Effects of inspiratory flow rate alterations on gas exchange during mechanical ventilation in normal lungs. Efficiency of end-inspiratory pause

Expiratory model-based method to monitor ARDS disease state

Introduction

Model-based methods can be used to characterise patient-specific condition and response to mechanical ventilation (MV) during treatment for acute respiratory distress syndrome (ARDS). Conventional metrics of respiratory mechanics are based on inspiration only, neglecting data from the expiration cycle. However, it is hypothesised that expiratory data can be used to determine an alternative metric, offering another means to track patient condition and guide positive end expiratory pressure (PEEP) selection.

Methods

Three fully sedated, oleic acid induced ARDS piglets underwent three experimental phases. Phase 1 was a healthy state recruitment manoeuvre. Phase 2 was a progression from a healthy state to an oleic acid induced ARDS state. Phase 3 was an ARDS state recruitment manoeuvre. The expiratory time-constant model parameter was determined for every breathing cycle for each subject. Trends were compared to estimates of lung elastance determined by means of an end-inspiratory pause method and an integral-based method. All experimental procedures, protocols and the use of data in this study were reviewed and approved by the Ethics Committee of the University of Liege Medical Faculty.

Results

The overall median absolute percentage fitting error for the expiratory time-constant model across all three phases was less than 10 %; for each subject, indicating the capability of the model to capture the mechanics of breathing during expiration. Provided the respiratory resistance was constant, the model was able to adequately identify trends and fundamental changes in respiratory mechanics.

Conclusion

Overall, this is a proof of concept study that shows the potential of continuous monitoring of respiratory mechanics in clinical practice. Respiratory system mechanics vary with disease state development and in response to MV settings. Therefore, titrating PEEP to minimal elastance theoretically results in optimal PEEP selection. Trends matched clinical expectation demonstrating robustness and potential for guiding MV therapy. However, further research is required to confirm the use of such real-time methods in actual ARDS patients, both sedated and spontaneously breathing.

Model-based respiratory mechanics to titrate PEEP and monitor disease state for experimental ARDS subjects

Modelling the respiratory mechanics of mechanically ventilated (MV) patients can provide useful information to guide MV therapy. Two model-based methods were evaluated based on data from three experimental acute respiratory distress syndrome (ARDS) induced piglets and validated against values available from ventilators. A single compartment lung model with integral-based parameter identification was found to be effective in capturing fundamental respiratory mechanics during inspiration. The trends matched clinical expectation and provided better resolution than clinically derived linear model metrics. An expiration time constant model also captured the same trend in respiratory elastance. However, the assumption of constant resistance and a slightly higher fitting error results in less insight than the single compartment model. Further research is required to confirm its application in titrating to optimal MV settings.

Pattern of ventilation during manual hyperinflation performed by physiotherapists

Summary The aim of this prospective observational study was to document patterns of ventilation during manual hyperinflation by physiotherapists. Manual hyperinflation with a Mapleson-F system was performed on the same patients on two consecutive days. Patterns of ventilation were recorded using a heated pneumotachometer, pressure transducer and custom designed data acquisition and analysis systems. The mean (SE) results were: inspiratory time 1.45 (0.10) s; volume delivered 1.23 (0.07) l; peak inspiratory and expiratory flow rate 1.51 (0.06) l.s(-1) and 3.26 (0.30) l.s(-1), respectively and I : E flow rate ratio 0.63 (0.05). All the physiotherapists achieved an increase in volume which was delivered within a safe and effective pressure range and without cardiovascular compromise. Most (26 out of 34 sessions) performed the technique in the way recommended for enhancing secretion clearance. This is the first study to document comprehensively the pattern of ventilation during manual hyperinflation and provides the basis for further clinical trials evaluating its effectiveness for secretion clearance and volume restoration.

Manual hyperinflation improves alveolar recruitment in difficult-to-wean patients

STUDY OBJECTIVES

To investigate the effect of manual hyperinflation (MH) in patients with atelectasis associated with ventilation support.

