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# (p. 75) Ventilator Modes and Breath Types

Ventilator Modes and Breath Types
Chapter:
Ventilator Modes and Breath Types
DOI:
10.1093/med/9780190670085.003.0005
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date: 11 July 2020

Mechanical ventilators allow the clinician to choose among many different ways of providing or assisting patient ventilation. The most important settings are the mode of ventilation and the type of mechanical breath. Although all machines provide very similar, if not identical, options, terminology varies widely among manufacturers (and authors). So it’s very difficult to read a review article or a textbook (even this one) and understand what the different settings on your ventilator actually do. There have been surprisingly few attempts to standardize ventilator terminology, although Chatburn and colleagues have proposed a “taxonomy” for mechanical ventilation (see “Additional Reading” at the end of this chapter). To give you some idea of the magnitude of this problem, Table 5.1 matches the classification scheme used in this book with the terms used by several ventilator companies and authors.

Table 5.1 Comparison of Terms and Abbreviations Used for Ventilator Modes and Breath Types

My classification

Medtronic (Puritan-Bennett)

Maquet

Drager

Hamilton

Other terms

CMV-VC

A/C-VC

VC

VC-AC

CMV

Volume A/C

CMV-PC

A/C-PC

PC

PC-AC

P-CMV

Pressure A/C

CMV-aPC

A/C-VC+

PRVC

PC-AC-VG

APVcmv

VTPC, VAPC

SIMV-VC

SIMV-VC

SIMV (VC) + PS

VC-SIMV

SIMV

SIMV-PC

SIMV-PC

SIMV (PC) + PS

PC-SIMV

P-SIMV

SIMV-aPC

SIMV-VC+

SIMV (PRVC) + PS

PC-SIMV-VG

APVsimv

SPONT-PS

SPONT-PS

PS

SPN-CPAP/PS

SPONT

SPONT-aPS

SPONT-VS

VS

SPN-CPAP/VS

VS

Bi-Level

Bi-Level

Bi-Vent

• PC-BiPap

• PC-APRV

• DuoPAP

• APRV

CMV = Continuous Mandatory Ventilation; SIMV = Synchronized Intermittent Mandatory Ventilation; SPONT and SPN = Spontaneous Ventilation; VC = Volume Control; PC and P = Pressure Control; aPC = Adaptive Pressure Control; aPS = Adaptive Pressure Support; A/C and AC = Assist-Control; VS = Volume Support; PRVC = Pressure-Regulated Volume Control; VG = Volume Guarantee; CPAP = Continuous Positive Airway Pressure; APRV = Airway Pressure Release Ventilation; APV = Adaptive Pressure Ventilation; VTPC = Volume-Targeted Pressure Control; VAPC = Volume-Assured Pressure Control

If you really want to understand the ventilator(s) used in your hospital, my advice is to make a trip to the respiratory therapy department and borrow (and read) the manufacturer’s user manual. I’m not kidding! The relevant sections are fairly short, and it will allow you to understand how your particular machine works and the terminology it uses.

As mentioned in Chapter 4, the mode of ventilation is the most basic ventilator setting and can be thought of simply as defining how the ventilator and the patient interact. It is usually the first parameter chosen by the clinician and influences many subsequent settings. The type of mechanical breath determines how the ventilator provides the pressure, volume, and flow needed to inflate the lungs. As you can see from Table 5.1, the mode and breath type are usually combined to form a complex abbreviation, such as CMV-VC and SPONT-PS.

## Modes of Mechanical Ventilation

### Continuous Mandatory Ventilation (CMV)

The CMV mode is also commonly referred to as assist-control (AC) ventilation. The CMV mode provides the patient with a clinician-selected number of guaranteed mechanical breaths each minute. These mandatory breaths can be triggered in two ways. The patient can initiate a breath simply by making an adequate inspiratory effort (patient-triggered breaths), or, in the absence of patient effort, the ventilator will deliver a mandatory breath at regular intervals (ventilator-triggered breaths).

These two triggering methods are shown in Figure 5.1. Note that the ventilator uses the set (mandatory) rate to divide each minute into equal breath intervals. For example, if the rate is set at 10 per minute, each interval is 60 seconds ÷ 10, or 6 seconds. In the absence of patient effort, a ventilator-triggered breath is provided at the end of each interval (Figure 5.1A). During active inspiration, however (Figure 5.1B), a mandatory breath is triggered by the first patient effort of each interval, and a ventilator-triggered breath is given only if the patient is apneic for an entire cycle.

