This is Part 4 of a 5 part series on fundamental concepts in ventilator management. See Part 3 is here.
We have discussed how to take over a patient’s work of breathing, now it’s time to discuss taking over control of respiration. This is a very important distinction in ventilator management: there are times when it is advantageous to allow a patient to control respiration while the vent just takes over the work of breathing. Diabetic ketoacidosis (DKA) is a classic example. Patients with DKA generally have intact respiratory control which maintains high minute volumes in an attempt to blow off CO2 to compensate for the metabolic acidosis. If the patient requires respiratory control, then it is the provider’s responsibility to manage the respiratory function through frequent monitoring of the acid-base status. Alternatively, the patient can be allowed to manage their respiratory function, in which case their respiratory rate and volume will automatically adjust to changes in their status. Traumatic brain injury is a case where it may become necessary to take over respiratory control, because the patient’s own respiratory control centers are possibly injured. The decision of whether or not to take control of respiration revolves around a single question: “Do I trust the patient to manage their own breathing?”
There are a variety of vent modes that offer you different levels of control over a patient’s breathing. I think of it as a spectrum, drawn below:
The modes on the ends of the spectrum are the simplest to understand, so we’ll start there. Continuous Positive Airway Pressure (CPAP) does exactly what it says: the vent maintains a specified positive airway pressure (PEEP) while the patient breathes. CPAP isn’t really a “ventilator” mode, since the vent isn’t actually ventilating the patient (i.e. delivering breaths). Control mode ventilation is the exact opposite: the vent delivers breaths of a set tidal volume at a set rate, regardless of what the patient wants to do. In this mode, the vent maintains a set PEEP, Vt, and respiratory rate. Patients generally do not tolerate Control mode ventilation well unless they are deeply sedated, or even paralyzed. Control mode ventilation should not be confused with the acronym “CMV”, since “CMV” refers to a different mode of ventilation altogether (Continuous Mandatory Ventilation). Control mode ventilation is rarely used these days, primarily by older gas operated transport vents.
The modes in the middle of the spectrum all respond to the patient’s respiratory effort, so it is important to have some familiarity with the “trigger” setting on the vent. Trigger refers to how sensitive the vent is to detecting inspiratory effort. There are two main styles of trigger: flow triggering and pressure triggering. In flow triggering, the vent maintains a constant flow through the vent circuit while monitoring how much gas is leaving through the exhalation port. When the patient tries to inhale, the vent detects that some of the gas has gone missing between the vent output and the exhalation port. The vent interprets this as the patient needs a breath, and then decides whether or not to deliver one based on the vent mode. Pressure triggering works by detecting a decrease in pressure in the vent circuit, which the vent interprets as an attempt to inhale. Flow triggering is generally more responsive to the patient and is easier for the patient to trigger, but it also wastes gas because of the constant flow through the circuit. Additionally, some transport vents don’t have the option of flow triggering. Pressure triggering requires more effort for the patient to trigger the breath, and also tends to be more susceptible to false triggering during transport. The usual flow triggering sensitivity is 1-3 liters per minute (patient needs to generate 1-3 LPM of inspiratory flow to trigger the vent) and the usual pressure triggering sensitivity is 1-2 cmH2O (patient needs to generate 1-2 cmH2O of inspiratory pressure to trigger the vent).
What happens when the vent is triggered? To explore this, first look at Assist/Control (AC) mode. In AC mode, the patient will receive a breath every time they trigger the vent. If the patient fails to trigger a breath in a set period of time, the vent will initiate a breath for the patient. In this mode, the set respiratory rate is the minimum rate that the patient is allowed to breathe. For example: a set rate of 12 breaths per minute comes out to 1 breath every 5 seconds. If the patient wants to breathe every 4 seconds (15 breaths per minute), then the vent will deliver 15 patient-initiated breaths per minute. If a patient fails to initiate a breath within 5 seconds of the previous breath ending, the vent will initiate a breath. AC mode is an excellent “general purpose” ventilator mode. If the patient is breathing at an appropriate rate, the vent will assist the patient with their breathing. If the patient is not breathing at an appropriate rate, the vent will take over control and breathe at an appropriate rate for the patient.
