Continuous on-line measurements of respiratory system, lung and chest wall mechanics during mechanic ventilation

Over-distension prediction via hysteresis loop analysis and patient-specific basis functions in a virtual patient model

BACKGROUND AND OBJECTIVE

Recruitment maneuvers (RMs) with subsequent positive-end-expiratory-pressure (PEEP) have proven effective in recruiting lung volume and preventing alveolar collapse. However, a suboptimal PEEP could induce undesired injury in lungs by insufficient or excessive breath support. Thus, a predictive model for patient response under PEEP changes could improve clinical care and lower risks.

METHODS

This research adds novel elements to a virtual patient model to identify and predict patient-specific lung distension to optimise and personalise care. Model validity and accuracy are validated using data from 18 volume-controlled ventilation (VCV) patients at 7 different baseline PEEP levels (0-12cmH₂O), yielding 623 prediction cases. Predictions were made up to ΔPEEP = 12cmH₂O ahead covering 6x2cmH₂O PEEP steps.

RESULTS

Using the proposed lung distension model, 90% of absolute peak inspiratory pressure (PIP) prediction errors compared to clinical measurement are within 3.95cmH₂O, compared with 4.76cmH₂O without this distension term. Comparing model-predicted and clinically measured distension had high correlation increasing to R² = 0.93-0.95 if maximum ΔPEEP ≤ 6cmH₂O. Predicted dynamic functional residual capacity (V_frc) changes as PEEP rises yield 0.013L median prediction error for both prediction groups and overall R² of 0.84.

CONCLUSIONS

Overall results demonstrate nonlinear distension mechanics are accurately captured in virtual lung mechanics patients for mechanical ventilation, for the first time. This result can minimise the risk of lung injury by predicting its potential occurrence of distension before changing ventilator settings. The overall outcomes significantly extend and more fully validate this virtual mechanical ventilation patient model.

Study of Tidal Volume and Positive End-Expiratory Pressure on Alveolar Recruitment Using Spiro Dynamics in Mechanically Ventilated Patients

Background and Aims

Ventilator setting in the intensive care unit patients is a topic of debate and setting of tidal volume (TV) should be patient-specific based on lung mechanics. In this study, we have evaluated to develop optimal ventilator strategies through continuous and thorough monitoring of respiratory mechanics during ongoing ventilator support to prevent alveolar collapse and alveolar injury in mechanically ventilated patients.

Methods

In our monocentric, randomized, observational study, we had recruited 60 patients and divided them into two groups of 30 each. In Group 1 patients, TV and positive end-expiratory pressure (PEEP) were set according to pressure-volume (P/V) curve obtained by the mechanical ventilator in a conventional manner (control group), and in Group 2, TV and PEEP were set according to P/V curve obtained by the mechanical ventilator using intratracheal catheter. PEEP and TV were set accordingly. TV, PEEP, and PaO₂/FiO₂ (P/F) ratio at days 1, 3, and 7, mortality within 7 days and mortality within 28 days were measured in each group and compared.

Results

We found a significant difference between PEEP and P/F ratio in both groups while intragroup comparison at days 1, 3, and 7. After the intergroup comparison of Group 1 and 2, we observed a significant difference of PEEP and P/F ratio between the groups at day 7 and not on day 1 or 3.

Conclusion

This study concludes that optimal PEEP is more accurate using an intratracheal catheter than the conventional method of deciding ventilator setting. Hence, it is recommended to use intratracheal catheter to obtain more accurate ventilator settings.

Robustness of two different methods of monitoring respiratory system compliance during mechanical ventilation

Robustness measures the performance of estimation methods when they work under non-ideal conditions. We compared the robustness of artificial neural networks (ANNs) and multilinear fitting (MLF) methods in estimating respiratory system compliance (C_RS) during mechanical ventilation (MV). Twenty-four anaesthetized pigs underwent MV. Airway pressure, flow and volume were recorded at fixed intervals after the induction of acute lung injury. After consecutive mechanical breaths, an inspiratory pause (BIP) was applied in order to calculate C_RS using the interrupter technique. From the breath preceding the BIP, ANN and MLF had to compute C_RS in the presence of two types of perturbations: transient sensor disconnection (TD) and random noise (RN). Performance of the two methods was assessed according to Bland and Altman. The ANN presented a higher bias and scatter than MLF during the application of RN, except when RN was lower than 2% of peak airway pressure. During TD, MLF algorithm showed a higher bias and scatter than ANN. After the application of RN, ANN and MLF maintain a stable performance, although MLF shows better results. ANNs have a more stable performance and yield a more robust estimation of C_RS than MLF in conditions of transient sensor disconnection.

