Comparison of static and dynamic measurements of intrinsic PEEP in mechanically ventilated patients

Setting positive end-expiratory pressure in the severely obstructive patient

PURPOSE OF REVIEW

The response to positive end-expiratory pressure (PEEP) in patients with chronic obstructive pulmonary disease (COPD) requiring mechanical ventilation depends on the underlying pathophysiology. This review focuses on the pathophysiology of COPD, especially intrinsic PEEP (PEEPi) and its consequences, and the benefits of applying external PEEP during assisted ventilation when PEEPi is present.

RECENT FINDINGS

The presence of expiratory airflow limitation and increased airway resistance promotes the development of dynamic hyperinflation in patients with COPD during acute respiratory failure. Dynamic hyperinflation and the associated development of PEEPi increases work of breathing and contributes to ineffective triggering of the ventilator. In the presence of airflow limitation, application of external PEEP during patient-triggered ventilation has been shown to reduce inspiratory effort, facilitate ventilatory triggering and enhance patient-ventilator interaction. To minimize the risk of hyperinflation, it is advisable to limit the level of external PEEP during assisted ventilation after optimization of ventilator settings to about 70% of the level of PEEPi (measured during passive ventilation).

SUMMARY

In patients with COPD and dynamic hyperinflation receiving assisted mechanical ventilation, the application of low levels of external PEEP can minimize work of breathing, facilitate ventilator triggering and improve patient-ventilator interaction.

Induction of dynamic hyperinflation by expiratory resistance breathing in healthy subjects - an efficacy and safety study

New Findings

What is the central question of this study?

The study aimed to establish a novel model to study the chronic obstructive pulmonary disease (COPD)‐related cardiopulmonary effects of dynamic hyperinflation in healthy subjects.

What is the main finding and its importance?

A model of expiratory resistance breathing (ERB) was established in which dynamic hyperinflation was induced in healthy subjects, expressed both by lung volumes and intrathoracic pressures. ERB outperformed existing methods and represents an efficacious model to study cardiopulmonary mechanics of dynamic hyperinflation without potentially confounding factors as present in COPD.

Abstract

Dynamic hyperinflation (DH) determines symptoms and prognosis of chronic obstructive pulmonary disease (COPD). The induction of DH is used to study cardiopulmonary mechanics in healthy subjects without COPD‐related confounders like inflammation, hypoxic vasoconstriction and rarefication of pulmonary vasculature. Metronome‐paced tachypnoea (MPT) has proven effective in inducing DH in healthy subjects, but does not account for airflow limitation. We aimed to establish a novel model incorporating airflow limitation by combining tachypnoea with an expiratory airway stenosis. We investigated this expiratory resistance breathing (ERB) model in 14 healthy subjects using different stenosis diameters to assess a dose–response relationship. Via cross‐over design, we compared ERB to MPT in a random sequence. DH was quantified by inspiratory capacity (IC, litres) and intrinsic positive end‐expiratory pressure (PEEPi, cmH₂O). ERB induced a stepwise decreasing IC (means (95% CI): tidal breathing: 3.66 (3.45–3.88), ERB 3 mm: 3.33 (1.75–4.91), 2 mm: 2.05 (0.76–3.34), 1.5 mm: 0.73 (0.12–1.58) litres) and increasing PEEPi (tidal breathing: 0.70 (0.50–0.80), ERB 3 mm: 11.1 (7.0–15.2), 2 mm: 22.3 (17.1–27.6), 1.5 mm: 33.4 (3.40–63) cmH₂O). All three MPT patterns increased PEEPi, but to a far lesser extent than ERB. No adverse events during ERB were noted. In conclusion, ERB was proven to be a safe and efficacious model for the induction of DH and might be used for the investigation of cardiopulmonary interaction in healthy subjects.

Continuous monitoring of intrinsic PEEP based on expired CO2 kinetics: an experimental validation study

Background

Quantification of intrinsic PEEP (PEEPi) has important implications for patients subjected to invasive mechanical ventilation. A new non-invasive breath-by-breath method (etCO₂D) for determination of PEEPi is evaluated.

Methods

In 12 mechanically ventilated pigs, dynamic hyperinflation was induced by interposing a resistance in the endotracheal tube. Airway pressure, flow, and exhaled CO₂ were measured at the airway opening. Combining different I:E ratios, respiratory rates, and tidal volumes, 52 different levels of PEEPi (range 1.8–11.7 cmH₂O; mean 8.45 ± 0.32 cmH₂O) were studied. The etCO₂D is based on the detection of the end-tidal dilution of the capnogram. This is measured at the airway opening by means of a CO₂ sensor in which a 2-mm leak is added to the sensing chamber. This allows to detect a capnogram dilution with fresh air when the pressure coming from the ventilator exceeds the PEEPi. This method was compared with the occlusion method.

Results

The etCO₂D method detected PEEPi step changes of 0.2 cmH₂O. Reference and etCO₂D PEEPi presented a good correlation (R² 0.80, P < 0.0001) and good agreement, bias − 0.26, and limits of agreement ± 1.96 SD (2.23, − 2.74) (P < 0.0001).

Conclusions

The etCO₂D method is a promising accurate simple way of continuously measure and monitor PEEPi. Its clinical validity needs, however, to be confirmed in clinical studies and in conditions with heterogeneous lung diseases.

Electronic supplementary material

The online version of this article (10.1186/s13054-019-2430-9) contains supplementary material, which is available to authorized users.

