Abstract
BACKGROUND:
Automode is a feature on Servo ventilators that automatically switches between mandatory and spontaneous breaths. Spontaneous breaths suppress mandatory breaths until apnea. The period from the last spontaneous breath to the first mandatory breath is automatically adjusted by a calculated apnea time limit based on a maximum apnea time setting, the mandatory breathing frequency setting, and the spontaneous breath count. The purpose of this study was to validate the apnea time algorithm by using simulated mechanical ventilation.
METHODS:
A Servo-u ventilator was connected to an ASL 5000 breathing simulator. Ventilator settings were the following: Automode (pressure control to pressure support), pressure control = 10 cm H2O; pressure support = 5 cm H2O; PEEP = 10 cm H2O; breathing frequency = 10, 12, 15, 20 breaths/min; maximum apnea time = 7 and 12 s. Simulator settings were the following: resistance = 10 cm H2O/L/s; compliance = 35 mL/cm H2O; flow trigger model: frequency = 20 breaths/min, trigger flow = 10 L/min, trigger duration = 800 ms. Flow waveforms were recorded, and the observed apnea time limit was compared with the calculated value. The outcome variable was error, defined as the difference between observed and calculated apnea times expressed as a percentage.
RESULTS:
The observed apnea time limit ranged from 3 to 12 s, depending on the mandatory frequency and the spontaneous breath count. The average error ranged from −2 to 0%.
CONCLUSIONS:
The measured apnea time for simulated ventilation settings was within 2% of calculated times. Automode allowed a spontaneous frequency lower than expected based on the mandatory frequency.
Keywords: Medical simulation, mechanical ventilation, patient circuit compensation, error, Automode
Introduction
Optimum mechanical ventilation requires determining which goal is most important (ie, safety, comfort, or liberation) and selecting a mode that best serves the goal.1 The ongoing adjustment of settings is then required. For liberation, a switch must be made from mandatory to spontaneous breaths, usually done manually. Staffing shortages and the increasing number of mechanically ventilated patients can delay needed mode changes and adversely affect patient outcomes. Automode, a feature included on Servo ventilators (Getinge, Gothenburg, Sweden), automatically switches between mandatory and spontaneous breaths. Specifically, it switches between a mode classified as continuous mandatory ventilation (all mandatory breaths) to a mode classified as continuous spontaneous ventilation (all spontaneous breaths).
Spontaneous breaths are defined as those for which inspiration is both triggered and cycled by the patient. Mandatory breaths are defined as those for which inspiration is either triggered or cycled by the machine.2 In addition, spontaneous breaths suppress mandatory breaths until an apnea occurs and mandatory breaths are re-introduced. Because spontaneous breaths can occur between mandatory breaths, Automode is a form of intermittent mandatory ventilation (IMV), specifically IMV type 2, as explained in detail elsewhere.3 As context for understanding IMV type 2, shown in Figure 1 is a flow chart that describes how the original IMV type 2 mode, called spontaneous/timed mode, switches between mandatory and spontaneous breaths. The period from the last spontaneous breath to the reappearance of the first mandatory breath, the apnea time limit, is determined by the mandatory breathing frequency setting, that is, it is the cycle time for mandatory breaths (Fig. 1).
Fig. 1.
Flow chart showing the operation of spontaneous/timed mode.
In contrast, in Figure 2 is shown the more complicated flow chart of Automode. The period from the last spontaneous breath to the reappearance of the first mandatory breath is determined by a calculated apnea time limit. The calculated apnea time limit is based, in part, on a maximum apnea time setting (7–12 s). However, unlike apnea settings on other modes, the calculated apnea time limit is automatically adjusted based on 3 variables: maximum apnea time setting, the mandatory breathing frequency setting, and the spontaneous breath count since the last mandatory breath (Fig. 1). This functionality is unusual and not explained clearly in any published literature that we are aware of.
Fig. 2.
Flow chart showing the operation of Automode on Servo ventilators. We deduced this flow chart from the equation provided by Getinge and observations of the mode performance.
A literature search by using the keyword Automode found many publications to be mostly descriptive of the automatic transition from mandatory to spontaneous breaths as a form of automatic weaning.4–7 The description of the mandatory breath reintroduction was poor, and the only study that described this in more detail was done using the Servo 300A, which did not have the equation for the calculated apnea time limit. Indeed, the only description of apnea time, the period from the last spontaneous breath to the first mandatory breath, is found in the Servo-u ventilator (Getinge, Gothenburg, Sweden) user manual (Servo-u ventilator system v1-1). But this description is short and does not include any equation for the calculation nor any visual guide graphic to make the end users aware of its unusual functionality.