DESIGN

Patients were randomized to either an experimental group or a control group.

SETTING

Pulmonary ICUs from two hospitals.

PATIENTS

Twenty-three patients with atelectasis associated with ventilation support.

INTERVENTIONS

The MH technique was at a rate of 8 to 13 breaths/min for a period of 20 min each session, three times per day for 5 days. The control group received their standard prescribed mechanical ventilation without supplemental MH. Sputum contents (wet/dry weight ratio, viscosity), respiratory system capacity (spontaneous tidal volume [Vt], maximal inspiratory pressure, rapid shallow breathing index [f/Vt], chest radiograph signs, and Pa(O2)/fraction of inspired oxygen [Fi(O2)]) were measured just prior to the MH at day 0 as baseline, and at day 3 and day 6 of the study.

MEASUREMENTS AND RESULTS

There were significant improvements in scores over the 6-day study in the experimental group compared to the control group in spontaneous Vt (p = 0.035) and chest radiograph signs (p = 0.040), and a trend toward improvement of f/Vt (p = 0.066) and Pa(O2)/Fi(O2) (p = 0.061) after adjustment for covariates. Other outcome variables did not differ significantly between the experimental and control groups.

CONCLUSIONS

MH performed on patients with atelectasis from ventilation support significantly improved alveolar recruitment.

The effects of three manual hyperinflation techniques on pattern of ventilation in a test lung model

Manual hyperinflation (MHI) is used by physiotherapists as a treatment technique in intubated patients. This study investigated the effect of three different MHI techniques using a Mapleson-C circuit configuration with a CIG Medishield valve on volume delivered (Vt), peak inspiratory (PIFR) and expiratory flow rates (PEFR), and peak airway pressure (PAP) in a test lung model. The protocols differed in the degree of valve closure and inclusion of an inspiratory pause. For protocols 1, 2 and 3 the measures were Vt-1.33 (0.21), 2.74 (0.13), 3.55 (0.12) litres; PAP-14.30 (0.82), 24.00 (0.47), 30.20 (0.92) cmH2O and PIFR-1.13 (0.05), 1.51 (0.15), 1.32 (0.09) l/s respectively. All pair comparisons were statistically significant except for PEFR (l/s), which was significantly lower for protocol 1 [1.62 (0.06)], compared to protocols 2 [2.01 (0.25)] and 3 [2.10 (0.19)] but not between protocols 2 and 3. Circuit and technique choice should be considered in relation to the specific therapeutic aim of treatment.

Manual hyperinflation: a description of the technique

The aim of this study was to describe the technique of manual hyperinflation as applied by physiotherapists. Ten physiotherapists volunteered to manually hyperinflate a test lung. The protocol randomly altered resuscitation circuits (Laerdal and Macgill) and respiratory compliance conditions. Measurements of tidal volume, airway pressure, inspiratory flow rate and inflation rate were recorded during each test condition. The therapists applied a mean (SD) tidal volume of 1.4 (0.2) L with a mean (SD) airway pressure of 20.6 (6.6) cmH2O. The technique was significantly different between each test condition and between all therapists. Therapists were observed to hyperventilate the test lung and also apply positive end expiratory pressure with the Macgill circuit. A description of the technique on patients is required prior to evaluation of the technique in determining its effect on patients.

Pressure-controlled and volume-cycled mechanical ventilation

Pressure and volume modes of mechanical ventilation are available as options in the current generation of ventilators, giving clinicians many choices when managing a mechanically ventilated patient. In volume cycled ventilation, tidal volume is set and airway pressures are measured, whereas in pressure-controlled ventilation, pressure is set and volume is measured. This article reviews the characteristics of these two ventilatory modes and discusses in detail conversion from one mode to the other. Pertinent clinical studies and recent direct comparisons of volume-cycled and pressure-controlled ventilation are reviewed.