Prolongation Of Inspiratory Time Research Articles

Physiological Effects of Surgical and N95 Masks During Exercise in the COVID-19 era

BackgroundDuring the COVID-19 pandemic, studies of the physiological effects of masking during exercise have been rare. MethodsTwelve healthcare workers performed a cardiopulmonary exercise test while wearing a surgical mask, an N95 mask, or no mask. Variables were collected at rest, warm-up, anaerobic threshold, and maximal exercise. ResultsFrom rest to maximal exercise, both the surgical and N95 masks decreased inspiratory flow, minute ventilation, and prolonged inspiratory time compared to the no mask condition. Oxygen uptake (VO2) and oxygen pulse (VO2/HR) decreased at rest, warm-up, and maximal exercise in both the surgical and N95 mask conditions (vs. no mask). At the anaerobic threshold, the surgical mask also led to a reduction of oxygen uptake and oxygen pulse compared to no mask. The maximal oxygen uptake (VO2% predicted) also decreased in both the surgical and N95 mask conditions. In addition, the severity of dyspnea increased, and exercise time decreased for both surgical and N95 masks. Compared to no mask, wearing an N95 mask led to lower breathing frequency and lower ventilation efficacy (assessed by VE/VCO2 and VE/VO2) from rest to maximal exercise (all p < 0.05 for trend). Wearing an N95 also led to retention of carbon dioxide (p < 0.05 for trend). ConclusionsWearing a surgical mask leads to a somewhat negative impact on cardiopulmonary function, and this effect is more serious with an N95 mask. Attention should be paid to exercise while wearing surgical or N95 masks.

End-Tidal Carbon Dioxide Pressure Measurement after Prolonged Inspiratory Time Gives a Good Estimation of the Arterial Carbon Dioxide Pressure in Mechanically Ventilated Patients.

Background: End-tidal carbon dioxide pressure (PetCO2) is unreliable for monitoring PaCO2 in several conditions because of the unpredictable value of the PaCO2–PetCO2 gradient. We hypothesised that increasing both the end-inspiratory pause and the expiratory time would reduce this gradient in patients ventilated for COVID-19 with Acute Respiratory Distress Syndrome and in patients anaesthetised for surgery. Methods: On the occasion of an arterial blood gas sample, an extension in inspiratory pause was carried out either by recruitment manoeuvre or by extending the end-inspiratory pause to 10 s. The end-expired PCO2 was measured (expiratory time: 4 s) after this manoeuvre (PACO2) in comparison with the PetCO2 measured by the monitor. We analysed 67 Δ(a-et)CO2, Δ(a-A)CO2 pairs for 7 patients in the COVID group and for 27 patients in the anaesthesia group. Results are expressed as mean ± standard deviation. Results: Prolongation of the inspiratory pause significantly reduced PaCO2–PetCO2 gradients from 11 ± 5.7 and 5.7 ± 3.4 mm Hg (p < 0.001) to PaCO2–PACO2 gradients of −1.2 ± 3.3 (p = 0.043) and −1.9 ± 3.3 mm Hg (p < 0.003) in the COVID and anaesthesia groups, respectively. In the COVID group, PACO2 showed the lowest dispersion (−7 to +6 mm Hg) and better correlation with PaCO2 (R2 = 0.92). The PACO2 had a sensitivity of 0.81 and a specificity of 0.93 for identifying hypercapnic patients (PaCO2 > 50 mm Hg). Conclusions: Measuring end-tidal PCO2 after prolonged inspiratory time reduced the PaCO2–PetCO2 gradient to the point of obtaining values close to PaCO2. This measure identified hypercapnic patients in both intensive care and during anaesthesia.

Breathing patterns in people with exercise-induced laryngeal obstruction.

