Expiratory Asynchrony Research Articles

Patient-ventilator asynchrony in preterm infants on nasal intermittent positive pressure ventilation

ObjectiveTo describe the incidence of patient-ventilator asynchrony and different types of asynchrony in preterm infants treated with non-synchronised nasal intermittent positive pressure ventilation (nIPPV).DesignAn observational study was conducted including preterm...

Effects of noninvasive proportional assist vs pressure support ventilation on respiratory work in chronic obstructive pulmonary disease patients with hypercapnia

Objective: To investigate the effect of noninvasive proportional assist ventilation (PAV) on respiratory work in chronic obstructive pulmonary disease(COPD) patients, in comparison to noninvasive pressure support ventilation(PSV). Methods: Ten severe COPD patients with hypercapnia during acute exacerbation were examined. The baseline inspiratory pressure of PSV (PS) and the assistance level of PAV(PA) were titrated by patients' tolerance. In addition to the baseline PS and PA, an additional decrease by 25% (PS-=75% PS, PA-=75% PA) or increase by 25% (PS+ =125% PS, PA+ =125% PA) of the assist level were applied to the patients. After the assessment of unassisted spontaneous breathing (SB), the patient was placed on the 6 levels of noninvasive-PSV and noninvasive-PAV in random sequence. Each level lasted at least 20 minutes. Respiratory rate (RR), tidal volume (Vt), and respiratory work(Wex, Wip and Wv) were measured. Asynchrony index (AI) was calculated. Results: During ventilation, Vt was significantly higher with each assist level than with SB. The Vt was significant increased with PS+ than with PA+ . An increase in expiratory work(Wex) and decrease in inspiratory work(Wip) were observed respectively, with the increasing assist level. The inspiratory muscles assessed by Wip were more unloaded at PS compared with PA [PS: (1.59±1.27) J/min vs PA: (4.99±3.48) J/min P<0.01]. However, the Wex was significantly higher with PS+ than with PA+ [PS+ : (1.17±0.54) J/min vs PA+ : (0.49±0.56)J/min, P<0.01]. The AI was increased with the increasing assist level of PSV [PS-: (0.46±0.57)%, PS: (1.36±1.24)% PS+ : (5.26±4.77)]. No asynchrony events were observed at PA- and PA. "Runaway" (expiratory asynchrony) was observed during PA+ [AI: (2.62±2.72)%]. Conclusions: Noninvasive-PAV can increase the Vt and decrease the Wip of the COPD patients with hypercapnia and avoid the over-assistance. The "Runaway" will occur at assist level higher than that set by tolerance. Physiological data can monitor the patient's responses and the ventilator-patient interaction, which may provide objective criteria for ventilator setting.

Patient-Ventilator Asynchrony During Assisted Ventilation in Children.

To describe the frequency and type of patient-ventilator asynchrony in mechanically ventilated children by analyzing ventilator flow and pressure signals. Prospective observational study. Tertiary PICU in a university hospital. Mechanically ventilated children between 0 and 18 years old and who were able to initiate and maintain spontaneous breathing were eligible for inclusion. Patients with congenital or acquired neuromuscular disorders, those with congenital or acquired central nervous system disorders, and those who were unable to initiate and maintain spontaneous breathing from any other cause were excluded. None. All patients were ventilated in a time-cycled, pressure-limited mode with flow triggering set at 1.0 L/min by using the Evita XL (Dräger, Lubeck, Germany). Patient-ventilator asynchrony was identified by a random 30-minute continuous recording and an offline analysis of the flow and pressure signals. Patient-ventilator asynchrony was categorized and labeled into four different groups: 1) trigger asynchrony (i.e., insensitive trigger, double triggering, autotriggering, or trigger delay), 2) flow asynchrony, 3) termination asynchrony (i.e., delayed or premature termination), and 4) expiratory asynchrony. Flow and pressure signals were recorded in 45 patients for 30 minutes. A total number of 57,651 breaths were analyzed. Patient-ventilator asynchrony occurred in 19,175 breaths (33%), and it was seen in every patient. Ineffective triggering was the most predominant type of asynchrony (68%), followed by delayed termination (19%), double triggering (4%), and premature termination (3%). Patient-ventilator asynchrony significantly increased with lower levels of peak inspiratory pressure, positive end-expiratory pressure, and set frequency. Patient-ventilator asynchrony is extremely common in mechanically ventilated children, and the predominant cause is ineffective triggering.