Monitoring of total positive end-expiratory pressure during mechanical ventilation by artificial neural networks

Ventilation treatment of acute lung injury (ALI) requires the application of positive airway pressure at the end of expiration (PEEP_app) to avoid lung collapse. However, the total pressure exerted on the alveolar walls (PEEP_tot) is the sum of PEEP_app and intrinsic PEEP (PEEP_i), a hidden component. To measure PEEP_tot, ventilation must be discontinued with an end-expiratory hold maneuver (EEHM). We hypothesized that artificial neural networks (ANN) could estimate the PEEP_tot from flow and pressure tracings during ongoing mechanical ventilation. Ten pigs were mechanically ventilated, and the time constant of their respiratory system (τ_RS) was measured. We shortened their expiratory time (TE) according to multiples of τ_RS, obtaining different respiratory patterns (R_pat). Pressure (P_AW) and flow (V'_AW) at the airway opening during ongoing mechanical ventilation were simultaneously recorded, with and without the addition of external resistance. The last breath of each R_pat included an EEHM, which was used to compute the reference PEEP_tot. The entire protocol was repeated after the induction of ALI with i.v. injection of oleic acid, and 382 tracings were obtained. The ANN had to extract the PEEP_tot, from the tracings without an EEHM. ANN agreement with reference PEEP_tot was assessed with the Bland-Altman method. Bland Altman analysis of estimation error by ANN showed -0.40 ± 2.84 (expressed as bias ± precision) and ±5.58 as limits of agreement (data expressed as cmH₂O). The ANNs estimated the PEEP_tot well at different levels of PEEP_app under dynamic conditions, opening up new possibilities in monitoring PEEP_i in critically ill patients who require ventilator treatment.

Neural versus pneumatic control of pressure support in patients with chronic obstructive pulmonary diseases at different levels of positive end expiratory pressure: a physiological study

INTRODUCTION

Intrinsic positive end-expiratory pressure (PEEPi) is a "threshold" load that must be overcome to trigger conventional pneumatically-controlled pressure support (PSP) in chronic obstructive pulmonary disease (COPD). Application of extrinsic PEEP (PEEPe) reduces trigger delays and mechanical inspiratory efforts. Using the diaphragm electrical activity (EAdi), neurally controlled pressure support (PSN) could hypothetically eliminate asynchrony and reduce mechanical inspiratory effort, hence substituting the need for PEEPe. The primary objective of this study was to show that PSN can reduce the need for PEEPe to improve patient-ventilator interaction and to reduce both the "pre-trigger" and "total inspiratory" neural and mechanical efforts in COPD patients with PEEPi. A secondary objective was to evaluate the impact of applying PSN on breathing pattern.

METHODS

Twelve intubated and mechanically ventilated COPD patients with PEEPi ≥ 5 cm H2O underwent comparisons of PSP and PSN at different levels of PEEPe (at 0 %, 40 %, 80 %, and 120 % of static PEEPi, for 12 minutes at each level on average), at matching peak airway pressure. We measured flow, airway pressure, esophageal pressure, and EAdi, and analyzed neural and mechanical efforts for triggering and total inspiration. Patient-ventilator interaction was analyzed with the NeuroSync index.

RESULTS

Mean airway pressure and PEEPe were comparable for PSP and PSN at same target levels. During PSP, the NeuroSync index was 29 % at zero PEEPe and improved to 21 % at optimal PEEPe (P < 0.05). During PSN, the NeuroSync index was lower (<7 %, P < 0.05) regardless of PEEPe. Both pre-trigger (P < 0.05) and total inspiratory mechanical efforts (P < 0.05) were consistently higher during PSP compared to PSN at same PEEPe. The change in total mechanical efforts between PSP at PEEPe0% and PSN at PEEPe0% was not different from the change between PSP at PEEPe0% and PSP at PEEPe80%.

CONCLUSION

PSN abolishes the need for PEEPe in COPD patients, improves patient-ventilator interaction, and reduces the inspiratory mechanical effort to breathe.

TRIAL REGISTRATION

Clinicaltrials.gov NCT02114567 . Registered 04 November 2013.

Management of COPD patients in the intensive care unit

Chronic obstructive pulmonary disease (COPD) is characterized by expiratory airflow limitation that is not fully reversible. Acute exacerbations in patients with moderate to severe COPD can cause severe hypoxia and persistent or severe respiratory acidosis, resulting in respiratory failure and the need for ventilator support. Acute respiratory failure, altered mental status, and hemodynamic instability associated with acute exacerbations of COPD are commonly encountered and require careful management in the intensive care unit (ICU). Noninvasive and invasive ventilator support in conjunction with pharmacotherapy can be lifesaving, although mortality remains high. It is important also to consider pulmonary rehabilitation and palliative care.

Helium in the adult critical care setting

Helium is a low-density inert gas whose physical properties are very different from those of nitrogen and oxygen. Such properties could be clinically useful in the adult critical care setting, especially in patients with upper to more distal airway obstruction requiring moderate to intermediate levels of FiO₂. However, despite decades of utilization and reporting, it is still difficult to give any firm clinical recommendation in this setting. Numerous case reports are available in the context of upper airway obstruction of different origins, but there is a lack of controlled studies for this indication. One study reported a helium-induced beneficial effect on surrogates of work of breathing after extubation in non-COPD patients, possibly in relation to laryngeal consequences of tracheal intubation. Physiological benefits of helium-oxygen breathing have been demonstrated in the context of acute severe asthma, but there is a lack of large controlled studies demonstrating an effect on pertinent clinical endpoints, except for a study reported only as an abstract, which mentioned a reduction in the intubation rate in helium-treated patients. Finally, there are a number of physiological studies in the context of COLD-COPD patients demonstrating a beneficial effect, mainly by a reduction in the resistive inspiratory work of breathing but also by a reduction in hyperinflation. Reduction of hypercapnia was mainly observed in spontaneously breathing and noninvasively ventilated helium-treated patients but not in intubated patients during controlled ventilation, suggesting that the decrease in PaCO₂ was mainly in relation to a diminution in CO₂ production, related to the diminution in work of breathing and not an improved alveolar ventilation. Moreover, there is little evidence that helium-oxygen could improve parameters of heterogeneity in such patients. Two RCTs were unable to demonstrate a reduction in the intubation rate in such setting, but they were likely underpowered. An adequately powered international multicentric study is ongoing and will help to determinate the exact place of the helium-oxygen mixture in the future. The place of the mixture during the weaning period will deserve further evaluation.

