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A ventilator is a machine that delivers a flow of gas for a certain amount of time by increasing proximal airway pressure, a process which culminates in a delivered tidal volume. Because of the imprecise, inconsistent, and outdated terminology used to describe modern ventilators, many clinicians often misunderstand exactly how a ventilator functions. Understanding the exact instructions that a ventilator follows to deliver a breath for the various modes of ventilation is crucial for optimal ventilator management. Anatomy of a Breath Breathing is a periodic event, composed of repeated cycles of inspiration and expiration. Each breath, defined as one cycle of inspiration followed by expiration, can be broken down into four components, known as phase variables. These phase variables determine when inspiration begins (trigger), how flow is delivered during inspiration (target), when inspiration ends (cycle), and proximal airway pressure during expiration (baseline) (Fig. 2.1).

Figure 2.1 Schematic of a breath cycle. The trigger variable determines when expiration ends and inspiration begins. The cycle variable determines when inspiration ends and expiration begins. The target variable determines flow during inspiration. The baseline variable determines proximal airway pressure during expiration. Key Concept #1 Ventilator phase variables: • Trigger: when inspiration begins • Target: how flow is delivered during inspiration • Cycle: when inspiration ends • Baseline: proximal airway pressure during expiration

Trigger The trigger variable determines when to initiate inspiration. Breaths can either be ventilator-triggered or patient-triggered. Ventilator-triggered breaths use time as the trigger variable. Patient-triggered breaths are initiated by patient respiratory efforts, utilizing pressure or flow for the trigger variable. Time Trigger With time triggering, the ventilator initiates a breath after a set amount of time has elapsed since the initiation of the previous breath. The most common manner to set the time trigger is by setting the respiratory rate (time = 1/rate). For example,

setting the ventilator respiratory rate to 12 breaths per minute is equivalent to setting the time trigger to 5 seconds because one breath every 5 seconds will result in 12 breaths per minute. When a breath is initiated by a time trigger, that breath is classified as a ventilator-triggered, or control, breath. Key Concept #2 • Control breath = ventilator-triggered breath • Trigger variable for control breath = time Patient Trigger Changes in pressure and flow in the circuit as a result of patient respiratory efforts are detected by the ventilator. When the patient makes an inspiratory effort, as discussed in Chap. 1, the diaphragm and inspiratory muscles contract, lowering pleural pressure, which ultimately reduces proximal airway pressure. This reduced airway pressure is transmitted along the ventilator tubing and measured by the ventilator. If a pressure trigger is set and the magnitude of the reduction in proximal airway pressure as measured by the ventilator is greater than the set pressure trigger, a breath will be initiated and delivered by the ventilator (Fig. 2.2). For flow-triggering, a continuous amount of gas flows from the inspiratory limb of the ventilator to the expiratory limb of the ventilator during the expiratory (baseline) phase. This flow is continuously measured by the ventilator. In the absence of any patient inspiratory efforts, the flow of gas leaving the ventilator through the inspiratory limb should equal the flow of gas returning to the ventilator through the expiratory limb. During a patient inspiratory effort, some of this flow will enter the patient instead of returning to the ventilator, and the ventilator will detect decreased flow into the expiratory limb. If this reduction in flow returning to the ventilator exceeds the set flow trigger, a breath will be initiated and delivered by the ventilator (Fig. 2.3).

Figure 2.2 Respiratory circuit demonstrating the pressure trigger mechanism. (a) Assuming that no external positive end-expiratory pressure is added, pressure in the respiratory circuit at baseline is 0 cm H2O. (b) A patient’s inspiratory effort will cause a decrease in the patient’s proximal airway pressure, leading to a decrease in airway pressure of the respiratory circuit, which can be detected by the ventilator. In this example, pressure in the respiratory circuit has decreased by 3 cm H2O. If the pressure trigger threshold is set at 3 cm H2O or less, this inspiratory effort would trigger the ventilator to deliver a breath. Pair proximal airway pressure

Figure 2.3 Respiratory circuit demonstrating the flow trigger mechanism. (a) A continuous amount of gas flows from the inspiratory limb to the expiratory limb of the ventilator. In this example, the continuous gas flow is 10 L/min. (b) A patient’s inspiratory effort will cause some of the flow to enter the patient instead of returning to the ventilator. In this example, 3 L/min of flow is entering the patient, resulting in 3 L/min less flow returning to the ventilator. If the flow trigger threshold is set at 3 L/min or less, this inspiratory effort would trigger the ventilator to deliver a breath.