Keep in mind, “appropriate rate” only means faster than what is programmed into the vent. If the patient tries to breathe 80 times a minute, then the vent will attempt to deliver 80 breaths per minute. This comes back to the primary question: “Do you trust the patient to manage their own respiratory function?” AC mode will prevent hypoventilation, but other than that, the patient is in full control of the respiratory rate. This can be a serious issue for patients with restrictive airway disease, because they often have significant “air hunger” that causes them to try and breathe excessively fast. As discussed before, excessive respiratory rates in restrictive lung disease can lead to breath stacking, which worsens air hunger, which further increases respiratory rate. This situation rapidly spirals out of control and can easily lead to tension pneumothoraces and death. This is another type of patient that should not be trusted with maintaining their own respiratory function.
Synchronized Intermittent Mandatory Ventilation (SIMV) represents a middle ground between AC and control mode ventilation. SIMV is basically control mode ventilation, except that the vent will try to line up the delivered breaths with the patient’s inspiratory efforts. In practice, the vent allows a small window around each scheduled breath when the patient is allowed to trigger the vent. If the vent is set to 12 breaths per minute, then approximately every 4 to 6 seconds, the patient will be allowed to trigger a breath. The average respiratory rate over the course of a minute will remain at 12. If the patient is breathing 24 times a minute, the vent will only deliver gas for the 12 patient triggered breaths that line up with the “breath schedule.” The vent will not prevent the patient from inhaling during the extra breaths, but the patient will have to pull air through the vent and vent circuit on their own if they want any breaths outside of those the vent has scheduled.
This last point is the major pitfall of SIMV. If the patient’s respiratory muscles are reasonably strong, they can generate significant negative intrathoracic pressure and expend a lot of energy trying to pull air through the vent. Heart failure patients in particular are at risk of decompensation when they are struggling to inhale against resistance, because the negative intrathoracic pressure acts like inverse CPAP while also increasing demands on the heart through increased work of breathing. We can get around this by providing the patient with some assistance pulling air through the vent, even if we aren’t delivering a full tidal volume breath. This is how SIMV with Pressure Support (PS) works. If the patient tries to inhale when a breath isn’t scheduled, the vent will give some positive pressure to reduce the patient’s effort. The higher the pressure support, the more assistance the patient gets.
This leads to an important point, consider the following in a thought experiment: Let’s put the patient in SIMV-pressure control, with PIP set to 15, PEEP set to 5, and PS set to 15 as well. In this hypothetical situation, the vent will behave almost exactly as if it were in Assist/Control-pressure control, because the vent initiated breaths will deliver the same volume as the patient initiated breaths. The resulting effect is that the patient can take a full Vt breath as often as they want, as long as they don’t drop below the set SIMV rate. There are some subtle differences between the PS breaths and the usual vent delivered breaths, but functionally, setting the vent this way is indistinguishable from setting it in AC.
This leaves one final mode, which is also the most difficult to master: Bi-level Positive Airway Pressure (BiPAP). This mode operates in a version of pressure control where every breath is a pressure support breath. When the vent detects that the patient is trying to inhale, it raises the pressure up to a specified inspiratory pressure. Functionally, the vent is behaving exactly like AC-pressure control at this point. Anytime the patient wants a breath, the vent delivers that breath by increasing the airway pressure to the set inspiratory pressure.
The difference between AC-pressure control and BIPAP is what ends the breath. In AC, SIMV, and control mode ventilation, the breath ends after the set I-time. Once the patient triggers a breath, the vent takes over control and delivers a full breath. In BIPAP, the breath terminates when the inspiratory flow drops below a certain threshold. In other words, BIPAP ends the breath when the patient stops inhaling. In a more direct sense, BIPAP is CPAP with pressure support breaths. Flow termination is what separates a pressure support breath from the usual vent breaths we’ve been discussing. Flow termination is difficult to truly master because it can take quite a bit of fiddling with the termination threshold setting to get the vent to end the breath when the patient wants the breath to end.
The biggest advantage of BIPAP is also its biggest drawback: it allows the patient to have full control over their breathing. The patient not only controls the respiratory rate, but they also control the I-time. If the patient wants to take a small breath, they’ll take a short and shallow breath and then stop inhaling. If they want a larger breath, then they’ll take a longer and deeper breath. This mode affords the patient maximum comfort, but it also requires you to be comfortable with handing over control of respiration to the patient.
This highlights one of the central principles in selecting a ventilator mode: the more control you have, the less comfortable the patient will be. Choosing an appropriate vent mode involves weighing the patient’s need for comfort against your need for control. If you need the control then don’t hesitate to take it, using sedation if necessary. Just be aware that taking over control of a patient’s respiration is a clinical decision that must be made in consideration for the patient’s best interest. Sedation is not the answer for poor vent management.
In the next section, we’ll start tying everything together with clinical scenarios to highlight some of the practical aspects of vent management.