Monitoring Of Respiratory Mechanics Research Articles

Important step of weaning from mechanical ventilation: accurate assessment of respiratory muscle strength

Mechanical ventilation (MV) is a powerful mean to rescue patients with respiratory failure. In view of the different etiology and basic respiratory function of patients with respiratory failure, weaning failure often occurs. Prolonged MV time is often accompanied by many complications. Thus, deeply understanding the pathophysiological changes of respiratory failure and strengthen monitoring of respiratory mechanics are helpful to optimize MV parameter settings, reduce ventilator-induced lung injury and wean from MV as early as possible. A successful weaning from MV depends on many factors, the most important factors are respiratory muscle strength, respiratory load and respiratory drive. Spontaneous breathing trial (SBT) is an important part of weaning process. The main purpose of implementing SBT is to screen patients and opportunities to weaning from MV, and find reversible reasons for not passing SBT. Because the accuracy of SBT in assessing weaning prognosis is about 85%, it is not adequate for difficult weaning patients. Standardized measurement of weaning indicators for patients with difficulty weaning is conducive to accurate assessment of respiratory muscle strength and improve the success rate of weaning from MV.

Respiratory distress associated with acute hydrothorax during transurethral electrocoagulation: a case report

BackgroundIn patients undergoing abdominal radiotherapy or transurethral surgery, bladder perforations are a possible complication. Likewise, pleural effusions due to a pleuroperitoneal leak caused by either a congenital or acquired diaphragmatic defect can also occur. We report a case in which a saline solution, which migrated into the abdominal cavity from a bladder perforation during transurethral electrocoagulation, further formed bilateral pleural effusions and caused rapid ventilation failure.Case presentationA patient undergoing radiation therapy and hormone therapy for prostate cancer underwent emergency surgery for electrocoagulation due to hematuria and a rapid drop in hemoglobin. The surgery began under general anesthesia, and we first noticed an increase in airway pressure and a decrease in dynamic lung compliance, followed by abdominal distension. Based on readouts from the respiratory mechanics monitor, we suspected lung abnormalities and performed a pulmonary ultrasound, leading to a diagnosis of bilateral pleural effusions, which we then drained.ConclusionsRespiratory mechanics monitoring is simple and can be performed at all times during anesthesia, and when combined with pulmonary ultrasound, diagnoses can be made quickly and prevent deaths.

Assessing the Asynchrony Event Based on the Ventilation Mode for Mechanically Ventilated Patients in ICU.

Respiratory system modelling can assist clinicians in making clinical decisions during mechanical ventilation (MV) management in intensive care. However, there are some cases where the MV patients produce asynchronous breathing (asynchrony events) due to the spontaneous breathing (SB) effort even though they are fully sedated. Currently, most of the developed models are only suitable for fully sedated patients, which means they cannot be implemented for patients who produce asynchrony in their breathing. This leads to an incorrect measurement of the actual underlying mechanics in these patients. As a result, there is a need to develop a model that can detect asynchrony in real-time and at the bedside throughout the ventilated days. This paper demonstrates the asynchronous event detection of MV patients in the ICU of a hospital by applying a developed extended time-varying elastance model. Data from 10 mechanically ventilated respiratory failure patients admitted at the International Islamic University Malaysia (IIUM) Hospital were collected. The results showed that the model-based technique precisely detected asynchrony events (AEs) throughout the ventilation days. The patients showed an increase in AEs during the ventilation period within the same ventilation mode. SIMV mode produced much higher asynchrony compared to SPONT mode (p < 0.05). The link between AEs and the lung elastance (was also investigated. It was found that when the AEs increased, the decreased and vice versa based on the results obtained in this research. The information of AEs and provides the true underlying lung mechanics of the MV patients. Hence, this model-based method is capable of detecting the AEs in fully sedated MV patients and providing information that can potentially guide clinicians in selecting the optimal ventilation mode of MV, allowing for precise monitoring of respiratory mechanics in MV patients.

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.

Interpretation of the transpulmonary pressure in the critically ill patient.