Exercise‐induced laryngeal obstruction (EILO) is common, but we lack readily available diagnostic tools. The larynx represents an important point of resistance in the airways, and we therefore hypothesized that EILO is associated with characteristic breathing patterns possible to record from a standard incremental ergospirometry test. We studied 24 individuals with moderate/severe EILO and 20 individuals with no‐EILO, mean (SD) age 17 (6.1) and 24 (6.4) years, respectively. EILO versus no‐EILO was verified from maximal continuous laryngoscopy treadmill exercise (CLE) tests, which also included ergospirometry. We described the relationships between minute ventilation (V˙E) versus tidal volume (V T) and V˙E versus carbon dioxide output (V˙CO2), using respectively quadratic and linear equations, and applied adjusted regression models to compare ergospirometry data and curve parameters. Compared to the no‐EILO group, the group with EILO had prolonged inspiratory time (T in), lower breathing frequency (B f), lower V˙E, and lower inspiratory flow rate (V˙in) at peak exercise. Mathematical modeling of the breathing pattern relationships was feasible in both groups, with similar coefficients of variation. For V˙E versus V T, the mathematical curve parameters were similar. For V˙E versus V˙CO2, the slope was similar but the intercept was lower in the EILO group. EILO was associated with prolonged T in, lower B f, V˙E, and V˙E. The relationship between V˙E versus V T was similar, whereas for V˙E versus V˙CO2, the slope was almost parallel but shifted downward for the EILO group. Most ergospirometry data overlapped, except V˙in which discriminated between EILO and no‐EILO in a promising way.

Utilization of Airway Pressure Release Ventilation as a Rescue Strategy in COVID-19 Patients: A Retrospective Analysis.

Background:Airway Pressure Release Ventilation (APRV) is a pressure controlled intermittent mandatory mode of ventilation characterized by prolonged inspiratory time and high mean airway pressure. Several studies have demonstrated that APRV can improve oxygenation and lung recruitment in patients with Acute Respiratory Distress Syndrome (ARDS). Although most patients with COVID-19 meet the Berlin criteria for ARDS, hypoxic respiratory failure due to COVID-19 may differ from traditional ARDS as patients often present with severe, refractory hypoxemia and significant variation in respiratory system compliance. To date, no studies investigating APRV in this patient population have been published. The aim of this study was to evaluate the effectiveness of APRV as a rescue mode of ventilation in critically ill patients diagnosed with COVID-19 and refractory hypoxemia.Methods:We conducted a retrospective analysis of patients admitted with COVID-19 requiring invasive mechanical ventilation who were treated with a trial of APRV for refractory hypoxemia. PaO2/FIO2 (P/F ratio), ventilatory ratio and ventilation outputs before and during APRV were compared.Results:APRV significantly improved the P/F ratio and decreased FIO2 requirements. PaCO2 and ventilatory ratio were also improved. There was an increase in tidal volume per predicted body weight during APRV and a decrease in total minute ventilation. On multivariate analysis, higher inspiratory to expiratory ratio (I: E) and airway pressure were associated with greater improvement in P/F ratio.Conclusions:APRV may improve oxygenation, alveolar ventilation and CO2 clearance in patients with COVID-19 and refractory hypoxemia. These effects are more pronounced with higher airway pressure and inspiratory time.

Effect of prolonged inspiratory time on gas exchange during robot-assisted laparoscopic urologic surgery.

Gas exchange disturbance may develop during urologic robotic laparoscopic surgery with the patient in a steep Trendelenburg position. This study investigated whether prolonged inspiratory time could mitigate gas exchange disturbances including hypercapnia. In this randomized cross-over trial, 32patients scheduled for robot-assisted urologic surgery were randomized to receive an inspiratory to expiratory time ratio (I:E) of 1:1 for the first hour of pneumoperitoneum followed by 1:2 for last period of surgery (groupA, n = 17) or I:E of 1:2 followed by 1:1 (groupB, n = 15). Arterial blood gas analysis, airway pressure and hemodynamic variables were assessed at four time points (T1: 10 min after induction of general anesthesia, T2: 1 h after the initiation of pneumoperitoneum, T3: 1 h after T2 and T4: at skin closure). The carry over effect of initial I:E was also evaluated over the next hour through arterial blood gas analysis. There was asignificant decrease in partial pressure of oxygen in arterial blood (PaO2) for both groups at T2 and T3 compared to T1 but in groupB the PaO2 at T4 was not decreased from the baseline. Partial pressure of carbon dioxide in arterial blood (PaCO2) increased with I:E of 1:2 but did not significantly increase with I:E of 1:1; however, there were no differences in PaO2 and PaCO2 between the groups. Decreased oxygenation by pneumoperitoneum was improved and PaCO2 did not increase after 1h of I:E of 1:1; however, the effect of equal ratio ventilation longer than 1h remains to be determined. There was no carryover effect of the two different I:E ratios.

