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8. Patient-Ventilator Asynchrony
8.1 Definition and Epidemiology
Mechanical ventilation is one of the most important life-supporting strategies for patients with critical illnesses, which aims to improve gas exchange and decrease the patient’s work of breathing and unload the respiratory muscles [1–3]. Optimal patient-ventilator interaction is essential to achieve these goals. Therefore, neuromuscular blockade was always administrated to eliminate the patient’s respiratory efforts and controlled ventilation was used in the old days. As assisted ventilatory modalities are employed and new generation ventilators are developed, neuromuscular blockade prescription is decreasing and the interaction between the patient and the ventilation attracts more attention.
Patient-ventilator asynchrony (PVA) is defined as a mismatch between the patient and ventilator in terms of inspiratory and expiration times . Although a variety of new ventilator modalities such as assist controlled ventilation (ACV), pressure-regulated volume control (PRVC), pressure support ventilation (PSV), and proportional assist ventilation (PAV) were developed, and more ventilator settings could also be adjusted for the needs of patients, including triggering sensitivity, rise time of ventilator delivery and cycling criteria, asynchronies are still very common, especially in critically ill patients.
PVA has been evaluated in some studies, with incidence varied from 16% to 80%, due to the differences in the detection methods used, evaluation periods, and population studied [5–7]. It occurs in all kinds of mechanical ventilation modes and all stages of mechanical ventilation, including the trigger phase, breath delivery phase, termination of inspiration, and expiratory phase, which is influenced by the condition of the patient (patient’s inspiratory drive, disease process), ventilator settings (trigger, sensitivity, flow delivery, and cycling criteria), and sedation levels [3, 8, 9]. PVA could be evaluated by an asynchrony index, defined as the number of asynchrony events divided by the total respiratory rate, which is the sum of ventilator cycles and wasted efforts and expressed as a percentage . And a high prevalence of asynchrony (AI >10%) may be associated with adverse outcomes, such as prolonged duration of mechanical ventilation and hospital stay and increased mortality [9, 10].
8.2 Detection Methods
There have been a lot of studies that evaluated the prevalence of patient-ventilator asynchrony, and airway pressure and flow signals were the most frequently used, which has been proved to be reliable . However, there are still some limitations. The interpretation of these waveforms depends on the experience of clinicians, type of asynchronies , and the reliance on the internal measurements of the ventilator. Some types of asynchronies may be missed or misread even by experienced clinicians [11–13]. Esophageal pressure, respiratory muscle electromyograms, transdiaphragmatic pressure measurement, and electrical activity of the diaphragm (EAdi) have also been used to detect asynchronies, which are more accurate but invasive and not routinely used during daily clinical practice.
8.2.1 Airway Pressure and Flow Waveforms
As a noninvasive method, interpretation of airway pressure and flow waveforms could help clinicians to recognize patient-ventilator asynchronies, and its accuracy and validity of detection of ineffective triggering and double triggering have been demonstrated comparable with esophageal pressure signals . Airway pressure and flow signals could be obtained at the bedside, and the clinicians could make proper adjustments in time. However, its validity of detection of auto-triggering, reverse triggering, and cycling asynchrony remains uncertain [12, 14].
8.2.2 Esophageal Pressure
Esophageal pressure (Pes) has been used as a surrogate for pleural pressure to monitor the patient’s respiratory drive and inspiratory effort, which could be obtained at the bedside. Usually, a negative Pes swing indicates an inspiratory effort, which triggers the ventilator to deliver one breath cycle. Asynchrony could be identified by comparing the time occurrence of the change in Pes with that of the airway pressure and flow. As a negative Pes swing indicates an inspiratory effort, which could also help to identify auto-triggering, reverse triggering, and cycling asynchronies accurately [3, 15].
