Patient-ventilator asynchrony during assisted mechanical ventilation

Is proportional-assist ventilation with load-adjustable gain factors a user-friendly mode?

OBJECTIVES

The aim of this study was to compare the number of interventions (ventilator settings and sedatives, analgesics and vasoactive medication dose manipulations) between critically ill patients on proportional-assist ventilation with load-adjustable gain factors (PAV+) and those on pressure support (PS).

DESIGN

Retrospective analysis of data from a previous randomized clinical trial.

METHODS

A total of 208 patients who were mechanically ventilated on controlled modes and met criteria for assisted breathing were randomized to receive either PS (n = 100) or PAV+ (n = 108). Changes in ventilator settings and in the dose of sedatives, analgesics, and vasoactive medications were identified during the period in which the patients were ventilated either with PS (30.4 +/- 17.4 h) or PAV+ (30.0 +/- 18.1 h) and classified as changes to facilitate weaning (CFW) or changes to respond to deterioration (CD).

RESULTS

The mean number of changes in ventilator settings was significantly higher with PS than that with PAV+ (10.7 +/- 5.7 vs. 8.9 +/- 4.6). With PS the proportion of these changes classified as CFW was significantly lower than that with PAV+ (59.8% vs. 69.2%). Dyssynchrony as a cause of CD was more likely to occur with PS than with PAV+ (42 vs. 3%). The mean number of changes in the dose of sedatives, analgesics, and vasoactive medications was higher with PS than with PAV+, the difference being significant only for sedatives (4.06 +/- 3.8 vs. 2.82 +/- 3.4).

CONCLUSIONS

Compared to PS, PAV+ is associated with fewer intervention in terms of ventilator settings and sedative dose changes.

Patient-ventilator asynchrony during non-invasive ventilation for acute respiratory failure: a multicenter study

OBJECTIVE

To determine the prevalence of patient-ventilator asynchrony in patients receiving non-invasive ventilation (NIV) for acute respiratory failure.

DESIGN

Prospective multicenter observation study.

SETTING

Intensive care units in three university hospitals.

METHODS

Patients consecutively admitted to ICU were included. NIV, performed with an ICU ventilator, was set by the clinician. Airway pressure, flow, and surface diaphragmatic electromyography were recorded continuously for 30 min. Asynchrony events and the asynchrony index (AI) were determined from visual inspection of the recordings and clinical observation.

RESULTS

A total of 60 patients were included, 55% of whom were hypercapnic. Auto-triggering was present in 8 (13%) patients, double triggering in 9 (15%), ineffective breaths in 8 (13%), premature cycling 7 (12%) and late cycling in 14 (23%). An AI > 10%, indicating severe asynchrony, was present in 26 patients (43%), whose median (25-75 IQR) AI was 26 (15-54%). A significant correlation was found between the magnitude of leaks and the number of ineffective breaths and severity of delayed cycling. Multivariate analysis indicated that the level of pressure support and the magnitude of leaks were weakly, albeit significantly, associated with an AI > 10%. Patient comfort scale was higher in pts with an AI < 10%.

CONCLUSION

Patient-ventilator asynchrony is common in patients receiving NIV for acute respiratory failure. Our results suggest that leaks play a major role in generating patient-ventilator asynchrony and discomfort, and point the way to further research to determine if ventilator functions designed to cope with leaks can reduce asynchrony in the clinical setting.

Variations in endotracheal cuff pressure in intubated critically ill patients: prevalence and risk factors

BACKGROUND AND OBJECTIVE

An endotracheal cuff pressure of 20-30 cmH(2)O is recommended. Underinflation and overinflation are associated with complications such as aspiration and tracheal wall damage. The aim of this study was to identify prevalence of, and risk factors for, endotracheal cuff underinflation and overinflation.

METHODS

Prospective observational cohort study. All critically ill patients intubated with a high-volume lowpressure endotracheal tube were eligible. After manual adjustment of cuff pressure at 25 cmH(2)O, continuous recording of cuff pressure and airway pressure was performed for 8 h. Underinflation and overinflation of the endotracheal cuff were defined as cuff pressure less than 20 cmH(2)O and more than 30 cmH(2)O, respectively. In all patients, the time spent with normal cuff pressure or with underinflation or overinflation of the endotracheal cuff was measured. Univariate and multivariate analyses were used to determine risk factors for cuff underinflation and overinflation.