The transition to extra‐uterine life by extremely preterm infants – handle with care

The transition to extra‐uterine life by extremely preterm infants – handle with care

Pathological Breathing Patterns After Pneumococcal Rhombencephalitis

Apneustic breathing is an abnormal breathing pattern characterized by a prolonged inspiratory time with an end-inspiratory pause versus a shorter expiratory time.[1][1] The termination of respiration is considered to be controlled by the pontine respiratory group (PRG). The apneustic breathing

The effects of prolonged inspiratory time during one‐lung ventilation: a randomised controlled trial

We evaluated the effects of a prolonged inspiratory time on gas exchange in subjects undergoing one-lung ventilation for thoracic surgery. One hundred patients were randomly assigned to Group I:E = 1:2 or Group I:E = 1:1. Arterial blood gas analysis and respiratory mechanics measurements were performed 10 min after anaesthesia induction, 30 and 60 min after initiation of one-lung ventilation, and 15 min after restoration of conventional two-lung ventilation. The mean (SD) ratio of the partial pressure of arterial oxygen to fraction of inspired oxygen after 60 min of one-lung ventilation was significantly lower in Group I:E = 1:2 compared with Group I:E = 1:1 (27.7 (13.2) kPa vs 35.2 (22.1) kPa, respectively, p = 0.043). Mean (SD) physiological dead space-to-tidal volume ratio after 60 min of one-lung ventilation was significantly higher in Group I:E = 1:2 compared with Group I:E = 1:1 (0.46 (0.04) vs 0.43 (0.04), respectively, p = 0.008). Median (IQR [range]) peak inspiratory pressure was higher in Group I:E = 1:2 compared with Group I:E = 1:1 after 60 min of one-lung ventilation (23 (22-25 [18-29]) cmH2O vs 20 (18-21 [16-27]) cmH2O, respectively, p < 0.001) and median (IQR [range]) mean airway pressure was lower in Group I:E = 1:2 compared with Group I:E = 1:1 (10 (8-11 [5-15]) cmH2O vs 11 (10-13 [5-16]) cmH2O, respectively, p < 0.001). We conclude that, compared with an I:E ratio of 1:2, an I:E ratio of 1:1 resulted in a modest improvement in oxygenation and decreased shunt fraction during one-lung ventilation.

Prolonged inspiratory time produces better gas exchange in patients undergoing laparoscopic surgery: A randomised trial

Laparoscopic surgery performed with a patient in the Trendelenburg position is known to have adverse effects on pulmonary gas exchange and respiratory mechanics. We supposed that prolonged inspiratory time can improve gas exchange at lower airway pressure. One hundred patients undergoing gynaecologic laparoscopic surgery were randomly assigned to one of four groups: conventional inspiratory-to-expiratory (I : E) ratio (Group 1 : 2), I : E ratio of 1 : 1 (Group 1 : 1), 2 : 1 (Group 2 : 1), or 1 : 2 with external positive end-expiratory pressure (PEEP) of 5 cmH2 O (Group 1 : 2 PEEP). Tidal volume was set to 6 ml/kg, and I : E ratio was adjusted at the onset of pneumoperitoneum. Arterial blood gas analysis with measurements of partial pressure of arterial oxygen/fraction of inspired oxygen (PaO2 /FiO2 ), and physiologic dead space-to-tidal volume ratio (VD /VT ) was performed 15 min after anaesthetic induction (T1), and 30 (T2) and 60 min (T3) after onset of CO2 insufflation. PaO2 /FiO2 at T3 in Groups 1 : 1, 2 : 1, and 1 : 2 PEEP were higher than Group 1 : 2. The partial pressure of arterial carbon dioxide at T3 in Group 2 : 1 was lower than the other groups. The VD /VT at T2 and T3 were lower in Groups 1 : 1 and 2 : 1 than Groups 1 : 2 and 1 : 2 PEEP. Peak or plateau airway pressure was higher in Group 1 : 2 PEEP than the other groups. A prolonged inspiratory time demonstrated a beneficial effect on oxygenation. Furthermore, it showed better CO2 elimination without elevating the peak or plateau airway pressure compared with applying external PEEP. In terms of gas exchange and respiratory mechanics, a prolonged inspiratory time appears to be superior to applying external PEEP in patients undergoing laparoscopic surgery in the Trendelenburg position.

