Triggering the Ventilator Expiration

The end of the patient's inspiratory time is difficult to determine for the ventilator and the time at which the ventilator terminates inspiration and opens the exhalation valve defines the beginning of the expiration process, for the ventilator, but also for the patient [26,27]. Regarding the criteria used for terminating the breath, i.e., the cycling criteria, little effort has been provided towards a specific recognition of the end of a patient's effort. During assist-control ventilation, the breath is terminated on a time criterion independent of patient effort. The ventilator primarily controls the flow, and the insufflation time depends on the peak flow setting and VT set on the ventilator by the clinician. The total inspiratory time can be prolonged by the addition of a pause or a plateau at end-insufflation. During pressure support ventilation (PSV), termination of the breath may be closer to a patient's neural signal than using a preset inspiratory time. The decelerating flow signal is used to determine the time at which the ventilator switches to expiration. Because the inspiratory flow should be influenced by the patient's effort at anytime of the breath, this criterion is influenced by a signal directly coming from the patient. Unfortunately, this off-switch criterion is influenced by complex interference [28,29]; the time constant of the respiratory system can vary the time at which the flow peaks, and, therefore, the time at which the flow threshold can be reached. For instance, in a patient with high respiratory resistance, the flow can become almost flat and a small percentage of the peak-flow will occur very late; the value of flow used as a threshold criterion can also make a large difference, especially in case of prolonged insufflation time. This latter parameter can now be adjusted on some ventilators, and the clinician must be aware of the possibility to generate or avoid dysynchrony at the end of the breath. The level of pressure applied at the opening of the respiratory system also influences the peak flow and the time of this peak; lastly, the remaining inspiratory effort at the end of the breath will also influence this criterion [30]. Many examples of dysynchrony occurring at the end

Mechanical Ventilation

Fig. 2 Example of dysynchrony due to an insufflation time shorter than the inspiratory effort. Tracings of airway pressure (Paw), flow and esophageal pressure (Pes) are shown. The inspiratory effort is prolonged beyond the end of the inspiration (dashed line) and creates a sudden drop of airway pressure at the onset of expiration, followed by a normal expiration.

Fig. 2 Example of dysynchrony due to an insufflation time shorter than the inspiratory effort. Tracings of airway pressure (Paw), flow and esophageal pressure (Pes) are shown. The inspiratory effort is prolonged beyond the end of the inspiration (dashed line) and creates a sudden drop of airway pressure at the onset of expiration, followed by a normal expiration.

of the breath have been described, especially for high levels of PSV and in patients with high respiratory resistance (Fig. 2). This factor is also crucial in case of leaks, such as during non invasive ventilation (NIV).

In addition, delayed expiration, as frequently observed with inadequate settings during assisted ventilation, may influence the level of dynamic hyperinflation and worsen PEEPi [26,27].

Tidal Volume, Peak-Flow, and Inspiratory Time

Adjusting the inspiratory time to match a patient's neural inspiratory time is potentially very important and is only automated, at least partially, with proportional assist ventilation. For the clinician, the first approach is to understand that the settings of volume, flow and inspiratory time are linked in a way that depends on the specific type of ventilator used. In particular, some ventilators propose to adjust the inspiration to expiration ratio in addition to the peak-flow setting. This is done with the purpose of calculating a plateau time or of recalculating an ideal inspiratory time during synchronized-intermittent mandatory ventilation (SIMV). Clinicians should be aware that they thus need to differentiate the insufflation time and the inspiratory time.

Since the seminal studies of Marini et al. [31, 32], of Ward et al. [7] and later of Cinnella et al. [8], the influence of the peak-flow setting on the patient's work of breathing has clearly been demonstrated as well as the importance of a proper adjustment for the clinician. In this regard, pressure-assisted modes deliver higher peak-flows than volume-controlled modes for a similar mean inspiratory flow and Vt. They may, therefore, be easier to adapt to patient comfort. For this reason, new ventilators have new servo controlled modes, where pressure-control or pressure-support is the way to deliver the breath but which are also volume-targeted. The intent is to offer the clinician both the safety of a pre-set volume and the comfort provided by a pressure-supported mode. Unfortunately, they bring more confusion than real benefits [33]. In case of increased ventilatory demand, they will react as an inversely proportional mode of assisted ventilation.