Adaptive support ventilation versus conventional ventilation for total ventilatory support in acute respiratory failure

OBJECTIVE

To compare the short-term effects of adaptive support ventilation (ASV), an advanced closed-loop mode, with conventional volume or pressure-control ventilation in patients passively ventilated for acute respiratory failure.

DESIGN

Prospective crossover interventional multicenter trial.

SETTING

Six European academic intensive care units.

PATIENTS

Eighty-eight patients in three groups: patients with no obvious lung disease (n = 22), restrictive lung disease (n = 36) or obstructive lung disease (n = 30).

INTERVENTIONS

After measurements on conventional ventilation (CV) as set by the patients' clinicians, each patient was switched to ASV set to obtain the same minute ventilation as during CV (isoMV condition). If this resulted in a change in PaCO(2), the minute ventilation setting of ASV was readjusted to achieve the same PaCO(2) as in CV (isoCO(2) condition).

MEASUREMENTS AND RESULTS

Compared with CV, PaCO(2) during ASV in isoMV condition and minute ventilation during ASV in isoCO(2) condition were slightly lower, with lower inspiratory work/minute performed by the ventilator (p < 0.01). Oxygenation and hemodynamics were unchanged. During ASV, respiratory rate was slightly lower and tidal volume (Vt) slightly greater (p < 0.01), especially in obstructed patients. During ASV there were different ventilatory patterns in the three groups, with lower Vt in patients with restrictive disease and prolonged expiratory time in obstructed patients, thus mimicking the clinicians' choices for setting CV. In three chronic obstructive pulmonary disease patients the resulting Vt was unacceptably high.

CONCLUSIONS

Comparison between ASV and CV resulted either in similarities or in minor differences. Except for excessive Vt in a few obstructed patients, all differences were in favor of ASV.

New ventilators for the ICU--usefulness of lung performance reporting

Monitoring the functional and mechanical properties of the lungs during positive pressure ventilation may assist in confirming the underlying pulmonary diagnosis, allow therapeutic interventions to be accurately assessed and provide information that ensures the optimal setting of the ventilator parameters and encourages timely weaning. This article reviews the range of lung function measurements, both continuous and intermittent, that may be undertaken during mechanical ventilation. The monitoring capability of ICU ventilators is increasing in complexity.

Alveolar recruitment in acute lung injury

Alveolar recruitment is one of the primary goals of respiratory care for acute lung injury. It is aimed at improving pulmonary gas exchange and, even more important, at protecting the lungs from ventilator-induced trauma. This review addresses the concept of alveolar recruitment for lung protection in acute lung injury. It provides reasons for why atelectasis and atelectrauma should be avoided; it analyses current and future approaches on how to achieve and preserve alveolar recruitment; and it discusses the possibilities of detecting alveolar recruitment and derecruitment. The latter is of particular clinical relevance because interventions aimed at lung recruitment are often undertaken without simultaneous verification of their effectiveness.

Expiratory flow limitation in morbidly obese postoperative mechanically ventilated patients

Although obesity promotes tidal expiratory flow limitation (EFL), with concurrent dynamic hyperinflation (DH), intrinsic PEEP (PEEPi) and risk of low lung volume injury, the prevalence and magnitude of EFL, DH and PEEPi have not yet been studied in mechanically ventilated morbidly obese subjects. In 15 postoperative mechanically ventilated morbidly obese subjects, we assessed the prevalence of EFL [using the negative expiratory pressure (NEP) technique], PEEPi, DH, respiratory mechanics, arterial oxygenation and PEEPi inequality index as well as the levels of PEEP required to abolish EFL. In supine position at zero PEEP, 10 patients exhibited EFL with a significantly higher PEEPi and DH and a significantly lower PEEPi inequality index than found in the five non-EFL (NEFL) subjects. Impaired gas exchange was found in all cases without significant differences between the EFL and NEFL subjects. Application of 7.5 +/- 2.5 cm H2O of PEEP (range: 4-16) abolished EFL with a reduction of PEEPi and DH and an increase in FRC and the PEEPi inequality index but no significant effect on gas exchange. The present study indicates that: (a) on zero PEEP, EFL is present in most postoperative mechanically ventilated morbidly obese subjects; (b) EFL (and concurrent risk of low lung volume injury) is abolished with appropriate levels of PEEP; and (c) impaired gas exchange is common in these patients, probably mainly due to atelectasis.