Pes could be obtained by a specific catheter with a balloon. To obtain accurate Pes measurements, the esophageal balloon should be placed in the lower two-thirds of the intrathoracic esophagus, which is determined by an occlusion test . However, the results could also be influenced by mediastinal loading, variations of esophageal elastance, and amplitude of the cardiac oscillations . Despite Pes monitoring is a minimally invasive procedure with accurate detection of asynchronies, it is not routinely used in the clinical setting.
8.2.3 Electrical Activity of the Diaphragm
The electrical activity of the diaphragm (EAdi) represents the patient’s neural respiratory drive, which could be used to monitor the onset of neural inspiration and expiration. EAdi could be obtained easily through a nasogastric tube with multiple electrodes and recorded by the software. Some studies have used EAdi to detect patient-ventilator asynchronies, which is more accurate and sensitive than airway pressure and flow waveforms only, especially in the detection of auto-triggering, reverse triggering, premature and delayed termination [18, 19]. It could also help to differentiate reverse triggering from double triggering, depending on the first inspiration that is triggered by the patient or the ventilator . However, considering its invasiveness, cost, technical challenges, and contraindications, such as severe coagulopathy, diagnosed or suspected esophageal varices, and history of esophageal or gastric surgery, EAdi is not routinely used to monitor asynchronies in the clinical practice. Some authors have demonstrated the linear relationship between EAdi and respiratory muscle surface electromyography (EMG), which may also be used to detect the activity of inspiratory muscles in patients under ventilation .
Definition of different types of patient-ventilator asynchronies was based on flow, airway pressure, and esophageal pressure waveforms
Refers to a patient’s inspiratory effort but not followed by a ventilator cycle, indicated by a decrease in pressure drop (≥0.5 cm H2O) associated with a simultaneous increase in the flow 
Refers to two cycles separated by a very short expiratory time, defined as less than one-half of the mean inspiratory time, and the first cycle being patient-triggered 
Refers to inspiratory efforts triggered by the ventilator in a repetitive and consistent manner 
Refers to an assisted breath delivered by the ventilator that is not triggered by the patient (without a prior airway pressure decrease) 
Refers to the ventilator’s flow setting is lower than patient’s demand, indicated by a dish-out of pressure waveform 
Refers to the ventilator’s flow setting is higher than the patient’s demand, indicated by a peaking of pressure waveform (overshoot) at the beginning of the inspiration 
Refers to the termination of the ventilator cycle despite the patient’s effort continued, indicated by a sharp decrease in the expiratory flow followed by an increase and then a decrease gradually to the baseline 
Refers to a patient’s expiratory effort starting before the ventilator switch to the expiratory phase, indicated by a spike in the airway pressure waveform and a rapid decrease of the inspiratory flow towards the end of mechanical inspiration 
Refers to the magnitude of the end-expiratory pressure in excess of the set extrinsic PEEP, indicated by a sudden onset of inspiratory flow prior to the expiratory flow curve returning to zero 
8.3.1 Triggering Asynchrony
Ineffective triggering refers to a patient’s inspiratory effort but not followed by a ventilator cycle, defined as a decrease in pressure drop (≥0.5 cm H2O) associated with a simultaneous increase in the flow . Ineffective triggering usually occurred among patients with chronic obstructive pulmonary disease (COPD), asthma, and those with intrinsic PEEP (PEEPi), as well as diminished respiratory drives, such as deep sedation and patients with brain stem injuries. Additionally, improper triggering thresholds of the ventilator could also lead to ineffective triggering. It can occur during both mechanical inspiratory phase and expiratory phase, under both pressure support ventilation and mandatory ventilation (Fig. 8.1). Most of the ineffective triggering can be detected by airway pressure and flow waveforms. Combined with pes or EAdi signals can improve the accuracy and sensitivity of detecting ineffective triggering than airway pressure and flow waveforms only [18, 19]. Ineffective triggering may be eliminated by reducing pressure support or inspiratory duration to decrease tidal volume and applying extrinsic PEEP to decrease the patient’s inspiratory effort to trigger the ventilator [23, 24].
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