Tracheal occlusions evoke respiratory load compensation and neural activation in anesthetized rats

Airway obstruction in animals leads to compensation and avoidance behavior and elicits respiratory mechanosensation. The pattern of respiratory load compensation and neural activation in response to intrinsic, transient, tracheal occlusions (ITTO) via an inflatable tracheal cuff are unknown. We hypothesized that ITTO would cause increased diaphragm activity, decreased breathing frequency, and activation of neurons within the medullary and pontine respiratory centers without changing airway compliance. Obstructions were performed for 2-3 breaths followed by a minimum of 15 unobstructed breaths with an inflatable cuff sutured around the trachea in rats. The obstruction procedure was repeated for 10 min. The brains of obstructed and control animals were removed, fixed, sectioned, and stained for c-Fos. Respiratory pattern was measured from esophageal pressure (P(es)) and diaphragm electromyography (EMG(dia)). The obstructed breaths resulted in a prolonged inspiratory and expiratory time, an increase in EMG(dia) amplitude, and a more negative P(es) compared with control breaths. Neurons labeled with c-Fos were found in brain stem and suprapontine nuclei, with a significant increase in c-Fos expression for the occluded experimental group compared with the control groups in the nucleus ambiguus, nucleus of the solitary tract, lateral parabrachial nucleus, and periaqueductal gray matter. The results of this study demonstrate tracheal occlusion-elicited activation of neurons in brain stem respiratory nuclei and neural areas involved in stress responses and defensive behaviors, suggesting that these neurons mediate the load compensation breathing pattern response and may be part of the neural pathway for respiratory mechanosensation.

Analysis of inflation breaths during face mask resuscitation in preterm infants

Purpose of the study: Positive pressure ventilation with a prolonged inspiratory time is recommended to establish a functional residual capacity and deliver appropriate tidal volume at preterm resuscitation. The aim of the study was to analyse the peak inspiratory pressure (PIP), tidal volume (TV), inspiratory time (Ti) and face mask leak during the first five inflation breaths at preterm resuscitation.

Postoperative Noninvasive Ventilation

Thesemodificationsoftherespiratoryfunctionoccur early after surgery and are more often transient andcould lead to ARF. The clinical result (severity of theARF) is the product of perioperative-related ventilatoryimpairment and severity of the preoperative pulmonarycondition. Maintenance of adequate oxygenation in thepostoperative period is of major importance, especiallywhen pulmonary complications such as ARF occur. Al-though invasive endotracheal mechanical ventilation hasremained the cornerstone of ventilatory strategy for manyyearsforsevereARF,severalstudieshaveshownthatmor-tality associated with pulmonary disease is largely relatedto complications of postoperative reintubation and me-chanicalventilation.Therefore,majorobjectivesforanes-thesiologists are first to prevent the occurrence of postop-erative complications and second to ensure oxygenadministration and carbon dioxide removal while avoid-ing intubation if ARF occurs. Noninvasive ventilation(NIV) does not require an artificial airway (endotrachealtube or tracheotomy), and its use is well established toprevent ARF occurrence (prophylactic treatment) or totreat ARF to avoid reintubation (curative treatment) (fig.1). Studies show that patient-related risk factors, such aschronic obstructive pulmonary disease, age older than 60yr, American Society of Anesthesiologists class of II orhigher, obesity, functional dependence, and congestiveheart failure, increase the risk for postoperative pulmo-nary complications.