Mechanical ventilation is a life-saving procedure, which takes over the function of the respiratory muscles while buying time for healing to take place. However, it can also promote or worsen lung injury, so that careful monitoring of respiratory mechanics is suggested to titrate the level of support and avoid injurious pressures and volumes to develop. Standard monitoring includes flow, volume and airway pressure (Paw). However, Paw represents the pressure acting on the respiratory system as a whole, and does not allow to differentiate the part of pressure that is spent di distend the chest wall. Moreover, if spontaneous breathing efforts are allowed, the Paw is the sum of that applied by the ventilator and that generated by the patient. As a consequence, monitoring of Paw has significant shortcomings. Assessment of esophageal pressure (Pes), as a surrogate for pleural pressure (Ppl), may allow the clinicians to discriminate between the elastic behaviour of the lung and the chest wall, and to calculate the degree of spontaneous respiratory effort. In the present review, the characteristics and limitations of airway and transpulmonary pressure monitoring will be presented; we will highlight the different assumptions underlying the various methods for measuring transpulmonary pressure (i.e., the elastance-derived and the release-derived method, and the direct measurement), as well as the potential application of transpulmonary pressure assessment during both controlled and spontaneous/assisted mechanical ventilation in critically ill patients.

Chest wall effect on the monitoring of respiratory mechanics in acute respiratory distress syndrome.

The respiratory system mechanics depend on the characteristics of the lung and chest wall and their interaction. In patients with acute respiratory distress syndrome under mechanical ventilation, the monitoring of airway plateau pressure is fundamental given its prognostic value and its capacity to assess pulmonary stress. However, its validity can be affected by changes in mechanical characteristics of the chest wall, and it provides no data to correctly titrate positive end-expiratory pressure by restoring lung volume. The chest wall effect on respiratory mechanics in acute respiratory distress syndrome has not been completely described, and it has likely been overestimated, which may lead to erroneous decision making. The load imposed by the chest wall is negligible when the respiratory system is insufflated with positive end-expiratory pressure. Under dynamic conditions, moving this structure demands a pressure change whose magnitude is related to its mechanical characteristics, and this load remains constant regardless of the volume from which it is insufflated. Thus, changes in airway pressure reflect changes in the lung mechanical conditions. Advanced monitoring could be reserved for patients with increased intra-abdominal pressure in whom a protective mechanical ventilation strategy cannot be implemented. The estimates of alveolar recruitment based on respiratory system mechanics could reflect differences in chest wall response to insufflation and not actual alveolar recruitment.

Evaluation of model-based methods in estimating respiratory mechanics in the presence of variable patient effort

Monitoring of respiratory mechanics is required for guiding patient-specific mechanical ventilation settings in critical care. Many models of respiratory mechanics perform poorly in the presence of variable patient effort. Typical modelling approaches either attempt to mitigate the effect of the patient effort on the airway pressure waveforms, or attempt to capture the size and shape of the patient effort. This work analyses a range of methods to identify respiratory mechanics in volume controlled ventilation modes when there is patient effort. The models are compared using 4 Datasets, each with a sample of 30 breaths before, and 2–3 minutes after sedation has been administered. The sedation will reduce patient efforts, but the underlying pulmonary mechanical properties are unlikely to change during this short time.Model identified parameters from breathing cycles with patient effort are compared to breathing cycles that do not have patient effort. All models have advantages and disadvantages, so model selection may be specific to the respiratory mechanics application. However, in general, the combined method of iterative interpolative pressure reconstruction, and stacking multiple consecutive breaths together has the best performance over the Dataset. The variability of identified elastance when there is patient effort is the lowest with this method, and there is little systematic offset in identified mechanics when sedation is administered.

Respiratory Mechanics in Acute Respiratory Distress Syndrome: A Quality Improvement Based Registry Project

The amount of pathophysiological impairment in patients with acute respiratory distress syndrome (ARDS) is variable and applying the same ventilator regimen to every patient is questionable. Monitoring of respiratory mechanics for the lung and chest wall allows an individualization of ventilator settings with a potential benefit for the patient. a registry with a large sample size will elicit helpful epidemiological information and may inform future recommendations. We therefore proposed a quality improvement (QI) program constituted by systematic assessment of respiratory mechanics and gas exchange response. the collected data are then introduced into a registry. We report here the preliminary results.