Assessment of a volume-dependent dynamic respiratory system compliance in ALI/ARDS by pooling breathing cycles

New methods were developed to calculate the volume-dependent dynamic respiratory system compliance (C(rs)) in mechanically ventilated patients. Due to noise in respiratory signals and different characteristics of the methods, their results can considerably differ. The aim of the study was to establish a practical procedure to validate the estimation of intratidal dynamic C(rs). A total of 28 patients from intensive care units of eight German university hospitals with acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) were studied retrospectively. Dynamic volume-dependent C(rs) was determined during ongoing mechanical ventilation with the SLICE method, dynostatic algorithm and adaptive slice method. Conventional two-point compliance C(2P) was calculated for comparison. A number of consecutive breathing cycles were pooled to reduce noise in the respiratory signals. C(rs)-volume curves produced with different methods converged when the number of pooling cycles increased (n ≥ 7). The mean volume-dependent C(rs) of 20 breaths was highly correlated with mean C(2P) (C(2P,mean) = 0.945 × C(rs,mean) - 0.053, r(2) = 0.968, p < 0.0001). The Bland-Altman analysis indicated that C(2P,mean) was lower than C(rs,mean) (-2.4 ± 6.4 ml cm(-1) H(2)O, mean bias ± 2 SD), but not significant according to the paired t-test (p > 0.05). Methods for analyzing dynamic respiratory mechanics are sensitive to noise and will converge to a unique solution when the number of pooled cycles increases. Under steady-state conditions, assessment of the volume-dependent C(rs) in ALI/ARDS patients can be validated by pooling respiratory data of consecutive breaths regardless of which method is applied. Confidence in dynamic C(rs) determination may be increased with the proposed pooling.

Pulmonary mechanics during mechanical ventilation

The use of mechanical ventilation has become widespread in the management of hypoxic respiratory failure. Investigations of pulmonary mechanics in this clinical scenario have demonstrated that there are significant differences in compliance, resistance and gas flow when compared with normal subjects. This paper will review the mechanisms by which pulmonary mechanics are assessed in mechanically ventilated patients and will review how the data can be used for investigative research purposes as well as to inform rational ventilator management.

Prolonged moderate pressure recruitment manoeuvre results in lower optimal positive end-expiratory pressure and plateau pressure

BACKGROUND

In acute lung injury (ALI)/acute respiratory distress syndrome (ARDS), recruitment manoeuvres (RMs) are used frequently. In pigs with induced ALI, superior effects have been found using a slow moderate-pressure recruitment manoeuvre (SLRM) compared with a vital capacity recruitment manoeuvre (VICM). We hypothesized that the positive recruitment effects of SLRM could also be achieved in ALI/ARDS patients. Our primary research question was whether the same compliance could be obtained using lower RM pressure and subsequent positive end-expiratory pressure (PEEP). Secondly, optimal PEEP levels following the RMs were compared, and the use of volume-dependent compliance (VDC) to identify successful lung recruitment and optimal PEEP was evaluated.

PATIENTS AND METHODS

We performed a prospective randomised cross-over study where 16 ventilated patients with early ALI/ARDS each were subjected to the two RMs, followed by decremental PEEP titration. Volume-dependent initial, middle and final compliance (C(ini) , C(mid) and C(fin) ) were determined. Electric impedance tomography and end-expiratory lung volume measurements were used to follow lung volume changes.

RESULTS

The maximum response in compliance, PaO₂/FIO₂, venous admixture and C(ini) /C(fin) after recruitment, during decremental PEEP, was at significantly lower PEEP and plateau pressure after SLRM than VICM. Fewer patients responded in gas exchange after the SLRM, which was not the case for lung mechanics. The response in C(ini) was more pronounced than in conventional compliance.

CONCLUSIONS

The same compliance increase is achieved with SLRM as with VICM, and lower PEEP can be used, with correspondingly lower plateau pressures. VDC seems promising to identify successful recruitment and optimal PEEP.

A new non-radiological method to assess potential lung recruitability: a pilot study in ALI patients

INTRODUCTION

Potentially recruitable lung has been assessed previously in patients with acute lung injury (ALI) by computed tomography. A large variability in lung recruitability was observed between patients. In this study, we assess whether a new non-radiological bedside technique could determine potentially recruitable lung volume (PRLV) in ALI patients.

