The Pre-brief
A patient with ARDS has been on mechanical ventilation for more than a month and is currently trached. He is on pressure support (PC-CSVs) at 15/5 cmH2O. There is a suspicion of respiratory muscle weakness and the main goal is gradual down-titration of ventilator support. On the aforementioned settings, there is excellent synchrony, the work of breathing is normal, and the gas exchange is good. Overnight, the patient is “switched to a rate for resting” (pressure control or PC-CMVs at 15/5 cmH2O). Do you agree with this rationale?
Introduction
The term ‘spontaneous mode’ is generally used for a mode that only allows spontaneous breath. By definition, a spontaneous breath is one that is both patient-triggered and patient-cycled. In a recent post, we discussed the two main ways by which patient effort can trigger a breath (flow triggering and pressure triggering). The term patient-cycled breath is a bit imprecise as it implies full control of the patient on breath cycling, which may not be entirely true. Also, it doesn’t provide any information on how the ventilator allows for it. In this post, we will try to address this topic further.
How is patient-cycling achieved?
Some of the major spontaneous modes in common use are: pressure support (PC-CSVs), volume support (PC-CSVa), proportional assist ventilation (PC-CSVr), and neurally adjusted ventilatory assist (PC-CSVr). In all but the last one, the breath is flow-cycled. In NAVA, the breath is cycled using cues from the electrical activity of the diaphragm (EAdi). Here, we will focus on flow cycling, especially in the context of the pressure support mode. A flow-cycled breath is cycled off when the inspiratory flow drops down to a certain percentage of peak inspiratory flow. This parameter is variably referred to as expiratory trigger sensitivity (ETS), End inspiration (%), ESENS, flow-cycle off criterion etc. The default setting is typically 25%. (Figure 1)
Imagine if the patient decides to take a larger and longer breath (sigh). The long duration of Pmus means that it takes longer for inspiratory flow to drop down to the ETS threshold. Hence, the I-time of this breath will be longer. This is how the patient exerts some control over breath cycling and I-time.
Cycle dyssynchrony with flow-cycling
Although flow-cycling allows some patient control, it is still not immune to cycle dyssynchrony. Apart from the ETS setting, cycle synchrony in these modes depends on the degree of set pressure support (Pvent), as well as the mechanics of the respiratory system.
The term over-assistance is used to describe the scenario where the set pressure support (Pvent) is disproportionately higher than patient effort (Pmus). This is a common cause of delayed cycling in pressure support (Figure 2). Over-assistance reduces both intensity and duration of Pmus, thereby increasing the chances of delayed cycling. The most extreme form of this is when the patient only performs enough effort to trigger the breath and the rest of the breath is passive. This issue is magnified in patients with long time constant of the respiratory system e.g. COPD patients (Figure 3). As inspiratory flow due to Pvent takes longer to decay, cycling off is delayed.
In summary, synchrony on pressure support can be increased by
(i) Setting appropriate ETS, and
(ii) Avoiding over-assistance.
Comparison of flow-cycling (spontaneous modes) and time-cycling (controlled modes):
Synchrony:
Despite the potential synchrony issues mentioned above, flow-cycling is much more likely to enhance synchrony compared to time-cycling (in “controlled modes” e.g. PRVC). As such, natural breathing is chaotic and doesn’t have a set I-time. Let’s reconsider the aforementioned example of a sigh (larger/longer Pmus): a perfectly physiological thing that we all do several times an hour. In controlled modes (with time-cycling), this will likely result in early cycling –> double triggering. This will not be the case in spontaneous modes (flow-cycling).
Degree of support:
A common argument to switch from a spontaneous mode to a controlled mode is that the latter provides more support. This is what happened in the case mentioned in the pre-brief. Let’s compare the two modes with more granularity. If the inflation pressure and PEEP are set the same, pressure control differs from pressure support in two ways:
(a) There is a backup rate
(b) Breaths are time-cycled as opposed to flow-cycled
A high backup rate is usually neither necessary nor advisable in patients who are not sedated. If the goals of mechanical ventilation are comfort and liberation, it is best for the patient to control the respiratory rate, as long as adequate gas exchange is maintained. Adding interspersed mandatory breaths will only worsen dyssynchrony and discomfort.
But what about the fact that the breaths in pressure support are flow-cycled as opposed to time-cycled. Does this fact imply that the “degree of support” is somehow lower than pressure control? The short answer is: it depends. As such, with longer I-time, the degree of support is likely to increase. The extent of this will depend on the underlying mechanics (see Figure 4 for details). As discussed, the I-time in pressure support will vary with each breath based on the duration of Pmus, degree of pressure support, and underlying mechanics. In pressure control, the I-time is set by the user. A given pressure support breath may thus provide lower, higher, or identical support compared to a given pressure control breath, depending on their relative I-times. E.g. If the ETS is set to 1%, the I-time of the pressure support breath will be long enough that increasing it will not provide any further ventilatory support.
It may have now become clear that switching from pressure support to pressure control to “rest the patient” is a fallacious argument that will likely worsen synchrony. If under-assistance is a concern, the set pressure support can always be dialed up to achieve a reasonable work of breathing. My favorite tool to use while titrating pressure support is ΔPocc, as discussed in the last post.
The Debrief
- Spontaneous modes allow the patient to exert some control over when the breath is cycled. This is most commonly achieved via flow-cycling.
- In flow-cycling, a breath is cycled when the inspiratory flow drops to a certain percentage of peak inspiratory flow that is set by the user (ETS).
- Spontaneous (flow-cycled) modes enhance cycle dyssynchrony compared to controlled (time-cycled) modes.
- Nonetheless, cycle dyssynchrony may still occur with flow-cycling. Fine-tuning of ETS and avoidance of under- or over-assistance are the key ways to enhance synchrony. ΔPocc can be used to titrate the degree of pressure support.
- In an awake spontaneously breathing patient, switching from pressure support to pressure control to “rest the patient” is a fallacious argument that will likely worsen synchrony.
References
- Brochard L, Telias I. Bedside Detection of Overassistance During Pressure Support Ventilation. Crit Care Med. 2018 Mar;46(3):488-490.
- Mojoli F, Iotti GA, Arnal JM, Braschi A. Is the ventilator switching from inspiration to expiration at the right time? Look at waveforms! Intensive Care Med. 2016 May;42(5):914-915.
- Ventilation simulator used for images: Xlung. Link – https://xlung.net/en/. Used with permission.