Figure 5.1 In the CMV and SIMV modes, if the patient is apneic (A), all mandatory breaths are ventilator-triggered and occur at the end of each calculated breath interval. If the patient breathes at or below the set rate (B), some mandatory breaths will be patient-triggered, while others will be ventilator-triggered. If the patient’s respiratory rate exceeds the set rate (C, D), all mandatory and spontaneous breaths will be triggered by the patient. In the CMV mode (C), spontaneous and mandatory breaths are identical. In the SIMV mode (D), they are always different.

Although the patient is guaranteed to receive a set number of mandatory breaths, the CMV mode allows any number of additional patient-triggered breaths, as shown in Figure 5.1C. Note that within each breath interval, these spontaneous breaths always follow a mandatory breath and that mandatory and spontaneous breaths are identical.

The CMV mode may be used with volume control (VC), pressure control (PC), and adaptive pressure control (aPC) breaths.

A summary of the CMV mode is provided in Box 5.1.

### Synchronized Intermittent Mandatory Ventilation (SIMV)

The SIMV mode is very similar to the CMV mode. In fact, if the patient is apneic or breathes at or below the set mandatory rate, the two modes are identical (Figure 5.1A and 5.1B). Like CMV, SIMV provides the patient with a clinician-selected mandatory breath rate, and a single mandatory breath, which can be either ventilator- or patient-triggered, is provided during each calculated breath interval. The difference between the two modes only appears when the patient triggers additional spontaneous breaths. In the CMV mode, spontaneous and mandatory breaths are the same; but in the SIMV mode, different breath types are always used (Figure 5.1D). Like CMV, mandatory breaths in the SIMV mode can be VC, PC, or aPC breaths. Spontaneous breaths, however, must be pressure support (PS) breaths. The SIMV mode is summarized in Box 5.2.

### Spontaneous Ventilation

The spontaneous mode provides no mandatory breaths. All mechanical breaths must be spontaneous and triggered by the patient (Figure 5.2). The spontaneous mode can be used only with PS or adaptive pressure support (aPS) breaths. Box 5.3 provides a summary of this mode.

Figure 5.2 In the spontaneous mode, there are no mandatory breaths, and all breaths must be triggered by the patient.

### Bi-level Ventilation

In this mode, patients cycle between a high and low clinician-set level of positive airway pressure (Figure 5.3). What distinguishes this mode from CMV with PC breaths is that patients may trigger spontaneous (pressure support) breaths at both pressure levels. Note that the pressure support level during high-pressure spontaneous breaths is limited to prevent dangerously high inspiratory pressures; as shown in Figure 5.3A, a common approach is to limit inspiratory pressure to the sum of the low pressure level and the set PS level. In the bi-level mode, ventilation occurs during spontaneous breaths and during the transitions between the two pressure levels. Airway pressure release ventilation (APRV) is a variant of this mode in which the low-pressure interval (the pressure release) is very brief, and the ratio of high to low pressure duration is greater than 1.0 (Figure 5.3B). Box 5.4 summarizes the bi-level mode.

Figure 5.3 (A) In the bi-level mode, airway pressure alternates between high and low clinician-set levels. The patient may trigger spontaneous breaths at both levels.

(B) Airway pressure release ventilation (APRV) is a variant of bi-level ventilation that has only brief periods of low pressure.

## Types of Mechanical Breaths

At the most basic level, the breaths available on ICU ventilators can be divided into two types. Volume-set breaths provide a clinician-selected tidal volume, whereas pressure-set breaths maintain a set, constant airway pressure throughout inspiration. As we will discuss in much greater detail, the pressure needed to deliver a set volume and the volume generated by a specified pressure depend on a number of factors that vary between patients and even in the same patient over time. So, volume-set breaths are pressure-variable, and pressure-set breaths are volume-variable. Each of the breath types discussed here can also be differentiated by its flow characteristics, by the signal that ends inspiration (cycling), and by the mode(s) with which it can be used.