METHODS

Sixteen mechanically ventilated patients with early ALI/ARDS were subjected to a recruitment manoeuvre and decremental PEEP titration. Electric impedance tomography, together with measurements of end-expiratory lung volume (EELV) and tracheal pressure, were used to determine PRLV. The method defines fully recruited open lung volume (OLV) as the volume reached at the end of two consecutive vital capacity manoeuvres to 40 cmH₂O. It also uses extrapolation of the baseline alveolar pressure/volume curve up to 40 cmH₂O, the volume reached being the non-recruited lung volume. The difference between the fully recruited and the non-recruited volume was defined as PRLV.

RESULTS

We observed a considerable heterogeneity among the patients in lung recruitability, PRLV range 11-47%. In a post hoc analysis, dividing the patients into two groups, a high and a low PRLV group, we found at baseline before the recruitment manoeuvre that the high PRLV group had lower compliance and a lower fraction of EELV/OLV.

CONCLUSIONS

Using non-invasive radiation-free bedside methods, it may be possible to measure PRLV in ALI/ARDS patients. It is possible that this technique could be used to determine the need for recruitment manoeuvres and to select PEEP level on the basis of lung recruitability.

Do newer monitors of exhaled gases, mechanics, and esophageal pressure add value?

The current understanding of lung mechanics and ventilator-induced lung injury suggests that patients who have acute respiratory distress syndrome should be ventilated in such a way as to minimize alveolar over-distension and repeated alveolar collapse. Clinical trials have used such lung protective strategies and shown a reduction in mortality; however, there is data that these "one-size fits all" strategies do not work equally well in all patients. This article reviews other methods that may prove useful in monitoring for potential lung injury: exhaled breath condensate, pressure-volume curves, and esophageal manometry. The authors explore the concepts, benefits, difficulties, and relevant clinical trials of each.

Dynamic respiratory mechanics in acute lung injury/acute respiratory distress syndrome: research or clinical tool?

PURPOSE OF REVIEW

Classic static measurements of lung mechanics have been used mainly for research purposes, but have not gained widespread clinical acceptance. Instead, dynamic measurements have been used, but interpretation of results has been hampered by lack of clear definitions. The review provides an overview of possible definitions and a description of methods for evaluating lung mechanics in acute lung injury/acute respiratory distress syndrome patients.

RECENT FINDINGS

Compliance measured using static techniques is significantly higher compared to measurements during ongoing ventilation. This indicates that lung mechanic properties depend on flow velocity during inflation and the time allowed for equilibration of viscoelastic forces. Thus, methods for evaluating lung mechanics should be clearly defined in terms of whether they are classically static, i.e. excluding resistance to flow and equilibration of viscoelastic forces, or truly dynamic, i.e. including flow resistance and unequilibrated viscoelastic forces. New techniques have emerged which make it possible to monitor lung mechanics during ongoing, therapeutic ventilation, 'functional lung mechanics', where the impact of flow resistance on tube and airway resistance has been eliminated, providing alveolar pressure/volume curves.

SUMMARY

Functional lung mechanics obtained during ongoing ventilator treatment have the potential to provide information for optimizing ventilator management in critically ill patients.

Maximal hysteresis: a new method to set positive end-expiratory pressure in acute lung injury?

BACKGROUND

No methods are superior when setting positive end-expiratory pressure (PEEP) in acute lung injury (ALI). In ALI, the vertical distance (hysteresis) between the inspiratory and expiratory limbs of a static pressure-volume (PV) loop mainly indicates lung recruitment. We hypothesized that PEEP set at the pressure where hysteresis is 90% of its maximum (90%MH) would give similar oxygenation, but less cardiovascular depression than PEEP set at the pressure at lower inflection point (LIP) on the inspiratory limb or at the point of maximal curvature (PMC) on the expiratory limb in ALI.

METHODS

In 12 mechanically ventilated pigs, ALI was induced in a randomized fashion by lung lavage, lung lavage plus injurious ventilation, or by oleic acid. From a static PV loop obtained by an interrupted low-flow method, the pressures at LIP [25 (25, 25) cmH(2)O, mean and 25, 75 percentiles], at PMC [24 (20, 24) cmH(2)O], and at 90% MH [19 (18, 19) cmH(2)O] were determined and used for the PEEP-settings. We measured lung inflation (by computed tomography), end-expiratory lung volume (EELV), airway pressures, compliance of the respiratory system (Crs), blood gases, cardiac output and arterial blood pressure.

RESULTS

There were no differences between the PEEP settings in EELV or oxygenation, but the 90%MH setting gave lower end-inspiratory pause pressure (P<0.025), higher Crs (P<0.025), less hyper-aeration (P<0.025) and better maintained hemodynamics.