The effect of mechanical ventilator settings during ventilator hyperinflation techniques: a bench-top analysis.

Ventilator hyperinflations are used by physiotherapists for the purpose of airway clearance in intensive care. There is limited data to guide the selection of mechanical ventilator modes and settings that may achieve desired flow patterns for ventilator hyperinflation. A mechanical ventilator was connected to two lung simulators and a respiratory mechanics monitor. Peak inspiratory (PIFR) and expiratory flow rates (PEFR) were measured during manipulation of ventilator modes (pressure support ventilation [PSV], volume-controlled synchronised intermittent mandatory ventilation [VC-SIMV] and pressure-controlled synchronised intermittent mandatory ventilation [PC-SIMV]) and ventilator settings (including set tidal volume, positive end-expiratory pressure, inspiratory flow rate, inspiratory pause, pressure support, inspiratory time and/or inflation pressure). Additionally, each trial was conducted with high (0.05 l/cmH2O) and low (0.01 l/cmH2O) compliance settings on the lung simulators. Each trial was dichotomised into success or failure under three categories (attainment of PIFR-PEFR less than or equal to 0.9, PEFR/PIFR greater than 17 l/min, PEFR greater than or equal to 40 l/min). A total of 232 trials were conducted (96 VC-SIMV, 96 PC-SIMV, 40 PSV). A greater proportion of VC-SIMV trials were ceased due to high peak inspiratory pressures (35%). However, VC-SIMV trials were more likely to be successful at meeting all three outcome measures (26 VC-SIMV trials, 7 PC-SIMV trials, 0 PSV trials). It was found that manipulation of settings in VC-SIMV mode appears more successful than PSV and PC-SIMV for ventilator hyperinflations.

Expiratory model-based method to monitor ARDS disease state

IntroductionModel-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.MethodsThree 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.ResultsThe 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.ConclusionOverall, 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.

Pleural effusion complicates monitoring of respiratory mechanics*

Although pleural effusion reduces respiratory system compliance by restricting the lungs, the effusion volume is partially accommodated by chest wall expansion. The implications for these opposing volume changes on airway pressure monitoring in ventilated patients with pleural effusion are unreported. We investigated the interactions among pleural effusion, positive end-expiratory pressure, and indices of respiratory mechanics in a swine model. Interventional animal model. Hospital animal research facility. Nine deeply anesthetized swine. The preparation included tracheostomy, arterial/venous catheter placement, and chest tube insertion. Animals were ventilated throughout the study at 9 mL/kg, and frequency was adjusted to maintain normocapnia (inspiratory:expiratory=1:2, FIO2=0.5) and positive end-expiratory pressure of 1 cm H2O and 10 cm H2O. Fluid was instilled into the right pleural space to simulate effusions of 13 mL/kg (pleural effusion 1) and 26 mL/kg (pleural effusion 2). Quantitative computerized tomography studies (in five animals) and functional residual capacity volumes (wash-in/wash-out technique) were obtained for each pleural effusion/positive end-expiratory pressure combination. Mean functional residual capacity compared to baseline at positive end-expiratory pressure of 1 cm H2O was decreased by pleural effusion 1 and pleural effusion 2 (-42%, -64%) and restored by positive end-expiratory pressure of 10 cm H2O (moderate) to +23% of baseline for pleural effusion 1 and +1% for pleural effusion 2. Plateau pressure increased and compliance decreased in response to pleural effusion 1 and pleural effusion 2. Moderate positive end-expiratory pressure applied during both pleural effusion quantities restored plateau pressure and tidal compliance to prepleural effusion values. Computed tomography studies revealed lung compression and tidal derecruitment cycles occurring with pleural effusion at positive end-expiratory pressure of 1 cm H2O, whereas a moderate positive end-expiratory pressure restored prepleural effusion functional residual capacity and prevented lung and intratidal derecruitment. When pleural effusion is present, respiratory mechanics must be interpreted cautiously and sufficient positive end-expiratory pressure should be applied to prevent extensive collapse and intratidal cycles of recruitment/derecruitment.