### Volume Control (VC)

Volume control breaths provide the patient with a clinician-selected, guaranteed tidal volume. On some ventilators, the maximum (peak) flow rate and flow profile (constant or descending ramp) must also be specified. On these machines, inspiratory time (TI) is not set, but instead depends on the selected tidal volume (VT) and the mean flow rate $(V˙mean)$, which, in turn, depends on the selected peak flow and flow profile.

$Display mathematics$
(5.1)

For example, a VC breath with a set volume of 0.5 L and a constant flow of 1 L/sec will have a TI of 0.5 second. Other ventilators provide only constant flow that is determined by the clinician-set TI and VT.

$Display mathematics$
(5.2)

So, a VT of 0.5 L with a TI of 0.5 second will generate a constant flow of 1 L/sec.

Volume control breaths are time-cycled. That is, the demand valve closes, inspiratory flow stops, and the expiratory valve opens once the set or calculated (Equation 5.1) inspiratory time has elapsed. Volume control breaths can be used only with the CMV and SIMV modes.

Volume control breaths are volume-set and pressure-variable, so let’s look at what determines how much pressure is needed. I actually spent a long time discussing this in Chapter 1 (see Figures 1.9, 1.11, and 1.14), although there, I didn’t specify that I was talking about VC breaths. Recall that the pressure generated by the ventilator (PAW) during a passive (i.e., no patient effort) VC breath must always equal the sum of the pressures needed to overcome viscous forces (PV), the increase in elastic recoil produced by the delivered volume (PER), and total end-expiratory pressure (PEEPT).

$Display mathematics$
(5.3)

Since PER is equal to the delivered volume (∆V) divided by the compliance of the respiratory system (CRS), and PV equals the product of airway resistance (R) and flow ($V˙$), we can rewrite this equation as:

$Display mathematics$
(5.4)

Figure 5.4 shows plots of PAW and alveolar pressure (PALV) versus time during passive VC breaths with constant inspiratory flow. As discussed in Chapter 1, PALV is equal to the sum of PER and PEEPT, and PV is the difference between PAW and PALV. Since flow is constant, volume, elastic recoil, and PALV increase linearly throughout inspiration. PAW also increases linearly and reaches its peak pressure (PPEAK) at end-inspiration. As predicted by Equation 5.4, both PPEAK and end-inspiratory PALV increase (with no change in PV) when tidal volume increases or compliance falls. When resistance or flow rises, PV and PPEAK increase without changing PALV. PPEAK and end-inspiratory PALV also increase when PEEP is present. A decrease in resistance, flow, volume, or PEEPT, and an increase in compliance have the opposite effects. Note that inspiratory time varies with tidal volume and flow, as predicted by Equation 5.1.

Figure 5.4 Schematic representation of the effect of low compliance (B), high volume (C), high resistance (D), high flow (E), and PEEPT (F) on baseline (A) airway pressure (PAW) and alveolar pressure (PALV) during volume control breaths with constant inspiratory flow. The pressure needed to balance elastic recoil (PER) and overcome viscous forces (PV) at end-inspiration and inspiratory time (TI) are shown. When PEEPT is zero (Figure 5.4A5.4E), PALV equals PER. When PEEP is present (Figure 5.4F), PALV equals the sum of PER and PEEPT.

Now let’s look at Figure 5.5, which shows how and why the shape of the PAW curve changes with the inspiratory flow profile. This was also discussed in Chapter 1. If resistance is constant, PV varies only with the rate of gas flow. If flow is constant (Figures 5.4 and 5.5A), PV will also be constant. If a descending ramp pattern of flow is used (Figure 5.5B), PV progressively falls, PALV approaches PAW, and PPEAK drops. Also notice that by reducing mean flow, a descending ramp profile increases the time required to deliver the set tidal volume (TI).

Figure 5.5 On some ventilators, volume control breaths allow the clinician to choose a constant (A) or a descending-ramp (B) flow profile. Peak airway pressure (PPEAK), the pressure needed to overcome viscous forces (PV), and inspiratory time (TI) change with the selected flow profile. If tidal volume and compliance are constant, the pressure needed to balance elastic recoil (PER) does not change.

So far, we have considered the factors that determine PAW only during a passive VC breath. When a patient inhales, the pressure produced by the diaphragm and other inspiratory muscles helps overcome elastic recoil and viscous forces. Since inspiratory flow cannot increase, the pressure in the ventilator circuit falls. As shown in Figure 5.6, increasing inspiratory effort causes a progressive change in the shape of the PAW-time curve and may even lower PPEAK.