CONCLUSION

In this porcine lung injury model, PEEP set at 90% MH gave better lung mechanics and hemodynamics, than PEEP set at PMC or LIP.

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.

Static pressure-volume curve characteristics are moderate estimators of optimal airway pressures in a mathematical model of (primary/pulmonary) acute respiratory distress syndrome

OBJECTIVE

To study the value of objective pressure-volume characteristics for predicting optimal airway pressures and the development of atelectasis and overstretching during a structured lung volume recruitment procedure with subsequent reduction in airway pressures.

METHODS

We used a mathematical model of a lung with adjustable characteristics of acute respiratory distress syndrome (ARDS) characteristics. Simulations were performed in five grades of ARDS in the presence of pure alveolar or combined alveolar-small airway closure as well complete or incomplete lung volume recruitability. For each simulation optimal end-expiratory pressure was determined. A static pressure-volume curve was constructed and objective characteristics of this curve calculated. The predictive value of these characteristics for end-expiratory atelectasis, overstretching, and optimal end-expiratory pressure was assessed.

RESULTS

Simultaneous alveolar recruitment and overstretching during inflation were more pronounced than alveolar derecruitment and overstretching during deflation. End-expiratory pressure needed to prevent significant alveolar collapse in severe ARDS resulted in maximal safe tidal volumes that may be insufficient for adequate ventilation using conventional mechanical ventilatory modes. Plateau pressures well below the "upper corner point" (airway pressure where compliance decreases) resulted in significant alveolar overstretching.

CONCLUSIONS

A recruitment maneuver followed by subsequent reduction in airway pressure limits end-expiratory atelectasis, overstretching, and pressure. None of the objective characteristics of the pressure-volume curve was predictive for end-expiratory atelectasis, overstretching, or optimal airway pressure.

Alveolar pressure monitoring: an evaluation in a lung model and in patients with acute lung injury

OBJECTIVES

We evaluated an algorithm for continuous on-line monitoring of alveolar pressure over time in a lung model with lower and upper inflection points and variable resistance ratios and in patients with acute lung injury. The algorithm is based on "static" pressure/volume curves obtained from tracheal pressure measurements under dynamic conditions.

DESIGN AND SETTING

Experimental and clinical evaluation of algorithm in a university hospital laboratory and intensive care unit.

PATIENTS

Ten patients undergoing postoperative respiratory therapy (feasibility of tracheal measurement) and ten patients with acute lung injury undergoing ventilator treatment (evaluation of algorithm).

MEASUREMENTS AND RESULTS

Direct tracheal pressure measurements with a catheter inserted through the endotracheal tube. Comparison of measured alveolar and the dynostatic alveolar pressure vs. time in a lung model with changes in five ventilatory parameters. Examples of clinical monitoring are reported. In the model there was excellent agreement between alveolar pressures obtained by the algorithm, the dynostatic alveolar pressure, and measured alveolar pressure at all ventilator settings. For inspiratory/expiratory resistance ratios between 1:2.1-2.1:1, the dynostatic alveolar pressure was within +/-1.5 cm H(2)O of measured alveolar pressure. In patients the technique for direct tracheal pressure measurement using a catheter inserted through the endotracheal tube functioned satisfactorily with intermittent air flushes for cleansing.

CONCLUSIONS

Using a thin tracheal pressure catheter inserted through the endotracheal tube alveolar pressure allows continuous bedside monitoring with ease and precision using the dynostatic algorithm. The method is unaffected by tube and connector geometry or by secretions.

The dynostatic algorithm accurately calculates alveolar pressure on-line during ventilator treatment in children

BACKGROUND

Monitoring of respiratory mechanics during ventilator treatment in paediatric intensive care is currently based on pressure and flow measurements in the ventilator or at the Y-piece. The characteristics of the tracheal tube will modify the pressures affecting the airways and alveoli in an unpredictable manner. The dynostatic algorithm (DSA), based on a one-compartment lung model, calculates the alveolar pressure during on-going ventilation. The DSA is based on accurate measurement of tracheal pressure. The purpose of this study was to test the validity of the DSA in a paediatric lung model and to apply the concept in an observational clinical study in children.

METHODS

We validated the DSA in a paediatric lung model with linear, nonlinear pressure flow and frequency-dependent characteristics by comparing calculated dynostatic (alveolar) pressures with directly measured alveolar pressures in the model and proximal plateau pressure with maximum alveolar pressure. Sixty combinations of ventilation modes, positive end expiratory pressures, inspiratory : expiratory ratios, volumes and frequencies were studied. A 0.25-mm fibreoptic pressure transducer in the tube lumen was used in combination with volume and flow from ventilator signals. Clinical measurements were performed in eight patients during anaesthesia and postoperative ventilator treatment.

