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

Respiratory drive: a journey from health to disease

Respiratory drive is defined as the intensity of respiratory centers output during the breath and is primarily affected by cortical and chemical feedback mechanisms. During the involuntary act of breathing, chemical feedback, primarily mediated through CO2, is the main determinant of respiratory drive. Respiratory drive travels through neural pathways to respiratory muscles, which execute the breathing process and generate inspiratory flow (inspiratory flow-generation pathway). In a healthy state, inspiratory flow-generation pathway is intact, and thus respiratory drive is satisfied by the rate of volume increase, expressed by mean inspiratory flow, which in turn determines tidal volume. In this review, we will explain the pathophysiology of altered respiratory drive by analyzing the respiratory centers response to arterial partial pressure of CO2 (PaCO2) changes. Both high and low respiratory drive have been associated with several adverse effects in critically ill patients. Hence, it is crucial to understand what alters the respiratory drive. Changes in respiratory drive can be explained by simultaneously considering the (1) ventilatory demands, as dictated by respiratory centers activity to CO2 (brain curve); (2) actual ventilatory response to CO2 (ventilation curve); and (3) metabolic hyperbola. During critical illness, multiple mechanisms affect the brain and ventilation curves, as well as metabolic hyperbola, leading to considerable alterations in respiratory drive. In critically ill patients the inspiratory flow-generation pathway is invariably compromised at various levels. Consequently, mean inspiratory flow and tidal volume do not correspond to respiratory drive, and at a given PaCO2, the actual ventilation is less than ventilatory demands, creating a dissociation between brain and ventilation curves. Since the metabolic hyperbola is one of the two variables that determine PaCO2 (the other being the ventilation curve), its upward or downward movements increase or decrease respiratory drive, respectively. Mechanical ventilation indirectly influences respiratory drive by modifying PaCO2 levels through alterations in various parameters of the ventilation curve and metabolic hyperbola. Understanding the diverse factors that modulate respiratory drive at the bedside could enhance clinical assessment and the management of both the patient and the ventilator.

How to ventilate obstructive and asthmatic patients

Exacerbations are part of the natural history of chronic obstructive pulmonary disease and asthma. Severe exacerbations can cause acute respiratory failure, which may ultimately require mechanical ventilation. This review summarizes practical ventilator strategies for the management of patients with obstructive airway disease. Such strategies include non-invasive mechanical ventilation to prevent intubation, invasive mechanical ventilation, from the time of intubation to weaning, and strategies intended to prevent post-extubation acute respiratory failure. The role of tracheostomy, the long-term prognosis, and potential future adjunctive strategies are also discussed. Finally, the physiological background that underlies these strategies is detailed.

Patient-ventilator dyssynchronies: Are they all the same? A clinical classification to guide actions

Patient ventilatory dyssynchrony (PVD) is a mismatch between the respiratory drive of the patient and ventilatory assistance. It is a complex event seen in almost all ventilated patients and at any ventilator mode, with uncertain significance and prognosis. Due to its different pathophysiological mechanisms, there is still not consensual classification to guide us in selecting the best treatment. In the present review we aimed to summarize some clinical data on PVD, and to propose a clinical classification based on the type of PVD, from potentially innocuous to clearly harmful PVD, which could help clinicians in the decision-making process from adjusting ventilator settings to deeply sedate or paralyze the patient. Clearly, further studies are needed addressing risk factors, physiologic mechanisms and direct consequences of PVD in order to help clinicians to design effective and proven strategies at the bedside.

Effect of external PEEP in patients under controlled mechanical ventilation with an auto-PEEP of 5 cmH2O or higher

Background

In some patients with auto-positive end-expiratory pressure (auto-PEEP), application of PEEP lower than auto-PEEP maintains a constant total PEEP, therefore reducing the inspiratory threshold load without detrimental cardiovascular or respiratory effects. We refer to these patients as “complete PEEP-absorbers.” Conversely, adverse effects of PEEP application could occur in patients with auto-PEEP when the total PEEP rises as a consequence. From a pathophysiological perspective, all subjects with flow limitation are expected to be “complete PEEP-absorbers,” whereas PEEP should increase total PEEP in all other patients. This study aimed to empirically assess the extent to which flow limitation alone explains a “complete PEEP-absorber” behavior (i.e., absence of further hyperinflation with PEEP), and to identify other factors associated with it.