It also does not make the distinction between the apnea time (an ongoing measurement [Fig. 1]) and calculated apnea time limit (the maximum value allowed for apnea time). The manual states “The ventilator system initially adapts with an increasing apnea time. This means that for the spontaneously triggering patient, the apnea time increases successively until the level set in the settings window for the maximal apnea time parameter is reached after 10 consecutive spontaneously triggered breaths. Exceeding the maximal apnea time setting without a sufficient patient effort will cause: In VS [volume support], a PRVC [pressure-regulated volume control] or VC [volume control] breath will be delivered according to the selected Automode functionality. In PS [pressure support], a PC [pressure control] breath will be delivered.” Setting the apnea time limit is a safety concern for all modes of ventilation. The fact that the apnea time limit in Automode is different from other modes and its use may result in unintended consequences, the purpose of this study was to describe and validate the calculated apnea time limit algorithm by using simulated mechanical ventilation.
QUICK LOOK.
Current Knowledge
In most modes of ventilation, the apnea time limit is set by the operator. In Automode the apnea time limit is automatically adjusted in a unique way not described in the literature.
What This Paper Contributes to Our Knowledge
The maximum apnea time algorithm automatically compensates for a reliable patient trigger rate. The apnea time during Automode is determined not by setting an absolute time (as in other modes) but instead is automatically adjusted according to an equation based on 3 variables: a maximum apnea time setting, the mandatory breathing frequency setting, and the spontaneous breath count since the last mandatory breath.
Methods
To validate and describe the functionality of apnea time in Automode, we compared the calculated apnea time limit by using an equation provided on request by Getinge and the observed apnea time limit produced by a Servo-u ventilator connected to a breathing simulator (Active Servo Lung, ASL 5000 sw3.6; IngMar Medical, Pittsburgh, Pennsylvania). The breathing simulator was initially programmed for a period of no inspiratory effort, during which Automode delivered mandatory breaths at the mandatory breathing frequency. Next, the simulator was programmed to introduce a specific number of spontaneous breaths, which abolished the mandatory breaths. This spontaneous breathing period was followed by another period of no inspiratory efforts (apnea) during which Automode reintroduced mandatory breaths again. The observed apnea time limit was defined as the time from the start of the last spontaneous breath until the start of the first mandatory breath. The observed apnea time limit was visually determined by using the flow versus time graph recorded by the ASL 5000.
Breathing Simulator
This study was conducted by using the ASL 5000 programmed to simulate an adult patient with severe ARDS.8 The simulator settings were the following: resistance = 10 cm H2O/L/s; compliance = 35 mL/cm H2O; simple flow trigger model: breathing frequency = 20 breaths/min, trigger flow = 10 L/min, trigger duration = 800 ms. The script was the following: spontaneous breath count = 0–12 breaths. Flow waveforms were recorded by the ASL 5000, and the observed apnea time limit was compared with the calculated apnea time limit. We verified that every inspiratory effort triggered a spontaneous breath to assure the accuracy of spontaneous breath count. We used a severe ARDS model because it resulted in a short expiratory time constant to reduce the risk of failed trigger efforts.
Ventilator
A Servo-u ventilator was used for all the experiments. The ventilator pre-check was performed with a heated wire circuit. The experiment was performed with an empty heated humidifier that was turned off. Automode has 3 different combinations: pressure control to pressure support, pressure-regulated volume control to volume support, and volume control to volume support, which share the same algorithm for the introduction of mandatory breaths after an apnea period. Automode pressure control to pressure support was selected for the study for simplicity, and it was programed with different pressure control and pressure support levels, so that mandatory breaths could be easily distinguished from spontaneous breaths. Automode pressure control to pressure support settings were the following: pressure control = 10 cm H2O, pressure support = 5 cm H2O, PEEP = 10 cm H2O, mandatory breathing frequency = 10, 12, 15, 20 breaths/min (10 breaths/min is the minimum mandatory frequency allowed), maximum apnea time setting = 7 and 12 s. For all experiments, was set at 0.21.
Outcome Variables
The calculated apnea time limit was used as the reference value and compared with the observed apnea time limit by using the ASL 5000 recorded waveform with the multi-parameter waveforms option in the post-run analysis tab. The outcome variable was error (%) = 100% × (observed apnea time limit – calculated apnea time limit)/calculated apnea time limit, averaged across the spontaneous breath count range.
Procedure
The calibration of the ASL 5000 simulator was verified before starting the study. The ventilator pre-use check was performed, which included an internal leakage test, a patient circuit test with patient circuit compliance, and flow and pressure transducer calibration. Each combination of mandatory frequency and spontaneous breath count for the 2 maximum apnea time settings tested (7 and 12 s) was considered an individual experiment. In total, 56 experiments were performed.