Measurement of lung mechanics at different lung volumes and esophageal levels in normal subjects: effect of posture change

Lung elastance and resistance increase in the supine posture. To evaluate the effects of change in posture on regional lung mechanics at different lung volumes, lung elastance and resistance were measured at graded volume subdivisions and three esophageal levels at seated and supine body positions, using the esophageal balloon technique. Volumes were adjusted to be the same in both postures. In general, lung elastance (both static and dynamic) tended to be higher in supine posture and uniform at all lung volumes, except at 80% vital capacity, where it increased sharply. The ratio of dynamic to static lung elastance was slightly higher at the cephalad esophageal level, where regional flow rates and relative volume expansion are lower. Lung resistance varied inversely with lung volume but was higher at corresponding volume subdivisions in the supine posture. It decreased at more cephalad esophageal levels, where volume expansion and flow are less. Thus, the increase in regional flow at low volume subdivisions (most marked in the supine position) also contributed to higher lung resistance at these volumes. These findings are explained on the basis of a combination of Newtonian physics as well as nonlinear viscoelastic properties of the lung as applied to regional flow and volume expansion.

Helium-oxygen reduces work of breathing in mechanically ventilated patients with chronic obstructive pulmonary disease

OBJECTIVE

To evaluate whether helium-oxygen mixture reduces inspiratory work of breathing (WOB) in sedated, paralyzed, and mechanically ventilated patients with acute exacerbation of chronic obstructive pulmonary disease (COPD).

DESIGN AND SETTING

Open, prospective, randomized, crossover study in the medical intensive care unit in a university hospital.

PATIENTS AND PARTICIPANTS

23 patients admitted for acute exacerbation of COPD and mechanically ventilated.

MEASUREMENTS

Total WOB (WOBt), elastic WOB (WOBel), resistive WOB (WOBres), and WOB due to PEEPi (WOBPeepi) were measured. Static intrinsic positive end expiratory pressure (PEEPi), static compliance (Crs), inspiratory resistance (Rins), inspiratory (tinsp) and expiratory time constant (texp) were also measured. These variables were compared between air-oxygen and helium-oxygen mixtures.

RESULTS

WOBt significantly decreased with helium-oxygen (2.34+/-1.04 to 1.85+/-1.01 J/l, p<0.001). This reduction was significant for WOBel (1.02+/-0.61 J/l to 0.87+/-0.47, p<0.01), WOBPeepi (0.77+/-0.38 J/l to 0.54+/-0.38, p<0.001), and WOBres (0.55+/-0.19 J/l to 0.44+/-0.24, p<0.05). PEEPi, Rins, tinsp and texp significantly decreased. Crs was unchanged.

CONCLUSIONS

Helium-oxygen mixture decreases WOB in mechanically ventilated COPD patients. Helium-oxygen mixture could be useful to reduce the burden of ventilation.

The pulmonary physician in critical care . 12: Acute severe asthma in the intensive care unit

Most deaths from acute asthma occur outside hospital, but the at-risk patient may be recognised on the basis of prior ICU admission and asthma medication history. Patients who fail to improve significantly in the emergency department should be admitted to an HDU or ICU for observation, monitoring, and treatment. Hypoxia, dehydration, acidosis, and hypokalaemia render the severe acute asthmatic patient vulnerable to cardiac dysrrhythmia and cardiorespiratory arrest. Mechanical ventilation may be required for a small proportion of patients for whom it may be life saving. Aggressive bronchodilator (continuous nebulised beta agonist) and anti-inflammatory therapy must continue throughout the period of mechanical ventilation. Recognised complications of mechanical ventilation include hypotension, barotrauma, and nosocomial pneumonia. Low ventilator respiratory rates, long expiratory times, and small tidal volumes help to prevent hyperinflation. Volatile anaesthetic agents may produce bronchodilation in patients resistant to beta agonists. Fatalities in acute asthmatics admitted to HDU/ICU are rare.

Controlled mechanical ventilation leads to remodeling of the rat diaphragm

Little is known about the structural response of the diaphragm to controlled mechanical ventilation. We examined effects of this intervention on muscle mass, myosin heavy chain isoforms, and contractile function in the rat diaphragm. Animals were mechanically ventilated for up to 4 days, and comparisons were made with normal control rats as well as spontaneously breathing animals anesthetized for the same duration as the mechanical ventilation group. The diaphragm-to-body weight ratio was significantly reduced in the mechanical ventilation group only. After mechanical ventilation, an increase in hybrid fibers coexpressing both type I (slow) and type II (fast) myosin isoforms was found within the diaphragm, which occurred at the expense of the pure type I fiber population. In contrast, the percentages of type I, type II, and hybrid fibers in the limb muscles (soleus and extensor digitorum longus) did not differ between experimental groups. The optimal length for force production, as well as maximal force-generating capacity of the diaphragm, was also significantly decreased in mechanically ventilated animals. We conclude that even short-term controlled mechanical ventilation produces significant remodeling and functional alterations of the diaphragm, which could impede efforts at discontinuing ventilatory support.

Effects of positive end-expiratory pressure on gas exchange and expiratory flow limitation in adult respiratory distress syndrome

OBJECTIVE

To assess the effects of different positive end-expiratory pressure (PEEP) levels (0, 5, 10, and 15 cm H2O) on tidal expiratory flow limitation (FL), regional intrinsic positive end-expiratory pressure (PEEPi) inhomogeneity, alveolar recruited volume (Vrec), respiratory mechanics, and arterial blood gases in mechanically ventilated patients with acute respiratory distress syndrome (ARDS).

DESIGN

Prospective clinical study.

SETTING

Multidisciplinary intensive care unit of a university hospital.

PATIENTS

Thirteen sedated, mechanically ventilated patients during the first 2 days of ARDS.