Methods

One hundred patients with auto-PEEP of at least 5 cmH₂O at zero end-expiratory pressure (ZEEP) during controlled mechanical ventilation were enrolled. Total PEEP (i.e., end-expiratory plateau pressure) was measured both at ZEEP and after applied PEEP equal to 80 % of auto-PEEP measured at ZEEP. All measurements were repeated three times, and the average value was used for analysis.

Results

Forty-seven percent of the patients suffered from chronic pulmonary disease and 52 % from acute pulmonary disease; 61 % showed flow limitation at ZEEP, assessed by manual compression of the abdomen. The mean total PEEP was 7 ± 2 cmH₂O at ZEEP and 9 ± 2 cmH₂O after the application of PEEP (p < 0.001). Thirty-three percent of the patients were “complete PEEP-absorbers.” Multiple logistic regression was used to predict the behavior of “complete PEEP-absorber.” The best model included a respiratory rate lower than 20 breaths/min and the presence of flow limitation. The predictive ability of the model was excellent, with an overoptimism-corrected area under the receiver operating characteristics curve of 0.89 (95 % CI 0.80–0.97).

Conclusions

Expiratory flow limitation was associated with both high and complete “PEEP-absorber” behavior, but setting a relatively high respiratory rate on the ventilator can prevent from observing complete “PEEP-absorption.” Therefore, the effect of PEEP application in patients with auto-PEEP can be accurately predicted at the bedside by measuring the respiratory rate and observing the flow-volume loop during manual compression of the abdomen.

Inhaled bronchodilator administration during mechanical ventilation: how to optimize it, and for which clinical benefit?

Bronchodilators are frequently used in ICU patients, and are the most common medications administered by inhalation during mechanical ventilation. The amount of bronchodilator that deposits at its site of action depends on the amount of drug, inhaled mass, deposited mass, and particle size distribution. Mechanical ventilation challenges both inhaled mass and lung deposition by specific features, such as a ventilatory circuit, an endotracheal tube, and ventilator settings. Comprehensive in vitro studies have shown that an endotracheal tube is not as significant a barrier for the drug to travel as anticipated. Key variables of drug deposition are attachments of the inhalation device in the inspiratory line 10 to 30 cm to the endotracheal tube, use of chamber with metered-dose inhaler, dry air, high tidal volume, low respiratory frequency, and low inspiratory flow, which can increase the drug deposition. In vivo studies showed that a reduction by roughly 15% of the respiratory resistance was achieved with inhaled bronchodilators during invasive mechanical ventilation. The role of ventilatory settings is not as clear in vivo, and primary factors for optimal delivery and physiologic effects were medication dose and device location. Nebulizers and pressurized metered-dose inhalers can equally achieve physiologic end points. The effects of bronchodilators should be carefully evaluated, which can easily be done with the interrupter technique. With the non-invasive ventilation, the data regarding drug delivery and physiologic effects are still limited. With the bilevel ventilators the inhalation device should be located between the leak port and face mask. Further studies should investigate the effects of inhaled bronchodilators on patient outcome and methods to optimize delivery of inhaled bronchodilators during non-invasive ventilation.

Ventilatory management in patients with chronic airflow obstruction

Mechanical ventilatory support allows patients who have CAO to gain time for pharmacologic treatment to work and to avoid and/or recover from respiratory muscle fatigue. The cornerstone to avoiding associated morbidity with mechanical ventilation in these patients is to prevent dynamic hyperinflation of the lung by limiting minute ventilation and maximizing time for expiration and by inducing synchronization between the patient and mechanical ventilator. When mechanical ventilation is necessary, NPPV should be considered first, whenever possible, in these patients. Patients who have CAO requiring mechanical ventilatory support have an increased risk of death following such an event. Therefore, careful followup is needed after hospital discharge.