Figure 5.6 The effect of varying levels of patient inspiratory effort on airway pressure (PAW) during a volume control breath. As effort increases (1 → 2 → 3), there is progressive alteration of the PAW–time curve.

A summary of volume control breaths is provided in Box 5.5.

### Pressure Control (PC)

When delivering PC breaths, the ventilator maintains a constant PAW throughout inspiration. The clinician sets this pressure by selecting the driving pressure (DP) that is applied at the onset of inspiration. Inspiratory pressure is then the sum of the DP and the set level of PEEP. Inspiratory time must also be specified. Pressure control breaths are therefore pressure-set and time-cycled. They can be used only with the CMV and SIMV modes.

Figure 5.7 illustrates the differences between the PAW, PALV, flow, and volume curves of passive VC and PC breaths. Let’s see why these differences occur. Remember from Chapter 1 that airway resistance is equal to the gradient between PAW and PALV divided by flow.

Figure 5.7 Schematic diagram showing the differences in airway pressure (PAW), alveolar pressure (PALV), flow, and volume during passive volume control (A) and pressure control (B) breaths. Since tidal volume and compliance are the same, the pressure needed to balance elastic recoil (PER) doesn’t change. Because flow stops before the end of inspiration, no pressure is needed to overcome viscous forces (PV) during the pressure control breath, and this lowers peak airway pressure.

$Display mathematics$
(5.5)

By rearranging this equation, we can calculate the flow at any time $(V˙t)$.

$Display mathematics$
(5.6)

Because PAW is constant, Equation 5.6 tells us that flow is highest at the beginning of inspiration when PALV is at its lowest level. Flow must then progressively fall as the lungs expand, PALV rises, and the gradient between PAW and PALV decreases. Flow will stop when PAW and PALV become equal. Because of the shape of the flow-time curve and because flow is simply volume per time, lung volume increases much more rapidly than during a VC breath. End-inspiratory PALV is the same in Figures 5.7A and 5.7B because the delivered volume is the same. But end-inspiratory PAW (PPEAK) is lower during PC breaths because flow (and PV) is minimal or absent.

Now let’s see how PC breaths are affected by changes in driving pressure, resistance, and compliance (Figure 5.8). We’ll start by focusing on inspiratory flow. Look again at Equation 5.6. If driving pressure (and PAW) is increased, flow must also increase throughout inspiration (Figure 5.8B). When resistance is high, flow must fall (Figure 5.8C), and this means that it takes longer for PAW and PALV to equilibrate. In fact, flow will persist at end-inspiration unless the set TI is sufficient to allow PALV to reach PAW. When compliance is low (Figure 5.8D), PALV rises quickly to reach PAW, and the duration of inspiratory flow is reduced.

Figure 5.8 Schematic diagram showing how an increase in driving pressure (B) and resistance (C) and a fall in compliance (D) alter baseline (A) airway pressure (PAW), alveolar pressure (PALV), flow, and volume during pressure control breaths.

Now let’s look at tidal volume. Recall from Chapter 1 that compliance (C) is the ratio of the volume change (∆V) produced by a change in pressure (∆P).

$Display mathematics$
(5.7)

If we rearrange this equation, you can see that the tidal volume (VT) delivered by each PC breath is determined by respiratory system compliance and the difference between PALV at the end of expiration (PALVee) and the end of inspiration (PALVei).

$Display mathematics$
(5.8)

Since PALV and PAW are usually equal at end-inspiration, Equation 5.8 can also be written as:

$Display mathematics$
(5.9)

This means that VT will increase with DP (Figure 5.8B) and fall with respiratory system compliance (Figure 5.8D). As you can see from Equation 5.8 and Figure 5.8C, VT will also fall if high resistance prevents PAW and PALV from equilibrating before the end of inspiration (i.e., PALVei < PAW).

The opposite effects from those shown in Figure 5.8 will occur with a decrease in DP, a fall in airway resistance, and an increase in respiratory system compliance.