RESULTS

Eight hundred and eight hours of cuff pressure recordings were analysed in 101 patients. Eighteen per cent of study patients spent 100% of recording time with normal (20-30 cmH(2)O) cuff pressure. Fifty-four per cent of study patients developed cuff underinflation, 73% developed cuff overinflation, and 44% developed both. Thirty- three per cent of study patients developed underinflation or overinflation for more than 30 min. Absence of sedation [odds ratio (95% confidence interval)=2.51 (1-6), P=0.03] and duration of prior intubation [1.16 (1.04-1.29), P<0.01] were independently associated with cuff underinflation. No risk factor for overinflation could be determined. The percentage of time spent with underinflation significantly (P<0.01) increased during the recording period.

CONCLUSION

Variations in endotracheal cuff pressure are common in ICU patients. Duration of prior intubation and absence of sedation are independently associated with increased risk for cuff underinflation.

A bench study of intensive-care-unit ventilators: new versus old and turbine-based versus compressed gas-based ventilators

OBJECTIVE

To compare 13 commercially available, new-generation, intensive-care-unit (ICU) ventilators in terms of trigger function, pressurization capacity during pressure-support ventilation (PSV), accuracy of pressure measurements, and expiratory resistance.

DESIGN AND SETTING

Bench study at a research laboratory in a university hospital.

METHODS

Four turbine-based ventilators and nine conventional servo-valve compressed-gas ventilators were tested using a two-compartment lung model. Three levels of effort were simulated. Each ventilator was evaluated at four PSV levels (5, 10, 15, and 20 cm H2O), with and without positive end-expiratory pressure (5 cm H2O). Trigger function was assessed as the time from effort onset to detectable pressurization. Pressurization capacity was evaluated using the airway pressure-time product computed as the net area under the pressure-time curve over the first 0.3 s after inspiratory effort onset. Expiratory resistance was evaluated by measuring trapped volume in controlled ventilation.

RESULTS

Significant differences were found across the ventilators, with a range of triggering delays from 42 to 88 ms for all conditions averaged (P < 0.001). Under difficult conditions, the triggering delay was longer than 100 ms and the pressurization was poor for five ventilators at PSV5 and three at PSV10, suggesting an inability to unload patient's effort. On average, turbine-based ventilators performed better than conventional ventilators, which showed no improvement compared to a bench comparison in 2000.

CONCLUSION

Technical performance of trigger function, pressurization capacity, and expiratory resistance differs considerably across new-generation ICU ventilators. ICU ventilators seem to have reached a technical ceiling in recent years, and some ventilators still perform inadequately.

Demand flow facilitates spontaneous breathing during high-frequency oscillatory ventilation in a pig model

OBJECTIVE

Maintenance breathing is advocated in mechanical ventilation, which is difficult for the high-frequency oscillatory (HFO) ventilation. To facilitate spontaneous breathing during HFO ventilation, a demand flow system (DFS) was designed. The aim of the present study was to evaluate the system.

DESIGN

Animal experiment.

SETTING

: University animal laboratory.

SUBJECTS

Eight pigs (47-64 kg).

INTERVENTIONS

Lung injury was induced by lung lavage with normal saline. After spontaneous breathing was restored HFO ventilation was applied, in runs of 30 minutes, with continuous fresh gas flow (CF) or the DFS operated in two different setups. Pressure to regulate the DFS was sampled directly at the Y-piece of the ventilator circuit (DFS) or between the endotracheal tube and measurement equipment at the proximal end of the endotracheal tube. In the end, animals were paralyzed. Breathing pattern, work of breathing, and gas exchange were evaluated.

MEASUREMENTS AND MAIN RESULTS

HFO ventilation with demand flow decreased breathing frequency and increased tidal volume compared with CF. Comparing HFO modes CF, DFS, and DFSPROX, total pressure-time product (PTP) was 66 cm H2O x sec x min (interquartile range 59-74), 64 cm H2O x sec x min (50-72), and 51 cm H2O x sec x min (41-63). Ventilator PTP was 36 cm H2O x sec x min (32-42), 8.6 cm H2O x sec x min (7.4-10), and 1 cm H2O x sec x min (-1.0 to 2.8). Oxygenation, evaluated by Pao2, was preserved when spontaneous breathing was maintained and deteriorated when pigs were paralyzed. Ventilation, evaluated by Paco2, improved with demand flow. Paco2 increased when using continuous flow and during muscular paralysis.