Bedside waveforms interpretation as a tool to identify patient-ventilator asynchronies

OBJECTIVE

During assisted modes of ventilatory support the ventilatory output is the final expression of the interaction between the ventilator and the patient's controller of breathing. This interaction may lead to patient-ventilator asynchrony, preventing the ventilator from achieving its goals, and may cause patient harm. Flow, volume, and airway pressure signals are significantly affected by patient-ventilator interaction and may serve as a tool to guide the physician to take the appropriate action to improve the synchrony between patient and ventilator. This review discusses the basic waveforms during assisted mechanical ventilation and how their interpretation may influence the management of ventilated patients. The discussion is limited on waveform eye interpretation of the signals without using any intervention which may interrupt the process of mechanical ventilation.

DISCUSSION

Flow, volume, and airway pressure may be used to (a) identify the mode of ventilator assistance, triggering delay, ineffective efforts, and autotriggering, (b) estimate qualitatively patient's respiratory efforts, and (c) recognize delayed and premature opening of exhalation valve. These signals may also serve as a tool for gross estimation of respiratory system mechanics and monitor the effects of disease progression and various therapeutic interventions.

CONCLUSIONS

Flow, volume, and airway pressure waveforms are valuable real-time tools in identifying various aspects of patient-ventilator interaction.

Oxygen consumption and PEEPe in ventilated COPD patients

The intrinsic positive-end-expiratory pressure (PEEPi) increases the inspiratory load, the cost of breathing and thus oxygen consumption (V(O2)). It has been shown that applying an extrinsic positive-end-expiratory pressure (PEEPe) reduces the inspiratory threshold load but the optimal PEEPe level is still in debate. We hypothesize that the best level of PEEPe could induce a decrease in V(O2) by reducing the V(O2) demands from PEEPi. Nine mechanically ventilated COPD patients were included. The level of PEEPe was determined in accordance with the static PEEPi. V(O2) was measured using an automatic gas analyser during synchronized intermittent mandatory ventilation (SIMV): without PEEPe, with a PEEPe equal to 50% of static PEEPi and with a PEEPe equal to 100% of static PEEPi. Static PEEPi appeared to be significantly correlated with the degree of airflow obstruction (FEV1) (P<0.05). Applying a PEEPe equal to static PEEPi resulted in a significant decrease in V(O2) (P<0.05) whereas the change in V(O2) proved to be unpredictable for a PEEPe level of 50% of static PEEPi. In conclusion, V(O2) decreases progressively when increasing PEEPe up to a level equal to 100% of static PEEPi. Thus, in mechanically ventilated COPD patients with a FEV1 < or = 1000 ml, applying a PEEPe of 5 cmH2O should be recommended.

Influence of expiratory loading and hyperinflation on cardiac output during exercise

Patients with obstructive lung disease are exposed to expiratory loads (ELs) and dynamic hyperinflation as a consequence of expiratory flow limitation. To understand how these alterations in lung mechanics might affect cardiac function, we examined the influence of a 10-cmH2O EL, alone and in combination with voluntary hyperinflation (ELH), on pulmonary pressures [esophageal (Pes) and gastric (Pg)] and cardiac output (CO) in seven healthy subjects. CO was determined by using an acetylene method at rest and at 40 and 70% of peak work. At rest and during exercise, EL resulted in an increase in Pes and Pg (7-18 cmH2O; P < 0.05) and a decrease in CO (from 5.3 ± 1.8 to 4.5 ± 1.4, 12.2 ± 2.2 to 11.2 ± 2.2, and 16.3 ± 3.3 to 15.2 ± 3.2 l/min for rest, 40% peak work, and 70% peak work, respectively; P < 0.05), which remained depressed after an additional 2 min of EL. With ELH, CO increased at rest and both exercise loads (relative to EL only) but remained below control values. The changes in CO were due to a reduction in stroke volume with a tendency for stroke volume to fall further with prolonged EL. There was a negative correlation between CO and the increase in expiratory Pes and Pg with EL ( R = -0.58 and -0.60; P < 0.01), whereas the rise in CO with subsequent hyperinflation was related to a more negative Pes ( R = 0.72; P < 0.01). In conclusion, EL leads to a reduction in CO, which appears to be primarily related to increases in expiratory abdominal and intrathoracic pressure, whereas ELH resulted in an improved CO, suggesting that lung inflation has little impact on cardiac function.

