Triggering and Inspiration

Ventilators provide positive-pressure assistance to a patient's inspiratory effort when the pressure in the ventilator circuit decreases by 1 to 2 cmH2O. The task of triggering the ventilator can require substantial effort [2]. Patients who struggle to reach the set sensitivity are unable to switch off their respiratory motor output immediately after successfully triggering the ventilator [3].As a result, considerable effort can be expended during the period of mechanical inflation that follows the trigger phase, the so-called post-trigger phase. The increased effort in the post-trigger phase may arise because of an inadequate level of positive pressure in the inspiratory limb during the period immediatelybefore and during the milliseconds after opening of the inspiratory valve [4]. In this situation, the increased effort during the post-trigger phase may offset the prime objective of the ventilator: to unload the respiratory muscles.

For a ventilator to function ideally, inspiratory assistance from the ventilator should coincide with the inspiratory effort of the patient. Most studies of patientventilator interaction have been based on indirect measurements, where the onset and offset of respiratory muscle activity have been estimated from recordings of airflow combined with airway, esophageal, or transdiaphragmatic pressures [2, 3, 5-7]. Parthasarathy and coworkers systematically evaluated the concordance between such indirect estimates and a more direct measurement of neural activity, namely the crural diaphragmatic electromyogram (EMG) [8]. Estimates of the duration of inspiration based on flow, esophageal pressure, and transdiaphragmatic pressures revealed substantial differences as compared with the duration of inspiration measured with the diaphragmatic EMG. The average differences ranged from 252 to 714 ms. The standard deviation of these differences ranged from 74 to 221 milliseconds. When inspiratory time measured on the recording of the diaphragmatic EMG was taken as the reference standard, the inspiratory time estimated from the transdiaphragmatic pressure (from the initial deflection of the signal until the signal returns to baseline) had a mean difference of 57% from the reference value and a scatter (+2 SD) of 87% (Fig. 1). Given the magnitude of these discrepancies, conclusions about patient ventilator interactions based on indirect estimates of inspiratory time are susceptible to considerable error.

Apart from the research importance of the discrepancies between indirect estimates of a patient's inspiratory time and the true value of inspiratory time, the discrepancies can adversely affect the operation of the ventilator. Because the ventilator's algorithms are based on recordings of flow and airway pressure, errors in estimating the onset of inspiratory time may give rise to delay in triggering the ventilator, and errors in estimating the duration of inspiratory time that may cause mechanical inflation to persist into expiration (Figs. 2 and 3). A delay in opening of the inspiratory valve may arise from a decreased respiratory drive [6, 9] or increased intrinsic positive end-expiratory pressure (PEEPi) [8]. In five critically ill patients receiving mechanical ventilation, significant delays were noted between the onset of patient's inspiratory effort (measured by crural diaphragmatic electromyogram) and the onset of inspiratory flow [8] (Figs. 1 and 2). The delay between the onset of inspiratory effort and the time the ventilator was triggered was correlated with the level of PEEPi of the breaths (r = 0.59). This observation suggests that when elastic recoil pressure at the end of expiration is high, the subsequent inspiratory effort also needs to be proportionally increased if the ventilator is to be successfully triggered. A ventilator that is designed to sense the patient's effort at a neural level (diaphragmatic electromyogram) instead ofsensing the final result of patient effort (changes in airway pressure or flow) should achieve better patient-ventilator synchrony [10]. The machine described by Sinderby and

Fig. 1. Representative tracings of the raw crural diaphragmatic electromyogram (EMG), the processed diaphragmatic EMG achieved by removing EKG artifacts by computer, the moving average (MA) of the processed diaphragmatic EMG, esophageal pressure (Pes), and flow in a patient. The relationship between an indirect estimate of the onset of neural inspiratory time and its onset on the diaphragmatic EMG signal was assessed by calculation of the phase angle, expressed in degrees. In this example, the onset of inspiratory time is estimated as occurring earlier (negative phase angle of 15 degrees) by Pes-based measurements and later (positive phase angle of 110 degrees) by flow-based measurements. The duration of inspiratory time as estimated by Pes (hatched horizontal bar) is longer than the true inspiratory time measured by the diaphragmatic EMG (note that the hatched bar is wider than 0 to 360 degrees on the solid black bar of the reference measurement). The duration of inspiratory time as estimated by flow-based measurements is shorter (clear horizontal bar) than the true inspiratory time measured by diaphragmatic EMG (note that the open white bar is narrower than 0 to 360 degrees on the solid black bar of the reference measurement). Modified from [8] with permission