CONCLUSIONS

In moderately lung-injured anesthetized pigs during HFO ventilation, demand flow facilitated spontaneous breathing and augmented gas exchange. Demand flow decreased total breathing effort as quantified by PTP. Imposed work caused by the HFO ventilator appeared totally reduced by demand flow.

Monitoring Patient/Ventilator Interactions: Manufacturer's Perspective

The introduction of reduced and more powerful electronics has allowed the transition of medical equipment such as respiratory support devices from the hospital to the patient’s home environment. Even if this move could be beneficial for the patient, the clinician ends up in a delicate situation where little or no direct supervision is possible on the delivered treatment.

Progress in technologies led to an improved handling of patient-device interaction: manufacturers are promoting new or improved ventilation modes or cycling techniques for better patient-ventilator coupling. Even though these ventilation modes have become more responsive to patient efforts, adversely they might lead to events such as false triggering, autotriggering, delayed triggering.

In addition, manufacturers are developing tools to enhance the follow-up, remotely or offline, of the treatment by using embedded memory in the respiratory devices. This logging might be beneficial for the caregiver to review and document the treatment and tune the settings to the patient’s need and comfort. Also, remote telemedicine has been raised as a potential solution for many years without yet overall acceptance due to legal, technical and ethical problems.

Benefits of new technologies in respiratory support devices give the technical foundation for the transition from hospital to home and reducing patient/ventilator asynchronies. Healthcare infrastructure has to follow this trend in terms of cost savings versus hospital stays.

Observational study of patient-ventilator asynchrony and relationship to sedation level

PURPOSE

Clinicians frequently administer sedation to facilitate mechanical ventilation. The purpose of this study was to examine the relationship between sedation level and patient-ventilator asynchrony.

MATERIALS AND METHODS

Airway pressure and airflow were recorded for 15 minutes. Patient-ventilator asynchrony was assessed by determining the number of breaths demonstrating ineffective triggering, double triggering, short cycling, and prolonged cycling. Ineffective triggering index (ITI) was calculated by dividing the number of ineffectively triggered breaths by the total number of breaths (triggered and ineffectively triggered). Sedation level was assessed by the following 3 methods: Richmond Agitation-Sedation Scale (RASS), awake (yes or no), and delirium (Confusion Assessment Method for the intensive care unit [CAM-ICU]).

RESULTS

Twenty medical ICU patients underwent 35 observations. Ineffective triggering was seen in 17 of 20 patients and was the most frequent asynchrony (88% of all asynchronous breaths), being observed in 9% +/- 12% of breaths. Deeper levels of sedation were associated with increasing ITI (awake, yes 2% vs no 11%; P < .05; CAM-ICU, coma [15%] vs delirium [5%] vs no delirium [2%]; P < .05; RASS, 0, 0% vs -5, 15%; P < .05). Diagnosis of chronic obstructive pulmonary disease, sedative type or dose, mechanical ventilation mode, and trigger method had no effect on ITI.

CONCLUSIONS

Asynchrony is common, and deeper sedation level is a predictor of ineffective triggering.

[Neurally adjusted ventilatory assist (NAVA). A new mode of assisted mechanical ventilation]

The aim of mechanical ventilation is to assure gas exchange while efficiently unloading the respiratory muscles and mechanical ventilation is an integral part of the care of patients with acute respiratory failure. Modern lung protective strategies of mechanical ventilation include low-tidal-volume ventilation and the continuation of spontaneous breathing which has been shown to be beneficial in reducing atelectasis and improving oxygenation. Poor patient-ventilator interaction is a major issue during conventional assisted ventilation. Neurally adjusted ventilator assist (NAVA) is a new mode of mechanical ventilation that uses the electrical activity of the diaphragm (EAdi) to control the ventilator. First experimental studies showed an improved patient-ventilator synchrony and an efficient unloading of the respiratory muscles. Future clinical studies will have to show that NAVA is of clinical advantage when compared to conventional modes of assisted mechanical ventilation. This review characterizes NAVA according to current publications on this topic.