Pattern of lung emptying and expiratory resistance in mechanically ventilated patients with chronic obstructive pulmonary disease

OBJECTIVES

To study the pattern of lung emptying and expiratory resistance in mechanically ventilated patients with chronic obstructive pulmonary disease (COPD).

DESIGN

A prospective physiological study.

SETTING

A 12-bed Intensive Care Unit.

PATIENTS

Ten patients with acute exacerbation of COPD.

INTERVENTIONS

At three levels of positive end-expiratory pressure (PEEP, 0, 5 and 10 cm H(2)O) tracheal (Ptr) and airway pressures, flow (V') and volume (V) were continuously recorded during volume control ventilation and airway occlusions at different time of expiration.

MEASUREMENTS AND RESULTS

V-V' curves during passive expiration were obtained, expired volume was divided into five equal volume slices and the time constant (tau) and dynamic deflation compliance (Crs(dyn)) of each slice was calculated by regression analysis of V-V' and post-occlusion V-Ptr relationships, respectively. In each volume slice the existence or not of flow limitation was examined by comparing V-V' curves with and without decreasing Ptr. For a given slice total expiratory resistance was calculated as tau/Crs(dyn), whereas expiratory resistance (Rrs) and time constant (tau(rs)) of the respiratory system were subsequently estimated taken into consideration the presence of flow limitation. At zero PEEP, tau(rs) increased significantly toward the end of expiration due to an increase in Rrs. PEEP significantly decreased Rrs at the end of expiration and resulted in a faster and relatively constant rate of lung emptying.

CONCLUSIONS

Patients with COPD exhibit a decrease in the rate of lung emptying toward the end of expiration due to an increase in Rrs. PEEP decreases Rrs, resulting in a faster and uniform rate of lung emptying.

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

OBJECTIVE

To compare the effects of He/O(2) and external PEEP (PEEPe) on intrinsic PEEP (PEEPi), respiratory mechanics, gas exchange, and ventilation/perfusion (V(A)/Q) in mechanically ventilated COPD patients.

DESIGN AND SETTING

Prospective, interventional study in the intensive care unit of a university hospital.

INTERVENTIONS

Ten intubated, sedated, paralyzed, mechanically ventilated COPD patients studied in the following conditions: (a) baseline settings made by clinician in charge, air/O(2), ZEEP; (b) He/O(2), ZEEP; (c) air/O(2), ZEEP; (d) air/O(2), PEEPe 80% of PEEPi. Measurements at each condition included V(A)/Q by the multiple inert gas elimination technique (MIGET).

RESULTS

PEEPi and trapped gas volume were comparably reduced by He/O(2) (4.2+/-4 vs. 7.7+/-4 cmH(2)O and 98+/-82 vs. 217+/-124 ml, respectively) and PEEPe (4.4+/-1.3 vs. 7.8+/-3.6 cmH(2)O and 120+/-107 vs. 216+/-115 ml, respectively). He/O(2) reduced inspiratory and expiratory respiratory system resistance (15.5+/-4.4 vs. 20.7+/-6.9 and 19+/-9 vs. 28.8+/-15 cmH(2)O l(-1)s(-1), respectively) and plateau pressure (13+/-4 vs. 17+/-6 cmH(2)O). PEEPe increased airway pressures, including total PEEP, and elastance. PaO(2)/FIO(2) was slightly reduced by He/O(2) (225+/-83 vs. 245+/-82) without significant V(A)/Q change.

CONCLUSIONS

He/O(2) and PEEPe comparably reduced PEEPi and trapped gas volume. However, He/O(2) decreased airway resistance and intrathoracic pressures, at a small cost in arterial oxygenation. He/O(2) could offer an attractive option in COPD patients with PEEPi/dynamic hyperinflation.