During VC breaths, increasing patient effort causes PAW to fall but does not change inspiratory flow or volume (Figure 5.6). During PC breaths, PAW remains constant, but flow and volume vary with patient effort. That’s because inspiratory effort lowers PALV and increases the PAW–PALV gradient. So, the greater the effort, the more flow and volume the patient will receive. Since PC breaths typically have a short set TI, though, the patient has relatively little time to influence flow and volume.

Now that you understand the basics of PC breaths, we have to discuss the effect of PEEP (Figure 5.9). Remember from Chapter 1 that PEEPT is PALV at end-expiration and the sum of intentionally added or extrinsic PEEP (PEEPE) and intrinsic PEEP (PEEPI). This allows us to rewrite Equation 5.9 as:

$Display mathematics$
(5.10)

Figure 5.9 Airway (PAW) and alveolar (PALV) pressure, flow, and volume curves during pressure control breaths with PEEP of 0 (A) extrinsic PEEP (PEEPE) of 5 cmH2O (B) and intrinsic PEEP (PEEPI) of 5 cmH2O (C). Driving pressure, flow, and volume are unchanged by PEEPE but fall when PEEPI is present.

Since the ventilator adjusts PAW for PEEPE (i.e., PAW = DP + PEEPE), the DP, flow, and tidal volume remain unchanged (Figure 5.9B). Unfortunately, the ventilator cannot detect or adjust for PEEPI. So, as PEEPI and PEEPT increase, the DP (PAW – PEEPT) and the delivered tidal volume (and flow) fall (Figure 5.9C). So watch out whenever you make a change that decreases expiratory time, such as increasing the mandatory rate or TI, because it may cause a significant drop in VT. Pressure-control breaths are summarized in Box 5.6.

This breath type, which is also referred to by a variety of other names, including pressure-regulated volume control (PRVC), volume control plus (VC+), volume-targeted pressure control (VTPC), and volume-assured pressure control (VAPC), is a hybrid of VC and PC breaths. In essence, aPC uses a pressure control breath to deliver a clinician-set tidal volume. Since mechanical breaths can be volume-set or pressure-set, but not both, you’re probably wondering how this is done. The answer is that, like PC breaths, aPC breaths maintain a constant PAW throughout inspiration, but the ventilator adjusts this pressure to deliver a target tidal volume.

When aPC is selected, the clinician sets both the tidal volume and the inspiratory time. Initially, the ventilator delivers a series of pressure control breaths until it determines the pressure needed to generate the set volume. Exhaled volume is then constantly monitored, and inspiratory airway pressure is adjusted to maintain this volume. If volume falls, PAW is increased; if volume increases, PAW is reduced. Like VC and PC breaths, aPC breaths can be used only with the CMV and SIMV modes.

You can see that aPC breaths overcome the major drawback of PC breaths—the marked change in delivered tidal volume (and minute ventilation) that can occur due to changes in compliance, resistance, patient effort, and PEEPI. Adaptive pressure control breaths compensate for these factors by adjusting PAW, thereby eliminating the volume variability inherent in PC breaths. Box 5.7 provides a summary of aPC breaths.

### Pressure Support (PS)

Like PC breaths, PS breaths provide a constant airway pressure that is the sum of a clinician-selected driving pressure (now called the pressure support level) and PEEPE. Unlike PC breaths, however, inspiratory time is not set. Instead, the ventilator cycles only when inspiratory flow falls below a low, preset value. So PS breaths are pressure-set and flow-cycled. Pressure support breaths are used in the SIMV, spontaneous ventilation, and bi-level modes.

Since PS breaths are pressure-set, inspiratory flow and tidal volume are influenced by the set pressure, respiratory system compliance and resistance, and PEEPI, just as they are with PC breaths; and, like PC breaths, inspiratory effort during PS breaths lowers PALV and increases the PAW–PALV gradient, which allows patients to influence inspiratory flow rate and tidal volume.

The big (actually huge) difference between these two breath types results from how they are cycled. Since inspiratory flow doesn’t stop until some minimum value has been reached, PS breaths give the patient total control over inspiratory time. So, for example, if the patient generates only the minimal effort needed to trigger the ventilator, TI will be short, and inspiratory flow and tidal volume will be determined only by the set PS level and respiratory system compliance and resistance. If, on the other hand, the patient puts a lot of effort into breathing, inspiration will not end until the patient stops inhaling, and inspiratory flow and volume will be much greater. The effect of patient effort on TI, flow, and volume is shown in Figure 5.10. Box 5.8 summarizes pressure support breaths.