Involuntary Cough Strength and Extubation Outcomes for Patients in an ICU

Removing the artificial airway is the last step in the mechanical ventilation withdrawal process. In order to assess cough effectiveness, a critical component of this process, we evaluated the involuntary cough peak flow (CPFi) to predict the extubation outcome for patients weaned from mechanical ventilation in ICUs. One hundred fifty patients were weaned from ventilators, passed a spontaneous breathing trial (SBT), and were judged by their physician to be ready for extubation in the Tri-Service General Hospital ICUs from February 2003 to July 2003. CPFi was induced by 2 mL of normal saline solution at the end of inspiration and measured using a hand-held respiratory mechanics monitor. All patients were then extubated. Of 150 enrolled patients for this study, 118 (78.7%) had successful extubation and 32 (21.3%) failed. In the univariate analysis, there were higher Acute Physiology and Chronic Health Evaluation (APACHE) II scores (16.0 vs 18.5, P = .018), less negative maximum inspiratory pressure (-45.0 vs -39.0, P = .010), lower cough peak flows (CPFs) (74.0 vs 42.0 L/min, P < .001), longer postextubation hospital stays (15.0 vs 31.5 days, P < .001), and longer postextubation ICU stays (1.0 vs 9.5 days, P < .001) in the extubation failures compared with the extubation successes. In the multivariate analysis, we found that a higher APACHE II score and a lower CPF were related to increasing risk of extubation failure (odds ratio [OR] = 1.13; 95% CI, 1.03-1.25; and OR = 0.95; 95% CI, 0.93-0.98, respectively). The receiver operator characteristic curve cutoff point for CPF was 58.5 L/min, with a sensitivity of 78.8% and specificity of 78.1%. CPFi as an indication of cough reflex has the potential to predict successful extubation in patients who pass an SBT.

Restoration of pulmonary compliance after laparoscopic surgery using a simple alveolar recruitment maneuver

Study Objective To test the hypothesis that a pulmonary maneuver designed to recruit additional alveoli (thereby decreasing atelectasis) applied before extubation can restore pulmonary compliance to baseline values. Design Cohort study. Setting Operating room of a university hospital. Patients 20 ASA physical status I and II patients scheduled to undergo laparoscopic radical nephrectomy. Interventions Participants received a balanced general anesthesia using intermittent positive pressure ventilation. A pulmonary recruitment maneuver was performed as a single manual inflation of the lungs to 40 cm H 2O, maintained for 10 seconds after release of pneumoperitoneum. Measurements Respiratory mechanics including dynamic compliance were measured continuously using the VenTrak respiratory mechanics monitor (VenTrak; Novametrix, Wallingford, CT, USA). Respiratory measures were recorded together with arterial blood gases after induction (T1), with the patient placed in the lateral “jackknife” position (T2), 10 and 120 minutes after CO 2 insufflation (T3 and T4), immediately after desufflation in the lateral and supine positions (T5 and T6), and 10 minutes after a pulmonary recruitment maneuver at the conclusion of surgery (T7). Outcome data were analyzed using analysis of variance for repeated measures; P < 0.05 was defined as statistically significant. Main Results On average, compliance decreased from an initial value of 63.5 to 52.6 mL/cmH 2O when patients were turned from the supine to the lateral position (T1 vs. T2; P < 0.001), and decreased further to 31.07 mL/cmH 2O after CO 2 insufflation (T2 vs. T3; P < 0.001). Compliance increased to 50.8 mL/cmH 2O after desufflation and 54.4 mL/cmH 2O after turning the patient to the supine position, but did not return to baseline levels until after performance of the pulmonary recruitment maneuver, 64.3 mL/cmH 2O (T6 vs. T7; P < 0.001, and T1 vs. T7; P = 0.73). Conclusions Respiratory mechanics do not fully return to baseline levels after desufflation following laparoscopy; however, lung compliance can be fully restored using a simple alveolar recruitment maneuver.