Patient-Ventilator Dyssynchrony: Clinical Significance and Implications for Practice

The cost of providing care to critically ill patients in the United States consumes roughly 15% of all health care dollars, or 1% of the gross national product.1 Contributing to this economic burden are patients admitted to the intensive care unit (ICU) who require mechanical ventilation and patients with complications from their dependence on this technology.2,3 In fact, 50% of ICU patients receive mechanical ventilation.4 Often, sedation is required to increase patients’ tolerance of the endotracheal tube, reduce anxiety, and facilitate sleep. In particular, sedation is used frequently to reduce patient-ventilator dyssynchrony (PVD).5–9 Sassoon and Foster10 define PVD as a mismatching between the patient’s breaths (neural) and ventilator-assisted breaths (phase asynchrony), as well as the inability of the ventilator’s flow delivery to match the patient’s flow demand (flow asynchrony). This definition suggests a faulty interaction between the patient and ventilator that is commonly managed by sedation and advanced ventilator modes and adjustments. The correction of PVD is complex and multifaceted given the current capabilities of traditional ventilators. An imperfect solution exists because the sensitivity and responsiveness of both the patient and the ventilator during the interaction is confounded by factors related to the patient and the ventilator. However, in light of the most serious complications (hypoxemia, barotrauma, prolonged mechanical ventilation, and discomfort) of PVD, and an imperfect solution for the resolution of PVD at the current time, nurses continue to face the challenge of preventing the consequences of PVD as well as complications due to oversedation or undersedation. In this article, we discuss the factors contributing to PVD; the manifestations, measurement, types, and causes of PVD; nursing implications; and future directions for improvement, with nursing research questions proposed for consideration.

Variation in inspiratory time and tidal volume with T-piece neonatal resuscitator: Association with operator experience and distraction

The most recent Neonatal Resuscitation Programme (NRP 5th edition) guidelines recognise the T-piece resuscitator (Neopuff) device as an acceptable method of administering a pre-selected peak inspiratory pressure (PIP) and positive end expiratory pressure (PEEP). While these are constant, other parameters are operator-dependent. Although in widespread clinical use, there is little published data on the use of the T-piece resuscitator in neonatal resuscitation. This study showed that despite fixed inflating pressures, less experienced operators used prolonged inspiratory times. Wide variation in mean airway pressure and tidal volume were seen in all operators.

Low cost of ventilation in the vagotomised alligator (Alligator mississippiensis)

Pulmonary ventilation requires energy, but the estimated costs of breathing in reptiles vary from 1 to 30% of resting metabolic rate. The low values have been estimated from changes in oxygen uptake during hypoxia or hypercapnia, but it remains possible that these treatments affected metabolism. We equipped alligators with masks for simultaneous measurements of ventilation and oxygen uptake during hypercapnia, hypoxia and bilateral vagotomy. Hypercapnia and hypoxia caused a marked increase in total ventilation, but oxygen uptake remained unchanged indicating a very low energetic cost of breathing. Upon vagotomy, breathing pattern changed to occasional and exceedingly deep breaths (76.1+/-11.1 ml kg(-1)) followed by buccal oscillations and shallower breaths (22.2+/-2.3 ml kg(-1)) interspersed between long non-ventilatory periods. This change in breathing pattern did not affect oxygen uptake. The duration of inspiration increased substantially upon vagotomy, so that inspiratory flow rate did not increase proportionally to tidal volume. These prolonged inspiratory times leads to less flow resistance and may explain that tidal volume could be increased substantially without a measurable energetic cost.