Figure 5.10 Airway (PAW) and alveolar (PALV) pressure, flow, and volume curves during pressure support breaths with minimal (A) and large (B) patient inspiratory effort. Flow, volume, and inspiratory time increase with patient effort.

Adaptive pressure support breaths, which are most commonly referred to as volume support breaths, are similar to aPC breaths because they deliver a clinician-set tidal volume while maintaining a constant PAW. Also, like aPC breaths, exhaled tidal volume is monitored, and PAW is adjusted to maintain the set volume. The difference is that aPS breaths use a pressure support rather than a pressure control breath. Adaptive pressure support breaths eliminate the potentially marked volume variability of PS breaths by altering PAW to compensate for changes in compliance, resistance, PEEPI, and, most importantly, patient effort. Adaptive pressure support breaths can be used only with the spontaneous ventilation mode. Box 5.9 provides a summary.

## How and When to Use Ventilator Modes and Breath Types

How do you decide which mode and breath type to use for a particular patient? Well, I’m going to make this simple—because it is.

The course of patients who require mechanical ventilation can be divided into two phases. In the first, which I’ll call the critical illness phase, the patient requires lots of support from the ventilator. If the patient survives, they usually improve to the point that mechanical ventilation may no longer be needed. I’ll call this the recovery phase.

For patients in the critical illness phase, CMV is by far the most commonly used mode of mechanical ventilation, and there are several good reasons for this. First, CMV provides a mandatory respiratory rate, so it can be set to guarantee a minimum, safe minute ventilation (and PaCO2 and pH). Second, by allowing additional spontaneous breaths, CMV lets the patient set the amount of ventilation needed for optimum CO2 clearance. Finally, CMV significantly reduces patient effort and work of breathing, because all mechanical breaths, whether mandatory or spontaneous, provide the same tidal volume and can be triggered by a minimal amount of inspiratory effort. This is obviously important in patients who are unable to maintain adequate spontaneous ventilation.

The other modes are simply less appropriate. Spontaneous ventilation isn’t safe because it doesn’t provide mandatory breaths. SIMV is also problematic because adequate minute ventilation depends on the patient’s ability to generate sufficient volume with each spontaneous breath. This, of course, also increases patient work of breathing. Bi-level ventilation has the same drawbacks as SIMV because it also uses PS breaths. It is occasionally used to improve PaO2 by opening or “recruiting” atelectatic alveoli in patients with ARDS (see Chapter 12), but there’s no evidence that it has a role in the routine management of these patients.

Once you’ve selected the CMV mode, you have three breath types to choose from: VC, PC, and aPC. If you think about it, VC breaths are ideal for critically ill patients because they provide a set tidal volume. This is essential for patients whose inspiratory effort, compliance, and resistance are likely to change repeatedly and unpredictably. Adaptive pressure control breaths are also acceptable, because they, too, are volume-set. Some clinicians favor aPC breaths because they believe that allowing some control over inspiratory flow improves patient comfort. Since PALV rises faster and mean PALV is higher than during VC breaths (Figure 5.7), aPC breaths may also increase alveolar recruitment and improve PaO2, and are sometimes preferentially used in patients with ARDS. Because they are volume-variable, PC breaths must be used with caution in critically ill patients.

What about patients who have entered the recovery phase of their illness? As I will discuss in Chapter 15, “spontaneous breathing trials” are used to determine whether mechanical ventilation can be discontinued. These can be truly spontaneous with no ventilator assistance (i.e., a “T-piece”), or the patient can be switched from CMV to spontaneous ventilation with a small amount (e.g., 5 cmH2O) of pressure support. I prefer the latter because it’s less time-consuming for the respiratory therapist, and all ventilator alarms remain active. Either way, the patient is considered for extubation if they demonstrate adequate spontaneous ventilation for 30 to 60 minutes. Patients who do poorly during a spontaneous breathing trial are returned to the CMV mode.

So, in the spirit of keeping it simple, here are my recommendations:

• Use CMV-VC or CMV-aPC in critically ill patients with respiratory failure.

• Assess the patient’s ability to breathe spontaneously by changing from CMV to spontaneous ventilation and adding a low level of pressure support.