Physiological effects of noninvasive positive ventilation during acute moderate hypercapnic respiratory insufficiency in children

INTRODUCTION

A prospective physiological study was performed in 12 paediatric patients with acute moderate hypercapnic respiratory insufficiency to assess the ability of noninvasive positive pressure ventilation (NPPV) to unload the respiratory muscles and improve gas exchange.

MATERIALS AND METHODS

Breathing pattern, gas exchange, and inspiratory muscle effort were measured during spontaneous breathing and NPPV.

RESULTS

NPPV was associated with a significant improvement in breathing pattern, gas exchange and respiratory muscle output. Tidal volume and minute ventilation increased by 33 and 17%, and oesophageal and diaphragmatic pressure time product decreased by 49 and 56%, respectively. This improvement in alveolar ventilation translated into a reduction in mean partial pressure in carbon dioxide from 48 to 40 mmHg (P = 0.01) and in respiratory rate from 48 to 41 breaths/min (P = 0.01). No difference between a clinical setting and a physiological setting of NPPV was observed. In conclusion, this study shows that NPPV is able to unload the respiratory muscles and improve clinical outcome in young patients admitted to the paediatric intensive care unit for acute moderate hypercapnic respiratory insufficiency.

Proportional assist ventilation with load-adjustable gain factors in critically ill patients: comparison with pressure support

OBJECTIVES

It is not known if proportional assist ventilation with load-adjustable gain factors (PAV+) may be used as a mode of support in critically ill patients. The aim of this study was to examine the effectiveness of sustained use of PAV+ in critically ill patients and compare it with pressure support ventilation (PS).

DESIGN AND SETTING

Randomized study in the intensive care unit of a university hospital.

METHODS

A total of 208 critically ill patients mechanically ventilated on controlled modes for at least 36 h and meeting certain criteria were randomized to receive either PS (n = 100) or PAV+ (n = 108). Specific written algorithms were used to adjust the ventilator settings in each mode. PAV+ or PS was continued for 48 h unless the patients met pre-defined criteria either for switching to controlled modes (failure criteria) or for breathing without ventilator assistance.

RESULTS

Failure rate was significantly lower in PAV+ than that in PS (11.1 vs. 22.0%, P = 0.040, OR 0.443, 95% CI 0.206-0.952). The proportion of patients exhibiting major patient-ventilator dyssynchronies at least during one occasion and after adjusting the initial ventilator settings, was significantly lower in PAV+ than in PS (5.6 vs. 29.0%, P < 0.001, OR 0.1, 95% CI 0.06-0.4). The proportion of patients meeting criteria for unassisted breathing did not differ between modes.

CONCLUSIONS

PAV+ may be used as a useful mode of support in critically ill patients. Compared to PS, PAV+ increases the probability of remaining on spontaneous breathing, while it considerably reduces the incidence of patient-ventilator asynchronies.

Reduction of patient-ventilator asynchrony by reducing tidal volume during pressure-support ventilation

OBJECTIVE

To identify ventilatory setting adjustments that improve patient-ventilator synchrony during pressure-support ventilation in ventilator-dependent patients by reducing ineffective triggering events without decreasing tolerance.

DESIGN AND SETTING

Prospective physiological study in a 13-bed medical intensive care unit in a university hospital.

PATIENTS AND PARTICIPANTS

Twelve intubated patients with more than 10% of ineffective breaths while receiving pressure-support ventilation.

INTERVENTIONS

Flow, airway-pressure, esophageal-pressure, and gastric-pressure signals were used to measure patient inspiratory effort. To decrease ineffective triggering the following ventilator setting adjustments were randomly adjusted: pressure support reduction, insufflation time reduction, and change in end-expiratory pressure.

MEASUREMENTS AND RESULTS

Reducing pressure support from 20.0 cm H(2)O (IQR 19.5-20) to 13.0 (12.0-14.0) reduced tidal volume [10.2ml /kg predicted body weight (7.2-11.5) to 5.9 (4.9-6.7)] and minimized ineffective triggering events [45% of respiratory efforts (36-52) to 0% (0-7)], completely abolishing ineffective triggering in two-thirds of patients. The ventilator respiratory rate increased due to unmasked wasted efforts, with no changes in patient respiratory rate [26.5 breaths/min (23.1-31.9) vs. 29.4 (24.6-34.5)], patient effort, or arterial PCO(2). Shortening the insufflation time reduced ineffective triggering events and patient effort, while applying positive end-expiratory pressure had no influence on asynchrony.