Not all types of asynchrony are created equal

Dear Editor,We read with interest the articleentitled, ‘‘Asynchrony, neural drive,ventilator variability and COMFORT:NAVA versus pressure support inpediatric patients’’ [1]. The authorsfound that the use of NeurallyAdjusted Ventilatory Assist (NAVA)improved patient-ventilator syn-chrony and patient comfort comparedto pressure support whether deter-mined by the treating physician oroptimized by means of protocolizedadjustments in trigger sensitivity,inspiratory rise time and terminationcriteria, airway pressure and appliedPEEP.First, we appreciate the citation ofour previous work entitled ‘‘Neurallytriggered breaths reduce trigger delayand improve ventilator response timesin ventilated infants with bronchioli-tis’’ [2]. However, the commentary onour work is not accurate. We com-pared NAVA to volume supportrather than pressure support, and itwas in human infants with bronchi-olitis, not in a mechanical lungmodel. This difference is importantbecause few studies have been pub-lished examining the use of NAVA inpediatric patients.Second, in their study, the authorslimited their documentation ofasynchrony to autotriggered, double-triggered and non-triggered breaths(types of trigger asynchrony) as wellas an asynchrony index. In a morerecent study that used the samemethodology as our infant study, wealso found that the use of NAVAsignificantly reduced the overallincidence of asynchrony, particularlytrigger asynchrony in a recoveringlung injury animal model. However,we also found that the incidence offlow asynchrony, short cycles andmore common with neurally triggeredbreaths than with pneumatically trig-gered breaths [3]. Flow asynchronyoccurs if ventilator flow does notequal patient flow. This type ofasynchrony can be detrimental topatients because it can lead to auto-PEEP and other types of asynchrony.Short cycles may be viewed graphi-cally as double triggering and aredefined as inspiratory time less thanhalf the mean inspiratory time. Theseasynchronous breaths can result fromimproperly set termination criteriasuch that the ventilator inspiratorytime ends before the patient’s inspi-ratory effort is complete. The loss ofventilator support during inspirationcan result in increased work ofbreathing. Expiratory asynchrony, inparticular shortened expiratory time,can also promote air trapping andauto-PEEP [4]. We believe that thesetypes of asynchrony should not beoverlooked, as all types of asyn-chrony can lead to prolonged durationof ventilation [5].

New modes of mechanical ventilation in the preterm newborn: evidence of benefit

The introduction of modern mechanical ventilation in neonatal medicine in the 1960s was followed shortly thereafter by its use in premature infants with hyaline membrane disease. Most premature infants born...

Expiratory Asynchrony

Expiratory asynchrony is a universal phenomenon, and expiratory synchrony occurs only by chance. Expiratory asynchrony exists in all breath modes and has a significant impact on the patient's work of breathing and the weaning process. Advancements in ventilator designs and basic physiologic science could lead to the improvement of the expiratory asynchrony.

Prolonged neural expiratory time induced by mechanical ventilation in infants.

Mechanical ventilation may interfere with the spontaneous breathing pattern in infants because they have strong reflexes that play a large role in the control of breathing. This study aimed to answer the following questions: does a ventilator-assisted breath 1) reduce neural inspiratory time, 2) reduce the amplitude of the diaphragm electrical activity, and 3) prolong neural expiration, within the delivered breath? In 14 infants recovering from acute respiratory failure (mean age and weight were 2.3 +/- 1.3 mo and 3.95 +/- 0.82 kg, respectively), we measured 1) the electrical activity of the diaphragm with a multiple-array esophageal electrode, and 2) airway opening pressure, while patients breathed on synchronized intermittent mandatory ventilation (mean rate, 11.2 +/- 6.5 breaths/min). We compared neural inspiratory and expiratory times for the mandatory breaths and for the spontaneous breaths immediately preceding and following the mandatory breath. Although neural inspiratory time was not different between mandatory and spontaneous breaths, neural expiratory time was significantly increased (p < 0.001) for the mandatory breaths (953 +/- 449 ms) compared with the premandatory and postmandatory spontaneous breaths (607 +/- 268 ms and 560 +/- 227 ms, respectively). Delivery of the mandatory breath resulted in a reduction in neural respiratory frequency by 28.6 +/- 6.4% from the spontaneous premandatory frequency. The magnitude of inspiratory electrical activity of the diaphragm was similar for all three breath conditions. For the mandatory breaths, ventilatory assist persisted for 507 +/- 169 ms after the end of neural inspiratory time. Infant-ventilator asynchrony (both inspiratory and expiratory asynchrony) was present in every mandatory breath and constituted 53.4 +/- 26.2% of the total breath duration.

Expiratory Asynchrony in Proportional Assist Ventilation

One of the proposed advantages of proportional assist ventilation (PAV) has been the automatic synchrony between the end of the patient's inspiratory effort and the ventilator cycle (i.e., expiratory synchrony). However, recent clinical studies have shown a prolonged ventilator inspiratory time or even a "runaway" phenomenon with the normal use of PAV. We hypothesize that control-system delay may account for this, because in reality there is always some degree of delays between control-system's input and output in all ventilators. Computer simulation study to date has not taken into account the potential effect of control-system delay on expiratory synchrony. We therefore created a computer model in which the parameter of control-system delay time was introduced. We found that significant expiratory asynchrony may occur with this more realistic model of PAV. The ventilator flow termination may fall behind the completion of the patient inspiration by as long as 0.33 seconds under the selected simulation conditions. The inspiratory termination delay time is in proportion to the control-system delay time, the respiratory time constant, and the assist gain settings. In conclusion, this model indicates that due to the unavoidable control-system delay in the ventilators, expiratory asynchrony may be an inherent shortcoming associated with PAV.