The Effects of Airway Pressure and Inspiratory Time on Bacterial Translocation

Mechanical ventilation with high peak inspiratory pressure (PIP) induces lung injury and bacterial translocation from the lung into the systemic circulation. We investigated the effects of increased inspiratory time on translocation of intratracheally inoculated bacteria during mechanical ventilation with and without extrinsic positive end-expiratory pressure (PEEP). Rats were ventilated in pressure-controlled mode with 14 cm H2O PIP, 0 cm H2O PEEP, I:E ratio 1/2, and Fio2 1.0. Subsequently, 0.5 mL of 10(5) cfu/mL Pseudomonas aeruginosa was inoculated through tracheostomy and rats were randomly assigned to six groups; two low-pressure groups (LP)1/2, 14 cm H2O PIP, 0 cm H2O PEEP, I:E = 1/2, and LP2/1 14 cm H2O PIP, 0 cm H2O PEEP, I:E = 2/1; two high-pressure groups (HP)1/2, 30 cm H2O PIP, 0 cm H2O PEEP, I:E = 1/2, and HP2/1, 30 cm H2O PIP, 0 cm H2O PEEP, I:E = 2/1; two HP PEEP groups (HPP)1/2, 30 cm H2O PIP, 10 cm H2O PEEP, I:E = 1/2, and HPP2/1, 30 cm H2O PIP, 10 cm H2O PEEP, I:E = 2/1. Blood cultures were obtained every 30 min. The rats were killed and their lungs were processed. When compared with baseline values, Pao2 decreased in the LP1/2, LP2/1, HP1/2, and HP2/1 groups at the last time point, but the decline in Pao2 reached statistical significance in only the HP1/2 group. The bacterial translocation rate was greater in group HPP2/1 than group HPP1/2 (P = 0.01). We found that high PIP, with or without prolonged inspiratory time, increased the rate of bacterial dissemination. PEEP prevented bacterial translocation in the high PIP group. However, the protective effect of PEEP was lost when inspiratory time was prolonged.

A murine model of hyperdopaminergic state displays altered respiratory control

The dopamine transporter (DAT) protein plays an important role in the termination of dopamine signaling. We addressed the hypothesis that loss of DAT function would result in a distinctive cardiorespiratory phenotype due to the significant role of dopamine in the control of breathing, especially with respect to chemical control, metabolism, and thermoregulation. The DAT knockout mouse (DAT-/-) displays a state of functional hyperdopaminergia characterized by marked novelty driven hyperactivity. Certain behavioral and drug responses in these mice are reminiscent of endophenotypes of individuals with attention deficit hyperactivity disorders (ADHD). We performed experiments on conscious, unrestrained DAT-/- mice (KO) and littermate DAT+/+ wild-type (WT) controls. Ventilation was measured by the barometric technique during normoxia, hypoxia, or hypercapnia. We measured core body temperature and CO2 production as an index of metabolism. DAT-/- mice displayed a significantly lower respiratory frequency than WT mice, reflecting a prolonged inspiratory time. DAT-/- mice exhibited a reduced ventilatory response to hypoxia characterized by an attenuation of both the respiratory frequency and tidal volume responses. Both groups showed similar metabolic responses to hypoxia. Circadian measurements of body temperature were significantly lower in DAT-/- mice than WT mice during inactive periods. We conclude that loss of the DAT protein in this murine model of altered dopaminergic neurotransmission results in a significant respiratory and thermal phenotype that has possible implications for understanding of conditions associated with altered dopamine regulation.

A Simple Model Illustrating the Respiratory System’s Time Constant Concept

The variation of lung volume (ΔV) depends on the compliance ( C ) of the respiratory system and the variation of pressure (ΔP) to which the system is subjected ([1][1]). This may be noticed in the following equation: \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \

Leak Monitoring in Noninvasive Ventilation

Nasal mask ventilation has been shown to be effective, but outcomes do not always match expectations because of mouth leaks, patient-ventilator asynchrony, or decreased upper airway patency. These developments are detected when they lead ultimately to circuit leaks that lower the effectiveness of ventilation through pressure loss, poor inspiratory triggering, and prolonged inspiratory time. The quality of sleep is affected, and adverse effects and treatment intolerance may arise. A number of ways to detect leaks and their practical consequences are proposed in this article. We applied 310 leak-detection procedures to 177 patients who had disappointing clinical, gasometric, or polysomnographic outcomes of ventilation. The leak-detection procedures varied according to the type of ventilation and the supposed underlying pathophysiological mechanism. Significant leaks were detected in 132 patients (76%); therapeutic changes were then prescribed to optimize outcomes. We present a practical method to apply in patients with suboptimal ventilation outcomes. If leaks can be detected during treatment, the probable cause of treatment failure can sometimes be established and possible pathophysiological mechanisms better understood. With this knowledge, it may be possible to improve ventilation.

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.