Results
The study results for each experimental combination of mandatory frequency and maximum apnea time setting averaged across the range of simulated spontaneous breath counts are shown in Table 1. The average error ranged from −2 to 0%. The calculated apnea time limit for maximum apnea time = 7 s ranged from 3 s (mandatory frequency = 20 breath/min, spontaneous breath count = 0) to 7 s (mandatory frequency = 10 – 20 breaths/min, spontaneous breath count = 10–12). The calculated apnea time limit for the maximum apnea time = 12 s, ranged from 3 s (mandatory frequency = 20 breaths/min, spontaneous breath count = 0) to 12 s (mandatory frequency = 10–20 breaths/min, spontaneous breath count = 10–12). To make the functionality of the apnea time during Automode more understandable to the bedside clinician, we designed a nomogram for estimating the calculated apnea time limit (Fig. 2) based on the equation given in Figure 1. We also created a spreadsheet that runs in Microsoft Excel (Microsoft, Redmond, WA) to calculate exact values for the calculated apnea time limit (https://1drv.ms/x/s!AuFakBJODC3DhaUGJSOD3RlCSf6zLg?e=ida1Tz).
Table 1.
Study Results
Discussion
To our knowledge, this is the first study to investigate how Automode determines the apnea time limit. The term “apnea” comes from Greek and literally means absence of breath. The Servo ventilator operator's manual defines apnea time as “the time without a patient breathing effort that the system will allow to elapse . . . before the No patient effort alarm is activated and the ventilator switches to the backup mode.” The backup mode is one that provides mandatory breaths (continuous mandatory ventilation) to serve the goal of safety. Naturally, a backup mode is required for continuous spontaneous ventilation modes but not for continuous mandatory ventilation and IMV modes because the set mandatory frequency is the backup. For Servo ventilators, the apnea time limit setting controls the “No patient effort” alarm in all modes except for Automode. For continuous spontaneous ventilation modes (pressure support and volume support), the apnea time limit setting range is 2 to 45 s for children and 15 to 45 s for adults.
For IMV type 1 modes, the set mandatory frequency is the default backup frequency for mandatory breaths in the event of apnea. However, IMV type 2 provides a unique challenge. Because the ventilator is automatically switching between continuous mandatory ventilation and continuous spontaneous ventilation breath sequences, theoretically, there are at least 3 ways to set the apnea time limit. Either it is based on the mandatory frequency (ie, the apnea time limit is the mandatory breath cycle time, equal to 60 s/mandatory frequency), or it is preset to an arbitrary value as for continuous spontaneous ventilation modes, or it is controlled by an automatic algorithm (as in Automode). Perhaps the first appearance of IMV type 2 was the spontaneous/timed mode on Respironics ventilators (now Koninklijke Philips, Amsterdam, Netherlands).
For example, on the V60 ventilator (Koninklijke Philips), there is no apnea alarm per se but instead a low frequency alarm with a setting range of 1–89 breaths/min. In non-CPAP modes, this alarm is functionally off if set below the mandatory frequency setting. This means that by default, the apnea time limit is the cycle time of the mandatory breaths (as explained above). In contrast, on the bellavista ventilator (Vyaire Medical. Mettawa, Illinois), a specific apnea time limit is set for the spontaneous/timed mode, and it is associated with an alarm and a specific backup mode. For either the V60 or the bellavista, it is not possible for the spontaneous frequency to fall below the set mandatory frequency, regardless of how the apnea time interval is set.
The apnea time limit and the lowest possible spontaneous breath count are different in Automode. The calculated apnea time limit (as defined above) is based, in part, on a maximum apnea time setting, which has a range from 7 to 12 s. It represents the maximum time that the machine waits for the patient to trigger a spontaneous breath before re-introducing a machine-triggered mandatory breath. However, the ventilator re-introduces a mandatory breath at the maximum apnea time setting only if the ventilator counts 10 consecutive spontaneous breaths before the apnea. This is explained in Figure 2. Starting from the last mandatory breath, the algorithm first checks to see if there is a trigger signal from a patient inspiratory effort. If so, it delivers a spontaneous breath, it updates the value for the spontaneous breath count, it recalculates the apnea time limit, and resets the apnea time to zero. Then the algorithm continues to look for the next spontaneous breath. If no patient trigger signal is detected, then it continues to measure apnea time. Once the apnea time equals the calculated apnea time limit, the ventilator assumes that an important apnea has occurred, and it delivers a mandatory breath.