INTERVENTIONS

Detection of tidal FL and evaluation of total dynamic PEEP (PEEPt,dyn), total static PEEP (PEEPt,st), respiratory mechanics, and Vrec from pressure, flow, and volume traces provided by the ventilator. The average (+/-sd) tidal volume was 7.1 +/- 1.5 mL/kg, the total cycle duration was 2.9 +/- 0.45 secs, and the duty cycle was 0.35 +/- 0.05.

MEASUREMENTS

Tidal FL was assessed using the negative expiratory pressure technique. Regional PEEPi inhomogeneity was assessed as the ratio of PEEPt,dyn to PEEPt,st (PEEPi inequality index), and Vrec was quantified as the difference in lung volume at the same airway pressure between quasi-static inflation volume-pressure curves on zero end-expiratory pressure (ZEEP) and PEEP.

RESULTS

On ZEEP, seven patients exhibited FL amounting to 31 +/- 8% of tidal volume. They had higher PEEPt,st and PEEPi,st ( p<.001) and lower PEEPi inequality index ( p<.001) than the six nonflow-limited (NFL) patients. Two FL patients became NFL with PEEP of 5 cm H2O and five with PEEP of 10 cm H2O. In both groups, PaO2 increased progressively with PEEP. In the FL group, there was a significant correlation of PaO2 to PEEPi inequality index ( p=.002). For a given PEEP, Vrec was greater in NFL than FL patients, and a significant correlation of Pao to Vrec ( p<.001) was found only in the NFL group.

CONCLUSIONS

We conclude that on ZEEP, tidal FL is common in ARDS patients and is associated with greater regional PEEPi inhomogeneity than in NFL patients. With PEEP of 10 cm H2O, flow limitation with concurrent cyclic dynamic airway compression and re-expansion and the risk of "low lung volume injury" were absent in all patients. In FL patients, PEEP induced a significant increase in PaO2, mainly because of the reduction of regional PEEPi inequality, whereas in the NFL group, arterial oxygenation was improved satisfactorily because of alveolar recruitment.

Pressure-volume curves in acute respiratory distress syndrome: clinical demonstration of the influence of expiratory flow limitation on the initial slope

The presence of an initial segment with a low compliance on the static pressure-volume (PV) curve in patients with acute respiratory distress syndrome (ARDS) indicates that some lung compartments do not initially receive insufflated gas. We tested the hypothesis that an uneven distribution of time constants, producing a "slow compartment," was in part responsible for the change in compliance between the initial and the intermediate segment of the PV curve. In 16 patients with ARDS submitted to mechanical ventilation in volume-controlled mode with a supportive respiratory rate of 15 breaths/minute, we constructed the static PV curve on the first day of respiratory support and determined the intrinsic positive end-expiratory pressure (PEEPi4) during a prolonged end-expiratory pause (4 seconds). We also measured the volume of a "slow compartment" during a prolonged expiration (> 6 seconds), and determined an external PEEP (PEEPe) suppressing PEEPi4. Among the 16 patients studied, 11 exhibited a low inflection point, associated with a "slow compartment" of 172 +/- 83 ml, responsible for a PEEPi4 of 3 +/- 2 cm H2O. Conversely, the five remaining patients had a linear PV curve, associated with a minimal "slow compartment" of 28 +/- 10 ml, responsible for a negligible PEEPi4. We observed that individual slopes of the initial segment of the PV curve were inversely and significantly correlated with the proportion of the "slow compartment" (r = -0.85). We concluded that the shape of the inspiratory PV curve in ARDS might be dependent on the presence of a "slow compartment," and demonstrated that a low external PEEP appeared sufficient to achieve a substantial mechanical improvement in clinical practice.

On-line monitoring of intrinsic PEEP in ventilator-dependent patients

Measurement of the intrinsic positive end-expiratory pressure (PEEP(i)) is important in planning the management of ventilated patients. Here, a new recursive least squares method for on-line monitoring of PEEP(i) is proposed for mechanically ventilated patients. The procedure is based on the first-order model of respiratory mechanics applied to experimental measurements obtained from eight ventilator-dependent patients ventilated with four different ventilatory modes. The model PEEP(i) (PEEP(i,mod)) was recursively constructed on an inspiration-by-inspiration basis. The results were compared with two well-established techniques to assess PEEP(i): end-expiratory occlusion to measure static PEEP(i) (PEEP(i, st)) and change in airway pressure preceding the onset of inspiratory airflow to measure dynamic PEEP(i) (PEEP(i,dyn)). PEEP(i, mod) was significantly correlated with both PEEP(i,dyn) (r = 0.77) and PEEP(i,st) (r = 0.90). PEEP(i,mod) (5.6 +/- 3.4 cmH(2)O) was systematically >PEEP(i,dyn) and PEEP(i,st) (2.7 +/- 1.9 and 8.1 +/- 5.5 cmH(2)O, respectively), in all the models without external PEEP. Focusing on the five patients with chronic obstructive pulmonary disease, PEEP(i,mod) was significantly correlated with PEEP(i,st) (r = 0.71), whereas PEEP(i,dyn) (r = 0.22) was not. When PEEP was set 5 cmH(2)O above PEEP(i,st), all the methods correctly estimated total PEEP, i.e., 11.8 +/- 5.3, 12.5 +/- 5.0, and 12.0 +/- 4.7 cmH(2)O for PEEP(i,mod), PEEP(i,st), and PEEP(i,dyn), respectively, and were highly correlated (0.97-0.99). We interpreted PEEP(i,mod) as the lower bound of PEEP(i,st) and concluded that our method is suitable for on-line monitoring of PEEP(i) in mechanically ventilated patients.

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.