Volume and flow dependence of respiratory mechanics in mechanically ventilated COPD patients

Volume and flow dependencies of respiratory mechanics are examined in 10 COPD patients under mechanical ventilation (MV) at 3 levels of externally applied PEEP (PEEPe). Airways pressure (Paw), flow (V') and volume (V) data are analyzed according to (1) the linear and (2) a non-linear model, accounting for volume dependence of elastance and for flow and volume dependence of resistance. The models' fitness to data is assessed by the regression errors. Non-linear modelling fits significantly better to data, while the difference of fitness decreases with PEEPe. Linear mechanics are not significantly different between the 3 levels of PEEPe. A positive volume dependence of elastance observed at 0, decreases at 5 and increases again at 10 hPa of PEEPe. A seriously negative volume dependence of resistance at 0 turned to positive with PEEPe. These dependencies of respiratory mechanics during COPD under MV, show that the present non-linear respiratory mechanical monitoring may help for better and less risky adjustment of PEEPe.

Effect of CPAP on intrinsic PEEP, inspiratory effort, and lung volume in severe stable COPD

BACKGROUND

Intrinsic positive end expiratory pressure (PEEPi) constitutes an inspiratory threshold load on the respiratory muscles, increasing work of breathing. The role of continuous positive airway pressure (CPAP) in alleviating PEEPi in patients with severe stable chronic obstructive pulmonary disease is uncertain. This study examined the effect of CPAP on the inspiratory threshold load, muscle effort, and lung volume in this patient group.

METHODS

Nine patients were studied at baseline and with CPAP increasing in increments of 1 cm H(2)O to a maximum of 10 cm H(2)O. Breathing pattern and minute ventilation (I), dynamic PEEPi, expiratory muscle activity, diaphragmatic (PTPdi/min) and oesophageal (PTPoes/min) pressure-time product per minute, integrated diaphragmatic (EMGdi) and intercostal EMG (EMGic) and end expiratory lung volume (EELV) were measured.

RESULTS

Expiratory muscle activity was present at baseline in one subject. In the remaining eight, PEEPi was reduced from a mean (SE) of 2.9 (0.6) cm H(2)O to 0.9 (0.1) cm H(2)O (p<0.05). In two subjects expiratory muscle activity contributed to PEEPi at higher pressures. There were no changes in respiratory pattern but I increased from 9.2 (0.6) l/min to 10.7 (1.1) l/min (p<0.05). EMGdi remained stable while EMGic increased significantly. PTPoes/min decreased, although this did not reach statistical significance. PTPdi/min decreased significantly from 242.1 (32.1) cm H(2)O.s/min to 112.9 (21.7) cm H(2)O.s/min). EELV increased by 1.1 (0.3) l (p<0.01).

CONCLUSION

High levels of CPAP reduce PEEPi and indices of muscle effort in patients with severe stable COPD, but only at the expense of substantial increases in lung volume.

Effects of PEEP on inspiratory resistance in mechanically ventilated COPD patients

This study aimed to investigate the effect of increased lung volume with positive end-expiratory pressure (PEEP) on respiratory resistance in patients with chronic obstructive pulmonary disease (COPD). Ten patients with COPD were mechanically ventilated for acute respiratory failure. PEEP was set at 0, 5, 10 and 15 cm H2O. Using the rapid airway occlusion technique, the total inspiratory resistance of the respiratory system was partitioned into interrupter (Rint,rs) and additional effective (deltaRrs) resistances. At each level of PEEP, at constant inflation flow, the inflation volume (deltaV) was varied from 0.2-1 L, and, at constant deltaV, the inflation flow was varied from 0.2-1.2 L x s(-1). The changes in end-expiratory lung volume (deltaEELV) induced by PEEP were also measured. The difference between the EELV and the relaxation volume of the respiratory system (deltaFRC) increased significantly with PEEP of 10 and 15 cm H2O as compared to a PEEP of 0, the increase being associated with a significant reduction of Rint,rs. By contrast, deltaRrs was independent of deltaFRC. At constant deltaV, Rint,rs fitted Rohrer's equation (Rint,rs = K1 + K2 x flow). While K2 significantly declined with AFRC, K1 did not change. At all levels of PEEP, deltaRrs was not influenced by deltaFRC. With increasing lung volume induced by positive end-expiratory pressure, the inspiratory airway resistance decreased, whereas the viscoelastic behaviour of the respiratory system, as reflected by additional effective resistance, did not change.