Once it was shown that high inspiratory flows were necessary, it was subsequently shown that this had an influence on a patient's respiratory frequency [34, 35]. Laghi et al. elegantly showed that this was primarily due to modifications of inspiratory time [36]. When patients look uncomfortable during assisted ventilation, flow is commonly increased to achieve a better match with patient demand. This results in a decrease in ventilator inflation time and, may thus allow more time for exhalation. Because tachypnea can also ensue, whether the time for exhalation is really prolonged and hyperinflation decreased is doubtful. The consequences of increasing frequency on hyperinflation, therefore, remain to be studied. In patients with moderate to severe COPD non-invasively ventilated, and using Vt sufficient to ensure comfort, Laghi et al. found that alterations in imposed ventilator inflation time produced increases in frequency but also decreases in PEEPi and inspiratory effort, whether these changes in inspiratory time were achieved by increasing delivered inspiratory flow or by decreasing ventilator inspiratory pause [36]. The data by Laghi et al. show that a decrease in ventilator inflation time does indeed allow more time for exhalation despite the development of tachypnea. Higher inspiratory flow also decreases respiratory drive and effort, which can help to improve patientventilator interaction.

Pressure-support Ventilation

The debate about the best level of pressure support to set has been compared to a classic in critical care medicine, that on the best PEEP [37]. An individual adjustment of the pressure support is indeed important in order to maintain enough inspiratory muscle activity while avoiding risk of respiratory muscle fatigue. Some have advocated the need for simple, clinically applicable techniques for measuring the inspiratory effort at the bedside in mechanically ventilated patients [38, 39]. The P0.1 has been used as a surrogate for patient work ofbreathing and has been shown to parallel changes in effort during changes in pressure support level [38]. Foti et al. showed that the difference between airway pressure at the end of inspiration and the elastic recoil pressure of the respiratory system obtained with an end-inspiratory pause was a good estimate of the pressure developed by the inspiratory muscles at end inspiration [30]. This measurement can be performed by simply activating the inspiratory hold button on the ventilator, and can simply be taken from the analog airway pressure display. In a group of non-obstructed patients with acute respiratory failure, this index was also a good reflection of the overall patient effort [30]. This method is best suited for research purposes, however, and there is no universally accepted method to titrate PSV. Clinical examination, especially regarding the use of accessory muscles of inspiration, and measurements of respiratory frequency are probably the more appropriate methods [40, 41]. The optimal level of respiratory frequency may be difficult to determine individually, although a threshold around 30 breaths per minute is probably acceptable for many patients, as shown by measurements of respiratory efforts [40, 41] and as used in automated algorithms to drive a ventilator [1, 2]. Clinicians should be aware, however, that the frequency displayed by the ventilator may differ from the real frequency of the patient in case of missing efforts [42]. This is especially true when high levels ofpressure support are used in patients with COPD, and should be suspected by visual inspection of the flow-time curve.

Non-invasive Ventilation and Leaks

Although NIV is generally perceived as more comfortable for patients than invasive mechanical ventilation, mask (or interface) intolerance remains a major cause of NIV failure [43]. Failure rates range from below 10% to over 40%, despite the best efforts of skilled caregiver staff. Thus, improvements in mask design that enhance comfort and reduce complication rates are needed, with the presumption that they will lead to improved tolerance and reduced NIV failure rates [44]. Leaks create major dysynchrony that the clinician needs to recognize. In case of leaks, attempts to minimize the leaks must be performed and should include readjustment of the mask and decrease of the delivered pressures [45]. Once leaks persist, minimizing their consequences on patient-ventilator interaction becomes important. During PSV, adjustment of the cycling-off criterion or addition of an inspiratory time limit will help in avoiding prolonging the ventilator's inspiration long after the end of the patient's neural inspiratory time [46]. These useful settings are, however, sometimes difficult to access on the ventilator panel. In other cases, they are not provided, or can only be obtained indirectly.

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