Malfunction of the New Aisys® Anesthesia Machine

GE Healthcare, Life Support Solutions, Madison, Wisconsin. michael.mitton@med.ge.comGE Healthcare would like to thank Anesthesiology for the opportunity to respond to the Letter to the Editor by Drs. Wax and Neustein.In their letter, the authors discuss two issues pertaining to the Aisys® machine. The first concerns the function of the EZchange absorber manifold, the second concerns the effect of water on the inspiratory and expiratory flow sensors.We have been able to reproduce the leak described by the authors. The breathing circuit leaks experienced at Mt. Sinai (New York, NY) were the result of an inadequate seal between the manifold and the absorber canister; small frays on the drain port of the disposable absorber canister produced the inadequate seal. This issue may be identified during the Aisys® automatic system checkout procedure; we have verified this with affected canisters. Of course, if the canister is changed in the middle of a case, there would be no additional system checkout and a leak may result.We have not been able to reproduce a system leak when the EZchange manifold is in place without a disposable canister because the leak exists where the manifold and the disposable absorber canister connect, as suggested by the authors.We have taken a two-pronged approach to resolving this issue. First, we are addressing the root cause, the disposable absorber canister, by working with the third-party supplier to remedy the issue with the drain port flashing. Second, we are currently revising the drain seal on the EZchange module so this flashing, even if unchanged, will not affect the seal between the canister and the manifold.With respect to the moisture and flow sensor issue, the root cause was most likely the impact that moisture or water may have on the function of the flow sensor. Our flow sensors are a vital component of the Aisys® ventilator. Like anesthesia practice, these sensors have undergone extensive evolution since they were first introduced. As clinical anesthesia has moved toward lower and lower fresh gas flows, the impact of the increased humidity has necessitated a redesign of the basic flow sensor. At the time of the events, Mt. Sinai was using an earlier version of the flow sensor. The current flow sensor incorporates an offset in the area of the flow sensor flap that helps to overcome the issue associated with high moisture or water. This version can be readily recognized by the presence of grooves on the bezel of the flow sensor closest to the patient.The final issue the authors describe is the use of the D-Lite sensor to obtain spirometry instead of using the flow sensors. The D-Lite sensor and the direct patient monitoring of respiratory mechanics the use of the D-Lite may provide are a design feature of the Aisys®. Using the D-Lite will not resolve alarm issues produced by the inspiratory and expiratory flow sensor. The authors’ description of the ventilator alarms suggests that the old-style flow sensors they were using at the time may have either had some moisture on the inspiratory sensor or may not have been calibrated.The flow sensors should be calibrated daily simply by removing the flow sensor module, waiting for the “no insp flow sensor” and “no expiratory flow sensor” messages to appear, and then reattaching the flow sensor module. The alarm messages reported by Mt. Sinai would not have occurred with calibrated offset flow sensors.GE Healthcare commends the authors for their insightful and accurate Letter to the Editor.GE Healthcare, Life Support Solutions, Madison, Wisconsin. michael.mitton@med.ge.com

Online SLICE: a tool for continuous monitoring of respiratory mechanics

The clinical use of respiratory mechanics is limited due to complicated measurement methods and restricted bedside availability of the achievable information. For observational and interventional studies in the ICUs of the university clinics of Freiburg we developed a laptop-based tool, which provides online information about the mechanical state of the patient's respiratory system on a breath by breath basis.

Noninvasive monitoring of respiratory mechanics during sleep

The sleep apnoea-hypopnoea syndrome is characterised by recurrent obstructions of the upper airway, resulting in sleep disruption and arterial oxygen desaturations. Noninvasive assessment of respiratory mechanics during sleep is helpful in facilitating the diagnosis and treatment of patients with sleep apnoea-hypopnoea syndrome. This series summarises the different tools that are currently available to noninvasively assess respiratory mechanics during sleep breathing disturbances. These techniques are classified according to the main variable monitored: ventilation, breathing effort or airway obstruction. Changes in patient ventilation are assessed by recording flow or volume signals by means of pneumotachographs, thermistors or thermocouples, nasal prongs or thoraco-abdominal bands. Common tools to noninvasively assess breathing efforts are the thoraco-abdominal bands and the pulse transit time technique. Upper airway obstruction is noninvasively characterised by its upstream resistance and its critical pressure or by means of the forced oscillation technique. Given the technical and practical limitations of each technique, combining different tools improves the reliability and robustness of patient assessment during sleep.

The forced oscillation technique in clinical practice: methodology, recommendations and future developments.