Correcting static intrinsic positive end-expiratory pressure for expiratory muscle contraction. Validation of a new method

We have recently shown (Eur. Respir. J. 1997;10:522-529) that in spontaneously breathing and actively expiring patients, static intrinsic positive end-expiratory pressure (PEEPi,st) can be corrected for expiratory muscle contraction by subtracting the average expiratory rise in gastric pressure (Pga,exp rise), calculated from three breaths just prior to an airway occlusion, from the end-expiratory airway pressure (Paw) of the first occluded inspiratory effort (PEEPi,st avg). However, since in some patients there is substantial variability in the intensity of expiratory muscle activity and hence in Pga,exp rise, this method may be inaccurate because the Pga,exp rise of breaths preceding airway occlusion may differ from that of the first postocclusion breath. In the present study, we introduced a new method consisting of synchronous subtraction of Pga,exp rise from Paw, both occurring during airway occlusion (PEEPi,st sub). PEEPi,st sub and PEEPi,st avg were each compared with the reference PEEPi,st (PEEPi,st ref), which was obtained during muscular paralysis and simulation of the spontaneous breathing pattern by the ventilator. We found that, in 25 critically ill patients, PEEPi,st sub (mean +/- SD, 5.3 +/- 2.6 cm H(2)O) was nearly identical to PEEPi,st ref (5.4 +/- 2.4 cm H(2)O). Their mean difference was -0.06 cm H(2)O with limits of agreement -0.96 to 0.84 cm H(2)O, indicating a strong agreement between these methods. In contrast, mean difference of PEEPi,st avg and PEEPi,st ref was 0.73 cm H(2)O with limits of agreement -3.97 to 5.43 cm H(2)O, indicating lack of agreement. Coefficient of variation of Pga,exp rise was 14.3 +/- 7.2% (range, 5.2 to 28.3%). There was a good correlation between the coefficient of variation of Pga,exp rise and the difference between PEEPi,st avg and PEEPi,st ref (r = 0.909; p < 0.001). We conclude that PEEPi,st can be accurately measured in spontaneously breathing patients by synchronous subtraction of Pga,exp rise from Paw during airway occlusion.

Partitioning of respiratory system resistance in children with respiratory insufficiency

Using end-inspiratory airway occlusion, respiratory system resistance (Rrs) can be partitioned into a flow-resistive component (Rint), and an additional component (DeltaR), reflecting viscoelasticity and time constant inequalities. We studied flow and volume dependence of Rrs and its subdivisions (Rint and DeltaR) in 13 children, seven mechanically ventilated for pulmonary insufficiency (Group 1; six with parenchymal lung disease; one with lower airway obstruction) and six without primary lung disorder (Group 2). In comparison with healthy children, Rint was increased in the patient with lower airway obstruction and five of six patients without primary lung disorder but in only one of six with parenchymal lung disease. DeltaR was increased in all seven patients in Group 1 and in four of six patients in Group 2. The directions of changes in Rint and Rrs with increasing flow (isovolume conditions) and with increasing volume (isoflow conditions) were variable. DeltaR decreased exponentially (p < 0.05) with increasing flow in 11 of 13 subjects and increased with increasing tidal volume (VT) in 12 of 13. Thus, DeltaR was increased in most children on mechanical ventilation with or without primary lung disease; its volume and flow dependence were opposite to that of airway resistance.

A simple automated method for measuring pressure-volume curves during mechanical ventilation

Measurement of respiratory compliance is advocated for assessing the severity of acute respiratory failure (ARF). Recently, the administration of an automated constant flow of 15 L/min was proposed as a method easier to implement at the bedside than supersyringe or inspiratory occlusions methods. However, pressure-volume (P-V) curves were shifted to the right because of the resistive properties of the respiratory system. The aim of this study was to compare the P-V curves obtained using two constant flows-3 and 9 L/min-during volume-controlled mechanical ventilation with those obtained with the supersyringe and the inspiratory occlusions methods. Fourteen paralyzed patients with ARF were studied. The supersyringe and the inspiratory occlusions methods were performed according to usual recommendations. The new automated method was performed during volume-controlled mechanical ventilation by setting the inspiratory:expiratory ratio at 80%, the respiratory frequency at 5 breaths/min, and the tidal volume at 500 or 1,500 ml. These peculiar ventilatory settings were equivalent to administering a constant flow of 3 or 9 L/min during a 9.6-s inspiration. Esophageal and airway pressures were recorded. P-V curves obtained by the 3-L/min constant-flow method were identical to those obtained by the reference methods, whereas the P-V curve obtained by the 9-L/min constant flow was slightly shifted to the right. The slopes of the P-V curves and the lower inflection points were not different between all methods, indicating that the resistive component induced by administering a constant flow equal to or less than 9 L/min is not of clinical relevance. Because the 3-L/min constant-flow method is not artifacted by the resistive properties of the respiratory system and does not require any other equipment than a ventilator, it is an easy-to-implement, inexpensive, safe, and reliable method for measuring the thoracopulmonary P-V curve at the bedside.