The equation in Figure 2 shows that the calculated apnea time limit is lengthened above the cycle time for mandatory breaths in proportion to the spontaneous breath count, as illustrated in the nomogram in Figure 3. Notice that the equation for the calculated apnea time limit does not consider the spontaneous breathing frequency, just the spontaneous breath count. As a consequence, unlike IMV type 1, Automode, as a special form of IMV type 2, allows a spontaneous breath count lower than what the clinician-set mandatory frequency would otherwise define as an acceptable minimum breathing frequency. For example, suppose the clinician set a mandatory frequency of 15 breaths/min (cycle time for mandatory breaths = 60/15 = 4 s). For other IMV type 2 modes, this would mean that the time between spontaneous breaths could be no more than 4 s (otherwise a mandatory breath would be delivered), which corresponds to a spontaneous frequency of 60/4 = 15 breaths/min. Thus, as long as the spontaneous frequency was at least 15 breaths/min, mandatory breaths would be suppressed.
Fig. 3.
A nomogram for estimating the calculated apnea time limit, based on the mandatory breath frequency setting, and the maximum apnea time setting (called “Max. apnea time” in the operator's manual), for the observed spontaneous breath count, since the last mandatory breath. Example: the green solid line shows the mandatory breathing frequency setting of 10 breaths/minute (cycle time = 6 s) and maximum apnea time setting of 12 s. If the spontaneous breath count = 0, then the calculated apnea time limit will be 6 s (ie, equal to the cycle time). If the spontaneous breath count = 4, then the calculated apnea time limit will be extended to 8.4 s (as shown by the green dashed lines arrows). If the spontaneous breath count is ≥ 10, then the apnea time will be extended to 12 s (ie, equal to the maximum apnea time setting). For any other settings, draw a diagonal line from the mandatory frequency to the maximum apnea time setting, then draw a vertical line from the spontaneous breath count to the diagonal line. Finally, draw a horizontal line from the intersection of the vertical and diagonal lines to the apnea time scale.
However, in Automode, suppose the maximum apnea time were set at 12 s. This would mean that the time between spontaneous breaths could be up to 12 s (assuming that the spontaneous breath count was at least 10, so that calculated apnea time limit = 12 s, as shown in Fig. 3). A cycle time of 12 s corresponds to a spontaneous frequency of 60/12 = 5 breaths/min as shown in Fig. 4. Therefore, the clinician might expect the patient to be ventilating at no less than 15 breaths/min, when, in fact, the patient could be ventilating at 5 breaths/min. The minute ventilation could thus be 66% less than what the clinician expects, based on experience with other modes (or even less, depending on the pressure support settings). In this case, the maximum apnea time feature makes the form of IMV type 2 in Automode potentially less safe than other forms of IMV type 2, such as the spontaneous/timed mode. This highlights the necessity of setting the minimum frequency and minimum minute ventilation alarms properly. It also highlights the need to understand the distinction between IMV type 1 and IMV type 2 and the intricacies of the maximum apnea time of Automode. Note that, if this patient were in a continuous spontaneous ventilation mode, such as pressure support or volume support with an apnea time set at 20 s, then the patient would be breathing at a frequency of only 3 breaths/min and still not activating the apnea alarm.
Fig. 4.
Example of unexpected low spontaneous breathing frequency in Automode (see text for explanation).
The design of the maximum apnea time algorithm seems to be a form of artificial intelligence in the sense that it automatically compensates for a reliable patient trigger rate. In other words, the higher the number of spontaneous breaths before the apnea, the less the need for the mandatory breaths and hence the longer the allowed apnea time. However, there are no data to our knowledge that support any assumption of benefit for this feature. On the contrary, we have seen that, among our clinicians, it causes confusion and perhaps avoidance of using Automode. We used a severe ARDS model because it resulted in a short expiratory time constant to reduce the risk of failed trigger efforts. We also used an effort model with a constant spontaneous frequency as opposed to a randomly varying frequency as observed in real patients. Hence, our results are limited to these simulation parameters. Other simulation models could produce other results. However, we believe our study design was appropriate for the study purpose.
Conclusions
The apnea time during Automode is determined not by setting an absolute time (as in other modes) but instead is automatically adjusted according to an equation based on 3 variables: a maximum apnea time setting, the mandatory frequency setting, and the spontaneous breath count since the last mandatory breath. The actual apnea time observed for a variety of simulated ventilation settings was within 2% of that expected from the equation. The apnea time calculation does not consider the spontaneous frequency, as the apnea alarm for other modes does, just the spontaneous breath count. Therefore, unlike IMV type 1 and other forms of IMV type 2, Automode, will allow a spontaneous frequency lower than what the clinician might expect based on the mandatory frequency. This highlights the necessity of properly setting the minimum frequency and minimum minute ventilation alarms.
Footnotes
The location of the study was the Cleveland Clinic, Cleveland, Ohio.
A version of this paper was presented by Mr Garnero as an Editors' Choice abstract at AARC Congress 2023, held November 5–8, 2023 in Nashville, Tennessee.
Mr Chatburn discloses relationships with IngMar Medical, Ventis, and Stimdia. Mr Garnero has disclosed no conflicts of interest.
See the Related Editorial on Page 154
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