Mechanical ventilation in chronic obstructive lung disease

Exacerbations of COPD are a leading indication for MV in the intensive care unit. A thorough understanding of the pathophysiology of AVF in COPD is critical for physicians caring for these patients. In particular, physicians should understand DHI and use the ventilator and ancillary techniques to minimize its impact. Noninvasive positive-pressure ventilation should be considered strongly in relatively stable patients with an adequate mental status and manageable secretions. Once AVF resolves, patients should be removed from the ventilator as soon as is safe to do so to minimize the adverse effects of prolonged MV. An organized approach to weaning and identifying patients capable of independent breathing is crucial. Most patients with COPD and AVF benefit from MV and generally return to or approach their premorbid functional status. A significant subset, however, will not benefit from, or choose not to undergo, MV. Deciding upon appropriate therapeutic options for these patients relies heavily on effective communication between physician and patient. Comprehensive discussions before the development of AVF can assist decision-making after respiratory failure develops.

Effects of helium-oxygen on intrinsic positive end-expiratory pressure in intubated and mechanically ventilated patients with severe chronic obstructive pulmonary disease

OBJECTIVE

To test the hypothesis that replacing 70:30 nitrogen: oxygen (Air-O2) with 70:30 helium:oxygen (He-O2) can decrease dynamic hyperinflation ("intrinsic" positive end-expiratory pressure) in mechanically ventilated patients with chronic obstructive pulmonary disease (COPD), and to document the consequences of such an effect on arterial blood gases and hemodynamics.

DESIGN

Prospective, interventional study.

SETTING

Medical intensive care unit, university tertiary care center.

PATIENTS

Twenty-three intubated, sedated, paralyzed, and mechanically ventilated patients with COPD enrolled within 36 hrs after intubation.

INTERVENTIONS

Measurements were taken at the following time points, all with the same ventilator settings: a) baseline; b) after 45 mins with He-O2; c) 45 mins after return to Air-O2. The results were then compared to those obtained in a test lung model using the same ventilator settings. MAIN RESULTS (MEAN + SD): Trapped lung volume and intrinsic positive end-expiratory pressure decreased during He-O2 ventilation (215+/-125 mL vs. 99+/-15 mL and 9+/-2.5 cm H2O vs. 5+/-2.7 cm H2O, respectively; p < .05). Likewise, peak and mean airway pressures declined with He-O2 (30+/-5 cm H2O vs. 25+/-6 cm H2O and 8+/-2 cm H2O vs. 7+/-2 cm H2O, respectively; p < .05). These parameters all rose to their baseline values on return to Air-O2 (p < .05 vs. values during He-O2). These results were in accordance with those obtained in the test lung model. There was no modification of arterial blood gases, heart rate, or mean systemic arterial blood pressure. In 12/23 patients, a pulmonary artery catheter was in place, allowing hemodynamic measurements and venous admixture calculations. Switching to He-O2 and back to Air-O2 had no effect on pulmonary artery pressures, right and left ventricular filling pressures, cardiac output, pulmonary and systemic vascular resistance, or venous admixture.

CONCLUSION

In mechanically ventilated COPD patients with intrinsic positive end-expiratory pressure, the use of He-O2 can markedly reduce trapped lung volume, intrinsic positive end-expiratory pressure, and peak and mean airway pressures. No effect was noted on hemodynamics or arterial blood gases. He-O2 might prove beneficial in this setting to reduce the risk of barotrauma, as well as to improve hemodynamics and gas exchange in patients with very high levels of intrinsic positive end-expiratory pressure.