RESULTS

In the lung model we found a correlation coefficient between calculated and measured alveolar pressure of 0.93-0.99 with root mean square median values of 1 cm H2O. Distal plateau pressure agreed well with maximum alveolar pressure. In the clinical situation, the algorithm provided a breath-by-breath display of the volume-dependent lung compliance and the temporal course of alveolar pressure during uninterrupted ventilation.

CONCLUSIONS

Fibreoptic measurement of tracheal pressure in combination with the dynostatic calculation of alveolar pressure provides an on-line monitoring of the effects of ventilatory mode in terms of volume-dependent compliance, tracheal peak pressure and true positive end expiratory pressure.

Adjusting positive end-expiratory pressure and tidal volume in acute respiratory distress syndrome according to the pressure-volume curve

BACKGROUND

Management of acute respiratory distress syndrome (ARDS) patients implies the selection of the adequate ventilatory parameters, essentially PEEP and tidal volume (Vt), to prevent ventilator-induced lung injury. These parameters should be reset as the lung injury evolves. Among the different methods proposed for the adjustment of the ventilator, the measurement of the P-V curve has emerged as a useful, although debated, tool. Our aim has been to study the relationship between the different inflection points of the P-V curve in ARDS patients, and to assess the changes in the empiric PEEP and Vt (PEEP(emp), V(temp) following its use.

METHODS

P-V curves were measured in 27 patients (lung injury score [LIS] >or= 2, 69 measurements) by means of the low-flow continuous inflation method.

RESULTS

A lower inflection point (LIP) was found in all patients and, although it correlated with the PEEP(emp), there was only a fair concordance, so the PEEP was modified in 80% of the cases. The expiratory inflection point (EIP) was significantly lower than the LIP (6.3 +/- 1.7 vs. 8.1 +/- 3.2, P = 0.008). An upper inflection point was observed in 16 measurements (23%) and the Vt was reset in 20% of the cases. Both PEEP and Vt were readjusted on 10 occasions (14%). Only the EIP was significantly higher on the first 3 days of mechanical ventilation. The LIS was correlated with all the inflection points. There were no differences for any parameter independent of the cause of the ARDS (pulmonary/extrapulmonary).

CONCLUSIONS

The quasi-static measurement of the P-V curve is a simple method, easy to interpret, for objective adjustment of the ventilatory parameters in ARDS patients as the lung injury evolves. The implementation of this strategy may vary the empiric clinical practice. The role of the EIP for the evaluation of the severity of lung injury deserves further investigation.

Ventilator treatment in the Nordic countries. A multicenter survey

BACKGROUND

A 1-day point prevalence study was performed in the Nordic countries to identify ventilator-treatment strategies in the region.

MATERIAL AND METHODS

On 30 May 30 2001 all mechanically ventilated patients in 27 intensive care units (ICUs) were registered via the internet. The results are shown as medians (25th, 75th percentile).

RESULTS

One hundred and eight patients were included (69% male) with new simplified acute physiology score (SAPS) 48 (37,57) and 4.5 d (2,11) of ventilator treatment. The most frequent indication for ventilator treatment was acute respiratory failure (73%). Airway management was by endotracheal tube (64%), tracheostomy (32%) and facial mask (4%). Pressure regulated ventilator modes were used in 86% of the patients and spontaneous triggering was allowed in 75%. The tidal volume was 7 ml/kg (6,9), peak inspiratory pressure 22 cmH2O (18,26) and positive end-expiratory pressure (PEEP) 6 cmH2O (6,9). FiO2 was 40% (35,50), SaO2 97% (95-98), PaO2 11 kPa (10,13), PaCO2 5.4 kPa (4.7,6.3), pH 7.43 (7.38,7.47) and BE 2.0 mmol/l (- 0.5,5). The PaO2/FiO2 ratio was 220 mmHg (166,283). The peak inspiratory pressure (r=0.37), mean airway pressure (r=0.36), PEEP (r=0.33), tidal volume (r=0.22) and SAPS score (r=0.19) were identified as independent variables in relation to the PaO2/FiO2 ratio.

CONCLUSION

The vast majority of patients were ventilated with pressure-regulated modes. Tidal volume was well below what has been considered conventional in recent large trials. Correlations between the parameters of gas exchange, respiratory mechanics, ventilator settings and physiological status of the patients was poor. It appears that blood gas values are the main tool used to steer ventilator treatment. These results may help to design future interventional studies of ventilator treatment.