Clinical examination reliably detects intrinsic positive end-expiratory pressure in critically ill, mechanically ventilated patients

Critically ill patients requiring mechanical ventilation often develop intrinsic positive end-expiratory pressure (PEEPi). Methods for its detection include an expiratory flow waveform display (not always available), an esophageal pressure transducer (invasive), or a relaxed or paralyzed patient. We sought to determine the accuracy of clinical examination for detecting PEEPi. Examiners blinded to waveform analysis assessed patients for the presence of PEEPi by inspection/palpation and auscultation. If either inspection/palpation or auscultation demonstrated PEEPi, it was said to be present by clinical exam. Clinicians with various levels of experience (attending, resident, student) made 503 observations of 71 patients. Sensitivity (SENS), specificity (SPEC), positive predictive value (PPV), negative predictive value (NPV), and likelihood ratios were determined for inspection/palpation, auscultation, and clinical exam. PEEPi was present during 69.8% of observations. SENS, SPEC, and PPV of clinical exam were 0.72, 0.91, and 0.95 respectively for the examiners as a whole. Likelihood ratio for PEEPi detection by clinical exam was 8.35. Attending intensivists displayed SPEC and PPV of 1.0. NPV was only 0.58 (likelihood ratio 0.31). We conclude that the clinical exam is very good for detecting PEEPi at all experience levels; and further, that the clinical exam is only modestly useful for ruling out PEEPi, therefore, other tests should be used if PEEPi is not detected by clinical exam.

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.

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.

The tension-time index and the frequency/tidal volume ratio are the major pathophysiologic determinants of weaning failure and success

We have previously shown (Am. J. Respir. Crit. Care Med. 1995;152:1248-1255) that in patients needing mechanical ventilation, the load imposed on the inspiratory muscles is excessive relative to their neuromuscular capacity. We have therefore hypothesized that weaning failure may occur because at the time of the trial of spontaneous breathing there is insufficient reduction of the inspiratory load. We therefore prospectively studied patients who initially had failed to wean from mechanical ventilation (F) but had successful weaning (S) on a later occasion. Compared with S, during F patients had greater intrinsic positive end-expiratory pressure (6. 10 +/- 2.45 versus 3.83 +/- 2.69 cm H2O), dynamic hyperinflation (327 +/- 180 versus 213 +/- 175 ml), total resistance (Rmax, 14.14 +/- 4.95 versus 11.19 +/- 4.01 cm H2O/L/s), ratio of mean to maximum inspiratory pressure (0.46 +/- 0.1 versus 0.31 +/- 0.08), tension time index (TTI, 0.162 +/- 0.032 versus 0.102 +/- 0.023) and power (315 +/- 153 versus 215 +/- 75 cm H2O x L/min), less maximum inspiratory pressure (42.3 +/- 12.7 versus 53.8 +/- 15.1 cm H2O), and a breathing pattern that was more rapid and shallow (ratio of frequency to tidal volume, f/VT 98 +/- 38 versus 62 +/- 21 breaths/min/L). To clarify on pathophysiologic grounds what determines inability to wean from mechanical ventilation, we performed multiple logistic regression analysis with the weaning outcome as the dependent variable. The TTI and the f/VT ratio were the only significant variables in the model. We conclude that the TTI and the f/VT are the major pathophysiologic determinants underlying the transition from weaning failure to weaning success.

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.

Static intrinsic PEEP in COPD patients during spontaneous breathing

Intrinsic positive end-expiratory pressure (PEEPi) is routinely determined under static conditions by occluding the airway at end-expiration (PEEPi,st). This procedure may be difficult in patients with chronic obstructive pulmonary disease (COPD) during spontaneous breathing, as both expiratory muscle activity and increased respiratory frequency often occur. To overcome these problems, we tested the hypothesis that the difference between maximum airway opening (MIP) and maximum esophageal (Ppl max) pressures, obtained with a Mueller maneuver from the end-expiratory lung volume (EELV), can accurately measure PEEPi,st. Using this method, we found that, in eight ventilator-dependent tracheostomized COPD patients (age 71+/-7 yr), PEEPi,st averaged 13.0+/-2.9 cm H2O. That measurement was validated by comparison with a reference static PEEPi (PEEPi,st-Ref) taken at the same EELV adopted by patients during spontaneous breathing, and measured on the passive quasi-static pressure-volume (P/V) curve of the respiratory system, obtained during mechanical ventilation. PEEPi,st-Ref averaged 13.1+/-3.0 cm H2O, i.e., a value essentially equal to PEEPi,st measured by means of our technique. We conclude that PEEPi,st can be accurately assessed in spontaneous breathing COPD patients by the difference between MIP and Ppl max during the Mueller maneuver.

Monitoring changes in lung air and liquid volumes with electrical impedance tomography

Electrical impedance tomography (EIT) uses electrical measurements at electrodes placed around the thorax to image changes in the conductivity distribution within the thorax. This technique is well suited to studying pulmonary function because the movement of air, blood, and extravascular fluid induces significant conductivity changes within the thorax. We conducted three experimental protocols in a total of 19 dogs to assess the accuracy with which EIT can quantify changes in the volumes of both gas and fluid in the lungs. In the first protocol, lung volume increments from 50 to 1,000 ml were applied with a large syringe. EIT measured these volume changes with an average error of 27 +/- 6 ml. In the second protocol, EIT measurements were made at end expiration and end inspiration during regular ventilation with tidal volume ranging from 100 to 1,000 ml. The average error in the EIT estimates of tidal volume was 90 +/- 43 ml. In the third protocol, lung liquid volume was measured by instilling 5% albumin solution into a lung lobe in increments ranging from 10 to 100 ml. EIT measured these volume changes with an average error of 10 +/- 10 ml and was also able to detect into which lobe the fluid had been instilled. These results indicate that EIT can noninvasively measure changes in the volumes of both gas and fluid in the lungs with clinically useful accuracy.