CONCLUSIONS

Markedly reducing pressure support or inspiratory duration to reach a tidal volume of about 6 ml/kg predicted body weight eliminated ineffective triggering in two-thirds of patients with weaning difficulties and a high percentage of ineffective efforts without inducing excessive work of breathing or modifying patient respiratory rate.

[Design of a lung simulator for teaching lung mechanics in mechanical ventilation]

Over the last 10 years, noninvasive ventilation has become a treatment option for respiratory insufficiency in pulmonology services. The technique is currently included in pulmonology teaching programs. Physicians and nurses should understand the devices they use and the interaction between the patient and the ventilator in terms of respiratory mechanics, adaptation, and synchronization. We present a readily assembled lung simulator for teaching purposes that is reproducible and interactive. Based on a bag-in-box system, this model allows the concepts of respiratory mechanics in mechanical ventilation to be taught simply and graphically in that it reproduces the patterns of restriction, obstruction, and the presence of leaks. It is possible to demonstrate how each ventilation parameter acts and the mechanical response elicited. It can also readily simulate asynchrony and demonstrate how this problem can be corrected.

Automatic detection of ineffective triggering and double triggering during mechanical ventilation

OBJECTIVE

Imperfect patient-ventilator interaction is common during assisted ventilation, and the detection of clinically relevant mismatching requires visual monitoring of the ventilator screen. We have assessed the feasibility, sensitivity and specificity of an algorithm embedded in a ventilator system that is able to automatically detect the occurrence of ineffective triggering and double triggering in real time.

DESIGN

Prospective study.

SETTING

Respiratory intensive care unit.

METHODS

Twenty patients undergoing pressure-support ventilation, either non-invasively (NIV, n=10) or conventionally ventilated (n=10), were studied.

MEASUREMENTS

The detection of ineffective triggering and double triggering from the algorithm was compared by two operators with the "real" occurrence of the phenomena as assessed using the transdiaphragmatic pressure (Pdi).

RESULTS

Seven of the 20 patients exhibited gross mismatching, while in the remaining patients patient-ventilator mismatching was artificially induced using a pressure control, with a low respiratory rate. Ineffective triggering and double triggering were identified by the operators in 507 and 19 of the 3343 analyzed breaths, respectively. False positives were significantly more frequent in the NIV group than with conventional ventilation. The algorithm had an overall sensitivity of 91% and specificity of 97%. Specificity was statistically higher in the conventional ventilated group than with NIV (99% vs. 95%, p<0.05).

CONCLUSIONS

We have demonstrated the feasibility and efficacy of a new algorithm to detect the occurrence of impaired patient-ventilator interaction during mechanical ventilation in real time. This software may help the clinician in the identification of this problem, which has been shown to have important clinical consequences.

Nonassociative learning promotes respiratory entrainment to mechanical ventilation

Background

Patient-ventilator synchrony is a major concern in critical care and is influenced by phasic lung-volume feedback control of the respiratory rhythm. Routine clinical application of positive end-expiratory pressure (PEEP) introduces a tonic input which, if unopposed, might disrupt respiratory-ventilator entrainment through sustained activation of the vagally-mediated Hering-Breuer reflex. We suggest that this potential adverse effect may be averted by two differentiator forms of nonassociative learning (habituation and desensitization) of the Hering-Breuer reflex via pontomedullary pathways.

Methodology/Principal Findings

We tested these hypotheses in 17 urethane-anesthetized adult Sprague-Dawley rats under controlled mechanical ventilation. Without PEEP, phrenic discharge was entrained 1∶1 to the ventilator rhythm. Application of PEEP momentarily dampened the entrainment to higher ratios but this effect was gradually adapted by nonassociative learning. Bilateral electrolytic lesions of the pneumotaxic center weakened the adaptation to PEEP, whereas sustained stimulation of the pneumotaxic center weakened the entrainment independent of PEEP. In all cases, entrainment was abolished after vagotomy.