The forced oscillation technique (FOT) is a noninvasive method with which to measure respiratory mechanics. FOT employs small-amplitude pressure oscillations superimposed on the normal breathing and therefore has the advantage over conventional lung function techniques that it does not require the performance of respiratory manoeuvres. The present European Respiratory Society Task Force Report describes the basic principle of the technique and gives guidelines for the application and interpretation of FOT as a routine lung function test in the clinical setting, for both adult and paediatric populations. FOT data, especially those measured at the lower frequencies, are sensitive to airway obstruction, but do not discriminate between obstructive and restrictive lung disorders. There is no consensus regarding the sensitivity of FOT for bronchodilation testing in adults. Values of respiratory resistance have proved sensitive to bronchodilation in children, although the reported cutoff levels remain to be confirmed in future studies. Forced oscillation technique is a reliable method in the assessment of bronchial hyperresponsiveness in adults and children. Moreover, in contrast with spirometry where a deep inspiration is needed, forced oscillation technique does not modify the airway smooth muscle tone. Forced oscillation technique has been shown to be as sensitive as spirometry in detecting impairments of lung function due to smoking or exposure to occupational hazards. Together with the minimal requirement for the subject's cooperation, this makes forced oscillation technique an ideal lung function test for epidemiological and field studies. Novel applications of forced oscillation technique in the clinical setting include the monitoring of respiratory mechanics during mechanical ventilation and sleep.

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

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. 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. 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. 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.

Direct Measurement of Intratracheal Pressure in Pediatric Respiratory Monitoring

We describe a method based on a Fabry-Perot interferometer at the tip of an optic fiber with a diameter of 0.25 mm for direct measurement of tracheal pressure in pediatric respiratory monitoring. The response time of the pressure transducer and its influence on the resistance of pediatric endotracheal tubes (internal diameter, 2.5 to 5 mm) during constant and dynamic flow at different ventilator settings in a lung model were measured. The transducer was positioned at -1.5 (inside), 0, and +1.5 cm (outside) relative to the tip of the endotracheal tube and compared with a reference pressure inside the trachea. The clinical application of the transducer was tested in five pediatric patients. The response time of the transducer was 1.3 ms. The influence of the fiberoptic transducer on tube resistance was negligible during constant flow in inspiratory and expiratory directions for all endotracheal tubes tested. There was no difference in pressure measurements with the transducer positioned at or 1.5 cm below or above the tip of the endotracheal tube during dynamic measurements. During clinical circumstances insertion of the fiberoptic transducer was easy, recordings were stable, and the safety of the patient was not jeopardized. The fiberoptic transducer provided a reliable and promising way of monitoring tracheal pressure in intubated pediatric patients. The presence of the probe did not interfere with either pressure-flow relationship or patient care and safety. The technique is proposed for monitoring of respiratory mechanics and calculation of changes in tube resistance caused by kinking and secretions.

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

We present a concept of on-line, manoeuvre-free monitoring of respiratory mechanics during dynamic conditions, displaying calculated alveolar pressure/volume curves continuously and separating lung and chest wall mechanics. Prospective observational study. Intensive care unit of a university hospital. Ten ventilator-treated patients with acute lung injury. Different positive end-expiratory pressure (PEEP) and tidal volumes, low flow inflation. Previously validated methods were used to present a single-value dynostatic compliance for the whole breath and a dynostatic volume-dependent initial, middle and final compliance within the breath. A high individual variation of respiratory mechanics was observed. Reproducibility of repeated measurements was satisfactory (coefficients of variations for dynostatic volume-dependent compliance: < or =9.2% for total respiratory system, < or =18% for lung). Volume-dependent compliance showed a statistically significant pattern of successively decreasing compliance from the initial segment through the middle and final parts within each breath at all respiratory settings. This pattern became more prominent with increasing PEEP and tidal volume, indicating a greater distension of alveoli. No lower inflection point (LIP) was seen in patients with respiratory rate 20/min and PEEP at 4 cmH2O. A trial with low flow inflation in four of the patients showed formation of a LIP in three of them and an upper inflection in one. The monitoring concept revealed a constant pattern of successively decreasing compliance within each breath, which became more prominent with increasing PEEP and tidal volume. The monitoring concept offers a simple and reliable method of monitoring respiratory mechanics during ongoing ventilator treatment.