A model of the spontaneously breathing patient: applications to intrinsic PEEP and work of breathing

Intrinsic positive end-expiratory pressure (PEEPi) and inspiratory work of breathing (WI) are important factors in the management of severe obstructive respiratory disease. We used a computer model of spontaneously breathing patients with chronic obstructive pulmonary disease to assess the sensitivity of measurement techniques for dynamic PEEPi (PEEPidyn) and WI to expiratory muscle activity (EMA) and cardiogenic oscillations (CGO) on esophageal pressure. Without EMA and CGO, both PEEPidyn and WI were accurately estimated (r = 0.999 and 0.95, respectively). Addition of moderate EMA caused PEEPidyn and WI to be systematically overestimated by 141 and 52%, respectively. Furthermore, CGO introduced large random errors, obliterating the correlation between the true and estimated values for both PEEPidyn (r = 0.29) and WI (r = 0.38). Thus the accurate estimation of PEEPidyn and WI requires steps to be taken to ameliorate the adverse effects of both EMA and CGO. Taking advantage of our simulations, we also investigated the relationship between PEEPidyn and static PEEPi (PEEPistat). The PEEPidyn/PEEPistat ratio decreased as stress adaptation in the lung was increased, suggesting that heterogeneity of expiratory flow limitation is responsible for the discrepancies between PEEPidyn and PEEPistat that have been reported in patients with severe airway obstruction.

Elastic work of breathing during continuous positive airway pressure in intubated patients with chronic obstructive pulmonary disease (theoretical analysis and experimental validation)

BACKGROUND

Continuous positive airway pressure (CPAP) is known to decrease inspiratory work of breathing in patients with chronic obstructive pulmonary disease (COPD). This effect is primarily attributed to a reduction in inspiratory elastic work of breathing (Wi,el) related to a decrease in intrinsic positive end-expiratory pressure (PEEP).

METHODS

The aim of this study is to design a model for computation of Wi,el on the basis of respiratory mechanics in patients with COPD, at various intrinsic PEEP- and CPAP-levels. The model was used to estimate the optimal CPAP-level with respect to the intrinsic PEEP-level in terms of reduction of Wi,el. Calculations of the decrease in Wi,el due to CPAP obtained with the model were compared to changes in Wi,el and total work of breathing (Wi,tot) determined from respiratory measurements in patients with COPD.

RESULTS

Model calculations revealed that Wi,el was minimal whenever a CPAP-level equal to the intrinsic PEEP-level was applied. When a CPAP-level exceeding the intrinsic PEEP-level was applied, the reduction in Wi,el was less. Comparing these results to the respiratory measurements, a similar pattern in reduction of Wi,el and Wi,tot was established, although absolute values of the differences were smaller in the experimental data.

CONCLUSION

This study indicates that in order to reduce Wi,el in patients with COPD, intrinsic PEEP should be measured and the CPAP-level adjusted to the intrinsic PEEP-level.

Patient-ventilator interactions

Patient-ventilator synchrony is important in the management of the ventilator-dependent patient. Factors inherent to the patient and the ventilator influence patient-ventilator synchrony. Detection of patient-ventilator synchrony may require monitoring of airway pressure and flow waveforms.

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.

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.

Auto-positive end-expiratory pressure and dynamic hyperinflation

PEEP is indicated in patients with COPD only to unload the respiratory muscles from the auto-PEEP resulting from expiratory flow limitation. If auto-PEEP is not caused by flow limitation, application of PEEP will cause further hyperinflation, worsening respiratory mechanics, muscle activity, and hemodynamics. To assess the presence of expiratory flow limitation correctly, to measure auto-PEEP correctly, and to identify the maximal PEEP level to be used, measurements of flow and opening pressure must be obtained during a brief period of suspended respiratory muscle activity (obtained by sedation) with the patient's own breathing pattern reproduced accurately.

An intrinsic positive end-expiratory pressure lung model, with and without flow limitation

OBJECTIVE

To design an intrinsic positive end-expiratory pressure (PEEP) lung model that has the property of air flow limitation.

DESIGN

Mechanical lung model study of intrinsic PEEP.

SETTING

Lung models were set up in the research laboratory.

INTERVENTIONS

Intrinsic PEEP lung models were created with and without flow limitation. In the model with flow limitation, intrinsic PEEP was created by replacing a portion of the expiratory circuit with a collapsible Penrose tube and by placing this portion the circuit under water. The expiratory circuit became a part of the respiratory airway, with flow limitation occurring at the Penrose drain. In the model without flow limitation, intrinsic PEEP was generated with a fixed linear resistor, which was inserted in the expiratory circuit to produce a similar level of intrinsic PEEP. Multiple levels of external PEEP, both above and below the initial intrinsic PEEP, were applied.

MEASUREMENTS AND MAIN RESULTS

At each level of external PEEP, peak airway pressure, plateau airway pressure, isovolume air flow, internal lung pressure, and intrinsic PEEP were measured. Peak airway pressure, plateau pressure, and internal lung pressure were minimally affected if the external PEEP was less than the intrinsic PEEP in the lung model with flow limitation. Intrinsic PEEP was reduced with external PEEP. However, if intrinsic PEEP was induced without dynamic airway closure or flow limitation, any level of external PEEP caused an immediate increase in peak airway pressure, plateau airway pressure, and internal lung pressure and a decrease in isovolume flow. External PEEP has little effect on the levels of intrinsic PEEP.

CONCLUSIONS

We demonstrated two different models of an intrinsic PEEP lung model. The interactions between intrinsic PEEP and externally applied PEEP were different. The lung model with collapsible tube closely simulated the human respiratory system with flow limitation. This lung model may be useful for the future study of intrinsic PEEP and pulmonary mechanics.