Bronchodilator delivery with metered-dose inhaler during mechanical ventilation

The delivery of bronchodilators with metered-dose inhaler (MDI) in mechanically ventilated patients has attracted considerable interest in recent years. This is because the use of the MDI has several advantages over the nebulizer, such as reduced cost, ease of administration, less personnel time, reliability of dosing and a lower risk of contamination. A spacer device is fundamental in order to demonstrate the efficacy of the bronchodilatory therapy delivered by MDI. Provided that the technique of administration is appropriate, MDIs are as effective as nebulizers, despite a significantly lower dose of bronchodilator given by the MDI.

Ventilation of patients with asthma and obstructive lung disease

Mechanical ventilation in a patient with obstructive airway disease may be a lifesaving measure; however, it may also be associated with significant morbidity and mortality. It is important for a physician to be familiar with the potential complications of mechanical ventilation in this group of patients and to know how to avoid them by carefully applying safe ventilator strategies. The cornerstone of such strategies is to minimize minute ventilation, maximize time for expiration, and avoid hyperinflation of the lung. Several bedside parameters (iPEEP, VEI, Pplat) that reflect presence of gas trapping and potential hyperinflation may be measured. In addition to mechanical ventilation, management should include inhaled bronchodilators and systemic corticosteroid therapies. In the event controlled hypoventilation is necessary, sedation with or without the use of muscle relaxants may be required. Unconventional therapies such as the use of Heliox, magnesium sulfate, ketamine, and inhalational anesthetics may be attempted in severe cases that do not respond to conventional management. With appropriate use of ventilator strategies, a reduction in the mortality and morbidity of patients with obstructive airway disease requiring mechanical ventilation has recently been noted.

Measurements of PEEP-induced changes in lung volume: evaluation of a laser monitor

STUDY OBJECTIVE

To evaluate a newly developed laser monitor in the measurement of positive end-expiratory pressure (PEEP)-induced changes in end-expiratory lung volume (EELV) in spontaneously breathing subjects.

DESIGN

An open comparison between two different methods to assess breathing parameters.

SETTING

A respiratory research unit.

PARTICIPANTS

Six spontaneously breathing, healthy volunteers.

INTERVENTIONS

Stepwise increases of PEEP from 0 to 0.8 to 1.6 kPa during spontaneous breathing; repeated validation of the laser monitor in each subject.

MEASUREMENTS AND RESULTS

Pressure and flow were recorded at the airway opening. Abdominal wall displacement (AWD) measured by the laser sensor was recorded simultaneously. The time lag between the volume and the laser signals during baseline was 0.068+/-0.052 s and during maximal PEEP, 0.108+/-0.093 s. There was no baseline drift in either the PEEP or the AWD signals. Mean EELV decreased by 290 mL to 1,157 mL after the PEEP valve was removed. Within each individual, the ratio between EELV and AWD showed only small variations. Measurements of tidal volume (VT) and AWD showed good agreement at all PEEP levels. Mean VT decreased in all but one subject during PEEP. With the increase in PEEP, the end-expiratory abdominal baseline increased linearly over a large range of end-expiratory pressures with a flattening of the curve at high PEEP levels in all subjects.

CONCLUSIONS

The laser monitor is sufficiently accurate for measuring PEEP-induced changes in EELV during spontaneous breathing in healthy adult subjects. Monitors incorporating multiple laser sensors may have considerable clinical promise.

Applied PEEP during pressure support reduces the inspiratory threshold load of intrinsic PEEP

OBJECTIVES

Mechanical ventilation in patients with obstructive airway disease (OAD) is associated with the development of dynamic hyperinflation and intrinsic positive end-expiratory pressure (PEEPi). One of the effects of this form of PEEPi is to act as an inspiratory threshold load that can produce ineffective breath triggering, dyspnea, and muscle fatigue. Recently it has been shown that applying PEEP in the ventilator circuit can reduce this imposed triggering load. We wished to investigate this further by studying patients with OAD being weaned with pressure support (PS) ventilation. Our first objective was to determine the prevalence and magnitude of this form of PEEPi in OAD patients who were clinically judged to be capable of triggering mechanical ventilatory breaths. Our second objective was to attempt to reduce the triggering load by applying circuit PEEP and then observe the response of patient-ventilator interactions during the patient-triggered, pressure-limited PS breath.

