. This experimental study was performed to determine the nature and extent of damage to rat bronchial and alveolar epithelia following endotracheal instillation of polyacrylamide, hydrocarbon mixture (petroleum distillate), dodecane (C12H26), or normal saline. The rat lungs were examined grossly and microscopically 10 min and 24, 72, and 96 h after endotracheal instillation, following inflation and fixation with 10 percent buffered formaldehyde. Gross examination revealed congested, mottled visceral pleural surfaces in the rats treated with polyacrylamide and dodecane. There were no pleural exudates or effusions. Microscopically, vascular engorgement, bronchiolitis, and focal pneumonia were observed. Vascular engorgement was most pronounced at 72 to 96 h in rat lungs treated with polyacrylamide and dodecane and was moderate at 24 h in rats treated with petroleum distillate. Focal organizing pneumonia was marked at 96 h in rats treated with petroleum distillate, at 72 h in those treated with polyacrylamide, and at 24 h in those treated with dodecane. The saline-treated control animals showed no change. Our findings suggest that polyacrylamide, dodecane, and petroleum distillate are strong irritants to the airways. However, a direct obstructive/mechanical effect of the polyacrylamide upon the airway has not been excluded. Airway exposure to polyacrylamide may result in lung injury secondary to the polyacrylamide itself, its suspending agents, or both.

Aspiration injury due to polyacrylamide

Acute lung injury secondary to aspiration of polyacrylamide, a synthetic polymer used widely in industry, has not been previously described in man or animal. We report the case of a 26-year-old man who aspirated polyacrylamide gel while cleaning it out of a tank truck. Subsequently, severe airway obstruction and lung parenchymal damage developed, and the patient died. At autopsy, numerous polyacrylamide particles were found in the lungs, along with extensive bronchiolar and alveolar damage.

Reversal of endotoxin-mediated shock by NG-methyl-L-arginine, an inhibitor of nitric oxide synthesis

Septic shock is a life-threatening condition that results from exposure to bacterial endotoxin. It is manifested by cardiovascular collapse and mediated by the release of cytokines such as tumor necrosis factor. Some of these cytokines cause the release of vasoactive substances. In the present study, administration of 40 microgram/kg of bacterial endotoxin to dogs caused a 33% decrease in peripheral vascular resistance and a 54% fall in mean arterial blood pressure within 30 to 90 minutes. Vascular resistance and systemic arterial pressure returned to normal within 1.5 minutes after intravenous administration of NG-methyl-L-arginine (20 mg/kg), a potent and selective inhibitor of nitric oxide synthesis. L-Arginine reversed the effect of L-NMA and restored the endotoxin-induced hypotension. Although NG-methyl-L-arginine injection increased blood pressure in control dogs, the hypertensive effect was much greater in endotoxemic dogs (24.8 +/- 2.7 mmHg vs 47.8 +/- 6.8 mmHg, p = 0.01, n = 4). NG-Methyl-L-arginine caused only a modest increase in blood pressure in dogs made hypotensive by continuous intravenous infusion of nitroglycerin (17.1 +/- 5.0 mm Hg, n = 3). These findings suggest that nitric oxide overproduction is an important contributor to endotoxic shock. Moreover, our findings demonstrate for the first time, the utility of nitric oxide synthesis inhibitors in endotoxic shock and suggest that such inhibitors may be of therapeutic value in the treatment of septic shock.

Reliability of pulse oximetry in titrating supplemental oxygen therapy in ventilator-dependent patients

Pulse oximetry is widely used in the critical care setting, but few studies have examined its usefulness in clinical decision making. One area where pulse oximetry might be useful is in the titration of fractional inspired O2 concentration (FIO2) in ventilator-dependent patients. Unfortunately, documented guidelines for this use do not exist, and in a survey of directors of intensive care units, we found that they employed a wide range of target O2 saturation (SpO2) values. Consequently, we undertook a study to determine if SpO2 could be reliably substituted for measurements of arterial O2 tension (PaO2) when adjusting FIO2 in ventilator-dependent patients. We examined a number of SpO2 target values in 54 critically ill patients aiming for a PaO2 of greater than or equal to 60 mm Hg, while minimizing the risk of O2 toxicity. In white patients, we found that a SpO2 target of 92 percent was reliable in predicting a satisfactory level of oxygenation. However, in black patients, such a SpO2 reading was commonly associated with significant hypoxemia (PaO2 as low as 49 mm Hg), and a higher SpO2 target, 95 percent, was required. In addition, inaccurate oximetry readings (ie, greater than 4 percent difference between SpO2 and direct SaO2 measurements) were more common in black (27 percent) than in white patients (11 percent, p less than 0.05). In conclusion, a SpO2 target of 92 percent was reliable when titrating supplemental O2 in white patients receiving mechanical ventilation; however, in black patients, such a SpO2 reading was commonly associated with significant hypoxemia, and a higher SpO2 target, 95 percent, was required to ensure a satisfactory level of oxygenation.

NG-methyl-L-arginine inhibits tumor necrosis factor-induced hypotension: implications for the involvement of nitric oxide

Clinical assessment of the activity of tumor necrosis factor (TNF) against human cancer has been limited by a dose-dependent cardiovascular toxicity, most frequently hypotension. TNF is also thought to mediate the vascular collapse resulting from bacterial endotoxin. The present studies address the mechanism by which TNF causes hypotension and provide evidence for elevated production of nitric oxide, a potent vasodilator initially characterized as endothelium-derived relaxing factor. Nitric oxide is synthesized by several cell types, including endothelial cells and macrophages, from the guanidino nitrogen of L-arginine; the enzymatic pathway is competitively inhibited by NG-methyl-L-arginine. We found that hypotension induced in pentobarbital-anesthetized dogs by TNF (10 micrograms/kg, i.v., resulting in a fall in mean systemic arterial pressure from 124.7 +/- 7 to 62.0 +/- 22.9 mmHg; 1 mmHg = 133 Pa) was completely reversed within 2 min following administration of NG-methyl-L-arginine (4.4 mg/kg, i.v.). In contrast, NG-methyl-L-arginine failed to reverse the hypotensive response to an equivalent depressor dose of nitroglycerin, a compound that acts by forming nitric oxide by a nonenzymatic, arginine-independent mechanism. The effect of NG-methyl-L-arginine on TNF-induced hypotension was antagonized, and the hypotension restored, by administration of excess L-arginine (100 mg/kg, i.v.). Our findings suggest that excessive nitric oxide production mediates the hypotensive effect of TNF.