Mechanical Ventilator ImagesMechanical Ventilation Symbol

Fig. 2. The phase angle between the indirect estimate of the onset of neural inspiratory time and its reference measurement in each patient during mechanical ventilation; estimates from the esophageal pressure (Pes) tracings are shown as closed squares, estimates from flow tracings are shown as closed circles, and estimates from transdiaphragmatic pressure (Pdi) tracings are shown as closed triangles. The closed symbols represents the mean difference (bias) in phase angle; the open symbols to the right of each closed symbol represents the mean difference between the two measurements noted during the reproducibility testing of the reference measurement. The error bars represent ± 2 SD (twice the precision). A positive value in the phase angle indicates a delay in onset of inspiratory time for the flow-based measurements. Modified from [8] with permission

Fig. 2. The phase angle between the indirect estimate of the onset of neural inspiratory time and its reference measurement in each patient during mechanical ventilation; estimates from the esophageal pressure (Pes) tracings are shown as closed squares, estimates from flow tracings are shown as closed circles, and estimates from transdiaphragmatic pressure (Pdi) tracings are shown as closed triangles. The closed symbols represents the mean difference (bias) in phase angle; the open symbols to the right of each closed symbol represents the mean difference between the two measurements noted during the reproducibility testing of the reference measurement. The error bars represent ± 2 SD (twice the precision). A positive value in the phase angle indicates a delay in onset of inspiratory time for the flow-based measurements. Modified from [8] with permission coworkers shows promise in that regard. This machine has yet to undergo rigorous evaluation, especially within the ICU.

Some patients have a high elastic load, secondary to hyperinflation, and a low respiratory drive. As a result, inspiratory effort will be insufficient to successfully trigger the ventilator [6]. The increased use of bedside displays of pressure and flow tracing has led to a growing awareness of the frequency with which patients fail to trigger a ventilator [8-12]. When receiving high levels of pressure support or assist-control ventilation, a quarter to a third of a patient's inspiratory efforts may fail to trigger the machine [9]. The number of ineffective triggering attempts increases in direct proportion to the level of ventilator assistance [9]. Some authors have recommended reducing the level of pressure support as a means of decreasing the number of ineffective triggering attempts [13]. While this approach should decrease the number of failed triggering attempts, it is likely to be accompanied by a decrease in ventilator assistance. At present, there are no rules of how best to achieve a good balance.

In a study of factors contributing to ineffective triggering, a decrease in the magnitude of inspiratory effort at a given level of assistance was not the cause of

Mechanical Ventilator Images

Fig. 3. The relationship of neural expiratory time to mechanical expiratory time was assessed by measuring the phase angle, expressed in degrees. If neural activity began simultaneously with the machine, the phase angle was zero. If neural activity began after the offset of mechanical inflation, the phase angle was positive (60 degrees for Subject 1). If neural activity began before the machine, the phase angle was negative (-45 degrees for Subject 2). Modified from [12] with permission

Fig. 3. The relationship of neural expiratory time to mechanical expiratory time was assessed by measuring the phase angle, expressed in degrees. If neural activity began simultaneously with the machine, the phase angle was zero. If neural activity began after the offset of mechanical inflation, the phase angle was positive (60 degrees for Subject 1). If neural activity began before the machine, the phase angle was negative (-45 degrees for Subject 2). Modified from [12] with permission ineffective triggering. Indeed, effort was 38% higher during non-triggering attempts than during the triggering phase of attempts that successfully opened the inspiratory valve [9]. Significant differences, however, were noted in the characteristics of the breaths before the triggering and non-triggering attempts. Breaths before non-triggering attempts had a higher tidal volume (Vt) than did the breaths before triggering attempts, 486 ± 19 and 444 ± 16 ml, respectively, and a shorter expiratory time, 1.02 ± 0.04 and 1.24 ± 0.03 seconds, respectively. An abbreviated expiratory time does not allow the lung to return to its relaxation volume, leading to an increase in elastic recoil pressure. Indeed, PEEPi was higher at the onset of non-triggering attempts than at the onset of triggering attempts: 4.22 ± 0.26 versus 3.25 ± 0.23 cmH2O. Thus, non-triggering results from premature inspiratory efforts that are not sufficient to overcome the increased elastic recoil associated with dynamic hyperinflation [9].

An elevated PEEPi may result from an increase in elastic recoil pressure or expiratory muscle activity. The relative contributions of these two factors to ineffective triggering were investigated in healthy subjects receiving pressure support and in whom airflowlimitation was induced with a Starling resistor [12]. PEEPi was more than 30% higher at the onset of nontriggering attempts than before triggering attempts similar to the situation in critically ill patients [9]. The magnitude of expiratory effort, quantified as the expiratory increase in gastric pressure, did not differ before triggering and non-triggering attempts. In contrast, elastic recoil, estimated as gross PEEPi minus the increase in gastric pressure, was higher before non-triggering attempts than before triggering attempts: for example, 14.1 and 4.9 cmH2O, respectively, at pressure support of 20 cmH2O. That non-triggering was causedby the elastic recoil fraction of PEEPi, not that resulting fromexpiratory effort, suggests that external PEEP might be clinicallyuseful in reducing ineffective triggering.

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