DESIGN

Thirteen random patients with OAD who were receiving PS ventilation were studied by measuring airway pressures, airway gas flow, baseline esophageal pressure, esophageal pressure time products (PTP), and esophageal pressure changes before ventilator gas delivery began (delta Pes taken to represent PEEPi). Measurements were made at baseline and after stepwise increases in circuit PEEP up to the PEEPi.

RESULTS

We found measurable PEEPi in all patients (average +/- SD of 9.54 +/- 4.3 cm H2O) and it was > 10 cm H2O in seven patients. As would be predicted, we observed progressive reductions in PEEPi as applied PEEP was given. We also observed that the component of patient effort (PTP) related to overcoming PEEPi also decreased, but the PTP related to tidal volume (VT) did not. The VT associated with the set PS thus did not change with application of PEEP, nor did the breathing frequency.

CONCLUSION

PEEPi is common in OAD patients receiving mechanical ventilatory support. The imposed triggering load from PEEPi can be offset to large extent by circuit PEEP approaching the baseline PEEPi. Although total patient effort substantially falls with applied PEEP, the patient effort that combine with PS to effect VT does not.

Mechanical ventilation in obstructive lung disease

This article reviews selected topics relevant to the use of mechanical ventilation in patients with severe airflow obstruction. Areas discussed include the bedside assessment of respiratory system mechanics, the ventilatory determinants of dynamic pulmonary hyperinflation, the role of controlled hypoventilation with permissive hypercapnia, and the delivery of bronchodilators during mechanical ventilation.

Effects of breathing patterns on mechanically ventilated patients with chronic obstructive pulmonary disease and dynamic hyperinflation

OBJECTIVE

To examine the circulatory and respiratory effects of breathing pattern in patients with chronic obstructive pulmonary disease (COPD) and dynamic hyperinflation (DH) during controlled mechanical ventilation.

DESIGN

Prospective, controlled, randomized, non-blinded study.

SETTING

Respiratory intensive care unit of a university hospital.

PATIENTS

Nine patients with acute respiratory failure and DH due to acute exacerbations of COPD.

INTERVENTIONS

Keeping tidal volume and total breath duration (TTOT) constant, patients were ventilated at six different values of expiratory time (TE). TE changes were randomly induced by alterations of constant inspiratory flow (VI) and/or end-inspiratory pause (EIP). Patients were studied at three levels of VI(0.93 +/- 0.08, 0.72 +/- 0.06 and 0.55 +/- 0.04 l/s, mean +/- SE), with and without EIP (10% of TTOT).

MEASUREMENTS AND RESULTS

Lung volumes, airflows, airways pressures, oxygenation indices and dead space were measured. Alveolar pressure and airway resistance (Rmin), as well as the additional resistance (delta R) due to viscoelastic pressure dissipation and time-constant inequalities, were estimated by rapid airway occlusion during inflation. In seven out of nine patients, right-heart catheterization was performed and hemodynamic parameters were obtained at each value of TE. A significant decrease of intrinsic positive end-expiratory pressure (PEEPi), end-inspiratory static and mean (mPaw) airway pressures, end-expiratory lung volume above passive FRC (Vtrap), delta R and venous admixture and a significant increase of peak airway pressure, Rmin, stroke volume index and mixed venous PO2 (PvO2) were observed when VI increased. At each VI, the addition of EIP significantly decreased iso-volume expiratory flows and PvO2 and increased Vtrap and mPaw.

CONCLUSIONS

We conclude that in mechanically ventilated patients with COPD, the pattern of lung inflation and TE alteration have a significant impact on respiratory system mechanics, gas exchange and hemodynamics. Addition of EIP in patients with COPD may be detrimental.