Conclusions/Significance

Our results demonstrate an important functional role for pneumotaxic desensitization and extra-pontine habituation of the Hering-Breuer reflex elicited by lung inflation: acting as buffers or high-pass filters against tonic vagal volume input, these differentiator forms of nonassociative learning help to restore respiratory-ventilator entrainment in the face of PEEP. Such central sites-specific habituation and desensitization of the Hering-Breuer reflex provide a useful experimental model of nonassociative learning in mammals that is of particular significance in understanding respiratory rhythmogenesis and coupled-oscillator entrainment mechanisms, and in the clinical management of mechanical ventilation in respiratory failure.

Mechanical ventilation: let us minimize sleep disturbances

PURPOSE OF REVIEW

This review provides a background in mechanical ventilation and sleep.

RECENT FINDINGS

Sleep pattern in mechanically ventilated patients differs largely from physiological sleep. The ventilatory mode and the ventilatory settings could have an influence on the sleep quality and quantity. Pressure support ventilation can increase the sleep fragmentation and decrease the sleep quantity, due to central apneas when compared with assist control ventilation. An excessive level of ventilatory assistance during sleep promotes central apneas and ineffective efforts. These two respiratory events can trigger arousals and awakenings, thus altering the sleep quality and quantity in mechanically ventilated patients. Ventilatory settings adjusted according to the patient's effort during pressure support allow reducing the number of ineffective efforts and improve sleep quality when compared with a clinical adjustment. A physiological approach to set the ventilator and the ventilatory mode may improve sleep quality and quantity.

SUMMARY

Minimizing the sleep alterations in mechanically ventilated patients could be obtained by setting the ventilator in such a way to avoid hyperventilation during the sleep stage. The impact of sleep derangements in patient outcomes is, however, unknown.

A method for monitoring and improving patient: ventilator interaction

OBJECTIVE

To evaluate a new approach for monitoring and improving patient-ventilator interaction that utilizes a signal generated by the equation of motion, using improvised values for resistance and elastance obtained noninvasively.

DESIGN AND SETTING

Observational study in intensive care units in five European centers.

PATIENTS

We studied 21 stable patients instrumented with esophageal/gastric catheters for a previous study and ventilated alternately with pressure support (PSV) and proportional assist (PAV) ventilation with a Tyco 840 ventilator.

MEASUREMENTS AND RESULTS

Previously recorded digital files were analyzed in real-time by a prototype incorporating the new technology (PVI monitor, YRT, Winnipeg, Canada). Actual onsets (P(DI)-T(ONSET)) and ends (P(DI)-T(END)) of inspiratory efforts, ineffective efforts, and patient respiratory rate were identified visually from transdiaphragmatic or calculated respiratory muscle pressure. Monitor-identified T(ONSET) occurred 0.107 +/- 0.074 s after P(DI)-T(ONSET), substantially less than trigger delay observed with conventional triggering (0.326 +/- 0.086 s). End of effort was identified 0.097 +/- 0.096 s after P(DI)-T(END), significantly less than actual cycling-off delay during PSV (0.486 +/- 0.307 s) or PAV (0.277 +/- 0.084 s). The monitor detected 80% of ineffective efforts. There was excellent agreement between monitor-estimated respiratory rate and actual patient rate over a wide range (17-59/min) of patient rates (mean (+/- SD) of difference -0.2 +/- 1.9/min for pressure support and 0.2 +/- 0.9/min for proportional assist) even when large discrepancies existed (> 35/min) between patient and ventilator rates.

CONCLUSIONS

The proposed approach should make it possible to improve patient-ventilator interaction and to obtain accurate estimates of true patient respiratory rate when there is nonsynchrony.

Closed-loop ventilation: an emerging standard of care?

The rational for using closed loop ventilation is becoming strong and stronger. Studies are now available supporting the hypothesis that patient outcome is improved by using closed loop ventilation. In the highly sophisticated ICU world driven by the triumvirate of cost-efficiency, quality, and safety, closed loop ventilation will become definitely unavoidable. The challenge is how to make that change effortless, "friendly" and as fast as possible. Introducing novel graphical user interfaces and providing data displays that are pertinent, integrative and dynamic will reduce cognitive resources of the clinician and have the potential to make ventilation safer. They may be the key to adopt closed loop ventilation in everyday practice.