Automation of expiratory trigger sensitivity in pressure support ventilation.

Patients under pressure support ventilation sometimes encounter patient-ventilator asynchrony in the transition from inspiration to expiration, that is, expiratory asynchrony. This problem is caused by the incompatibility of the fixed level of expiratory trigger sensitivity termination criteria (i.e., flow termination criteria) in the ICU ventilators to various patient conditions. The user-adjustable expiratory trigger sensitivity implemented in some newly released ventilators has been experienced to be difficult to use and unable to adapt ever-changing patient conditions without user intervention, although it provides more flexibility. This article elucidates the rationale for automatic control of the expiratory trigger sensitivity and evaluates the automation system with a bench setup. The evaluation data suggest that good expiratory synchronies can be achieved through automatic adjustments of expiratory trigger sensitivity.

Analysis of the mechanisms of expiratory asynchrony in pressure support ventilation: a mathematical approach.

A mathematical model was developed to analyze the mechanisms of expiratory asynchrony during pressure support ventilation (PSV). Solving the model revealed several results. 1) Ratio of the flow at the end of patient neural inspiration to peak inspiratory flow (VTI/V(peak)) during PSV is determined by the ratio of time constant of the respiratory system (tau) to patient neural inspiratory time (TI) and the ratio of the set pressure support (Pps) level to maximal inspiratory muscle pressure (Pmus max). 2) VTI/V(peak) is affected more by tau/TI than by Pps/Pmus max. VTI/V(peak) increases in a sigmoidal relationship to tau/TI. An increase in Pps/Pmus max slightly shifts the VTI/V(peak)-tau/TI curve to the right, i.e., VTI/V(peak) becomes lower as Pps/Pmus max increases at the same tau/TI. 3) Under the selected adult respiratory mechanics, VTI/V(peak) ranges from 1 to 85% and has an excellent linear correlation with tau/TI. 4) In mechanical ventilators, single fixed levels of the flow termination criterion will always have chances of both synchronized termination and asynchronized termination, depending on patient mechanics. An increase in tau/TI causes more delayed and less premature termination opportunities. An increase in Pps/Pmus max narrows the synchronized zone, making inspiratory termination predisposed to be in asynchrony. Increasing the expiratory trigger sensitivity of a ventilator shifts the synchronized zone to the right, causing less delayed and more premature termination. Automation of expiratory trigger sensitivity in future mechanical ventilators may also be possible. In conclusion, our model provides a useful tool to analyze the mechanisms of expiratory asynchrony in PSV.

Respiratory inductive plethysmography in the evaluation of lower airway obstruction during methacholine challenge in infants.

Respiratory inductive plethysmography (RIP) is a simple technique for an objective, noninvasive assessment of thoracoabdominal asynchrony, which in turn is an indirect measure of airway obstruction. We evaluated different indices of asynchrony obtained by RIP before and after methacholine-induced airway obstruction. Bronchial obstruction was elicited by progressive doubling concentrations of methacholine until a > 15% fall in the transcutaneous oxygen tension (PtcO2) had developed. Maximal expiratory flow rates at functional residual capacity (FRC) (VmaxFRC) was obtained by the squeeze technique before and after the challenge. Fifteen infants with a history of wheezing were studied after sedation. Thoracoabdominal movements were recorded with RIP bands placed around either the upper or the lower ribcage (RC) and around the abdomen (AB). An inspiratory asynchrony index (IAI) and an expiratory asynchrony index (EAI) were calculated as determined by the lag of RC relative to AB at start of inspiration and of expiration, respectively. The total time in asynchrony (TTA: the percentage of time in which the RC and the AB signals were in opposite direction) and phi (an angle derived from a Lissajous loop) were also calculated. All subjects responded to the challenge. The median fall in PtcO2 following methacholine challenge was 23.6% and in VmaxFRC was 43%. A large scatter of baseline values was found for all indices with the exception of TTA. There was no correlation between TTA and age, length, or VmaxFRC. The IAI and EAI with the RC band in the upper position were the most sensitive indices, both within subjects (65% of the subjects had a significant change in IAI and 80% in EAI) and for the group as a whole (median values increased for IAI, P = 0.007, and for EAI, P = 0.017). TTA and phi were less sensitive, and a great discrepancy was observed between the two measurements. Poor results were obtained with the RC band in the lower position. No correlations were found between the changes in IAI and EAI, with the RC band around the lower chest and VmaxFRC. We conclude that IAI and EAI, measured with the RC band in the upper position and another band around the abdomen, can detect changes in thoracoabdominal asynchrony in most infants. The usefulness of assessing IAI and EAI in infants with acute lower airway obstruction needs to be determined.