When a breath is initiated by a pressure or flow trigger, that breath is classified as a patient-triggered, or assist, breath. The difference between pressure and flow triggers in modern ventilators is generally clinically insignificant. A patient can trigger the ventilator only during the expiratory (baseline) phase. Patient respiratory efforts during inspiration after a breath has been initiated will not trigger another breath. Key Concept #3 • Assist breath = patient-triggered breath • Trigger variable for assist breath = pressure or flow Assist-Control A patient trigger (assist) and a ventilator trigger (control) can be combined to create a hybrid trigger mode known as assist- control (A/C). With this hybrid trigger, both a control respiratory rate (time trigger) and either a pressure or flow trigger are set. If an amount of time as set by the time trigger has elapsed without a patient-triggered breath, the ventilator will initiate a “control” breath. However, if the patient triggers the ventilator, via the pressure or flow trigger, prior to elapsing of the time trigger, the ventilator will initiate an “assist” breath and the time trigger clock will reset. It is important to note that there are no differences in the other characteristics of a breath (i.e., target, cycle, and baseline) between a time- triggered “control” breath and a patient-triggered “assist” breath. “Assist” and “control” only describe whether the breath was triggered by the patient or by the ventilator, respectively. Key Concept #4 • A/C combines two triggers: patient trigger (assist) and ventilator trigger (control) • A/C refers only to the trigger, not to other phase variables

Many ventilators indicate whether the delivered breath was a “control” or “assist” breath, often with a flashing “A” or “C” on the display. Additionally, one can determine whether a delivered breath was a “control” or “assist” breath by examining the pressure curve on the ventilator screen. Patient- triggered “assist” breaths will have a negative deflection on the pressure curve right before inspiration, whereas time- triggered “control” breaths will not. A downward deflection of the pressure tracing for patient-triggered breaths is reflective of the patient inspiratory effort, resulting in a reduction in proximal airway pressure (Fig. 2.4). The actual respiratory rate of the ventilator will depend on the relationship between the time-triggered control rate and the rate of inspiratory effort by the patient. Assuming the intrinsic breathing pattern of the patient is regular, if the time trigger is set such that the control rate is 10 breaths per minute (one breath every 6 seconds), and the rate of patient inspiratory efforts is 20 breaths per minute (one breath every 3 seconds), then all of the breaths will be “assist” breaths because the patient will trigger the ventilator prior to the

Figure 2.4 Pressure tracing demonstrating a ventilator-triggered “control” breath and a patient-triggered “assist” breath. Proximal airway pressure is plotted on the vertical (y) axis, and time is plotted on the horizontal (x) axis. Note the downward deflection in the pressure tracing prior to the assist breath, indicating that a patient inspiratory effort triggered the ventilator.

time trigger elapsing. Therefore, the actual respiratory rate will be 20 breaths per minute. In this case, increasing the control respiratory rate on the ventilator from 10 to 15 breaths per minute (reducing the time trigger from 6 to 4 seconds) will have no effect on the respiratory rate if the patient continues to trigger the ventilator every 3 seconds. However, increasing the set respiratory rate to above 20 breaths per minute (decreasing the time trigger to below 3 seconds) will result in all of the breaths being time-triggered control breaths. The set time-triggered respiratory rate is essentially a “backup” rate—if the patient does not trigger the ventilator at a frequency higher than the backup rate, the ventilator will deliver time-triggered control breaths at the set backup respiratory rate. Most ventilators display the actual respiratory rate. If the actual respiratory rate is higher than the time-triggered “control” respiratory rate, there must be patient-triggered “assist” breaths present. For patients with irregular breathing patterns where the time between patient inspiratory efforts varies, there can be a combination of patient-triggered “assist” breaths and time-triggered “control” breaths. Target The target variable is probably the most misunderstood of the phase variables. Part of this confusion arises from the fact that other names are commonly used for this variable, including “control” and “limit.” The target variable regulates how flow is administered during inspiration. The variables most commonly used for the target include flow and pressure. Volume, specifically tidal volume, is technically not a target variable because it does not clarify how the flow is to be delivered—setting a tidal volume does not determine whether that volume is to be delivered over a short period of time (high flow rate) or a long period of time (low flow rate). Note that volume delivered per unit time, which is the definition of flow, is a target variable.

Key Concept #5 • Target variable can be flow or pressure • Volume is not a target variable (but can be a cycle variable)

The equation from Chap. 1 relating flow, pressure, and resistance of the respiratory system helps elucidate the role of the target variable: Q Q = flow = airalv PP R Pair = proximal airway pressure Palv = alveolar pressure R = airway resistance The target variable is the independent variable in this equation, its value set by the provider and dutifully achieved by the ventilator. The target can be either flow or proximal airway pressure, but not both at the same time. When either flow or proximal airway pressure is set by the ventilator as the target variable, the other variable becomes a dependent variable, its value determined by the target variable, resistance, and alveolar pressure. Flow Target With a flow target, flow is selected as the independent variable. The ventilator simply delivers the flow as set by the provider. Therefore, proximal airway pressure becomes dependent on flow (target variable), resistance, and alveolar pressure. The flow waveform pattern, which describes the pattern of gas flow, is also selected. The most commonly used flow waveforms are constant flow and decelerating ramp.

With the constant flow waveform pattern, also known as the square or rectangle waveform pattern, the inspiratory flow rate instantly rises to the set level and remains constant during the inspiratory cycle. With the decelerating ramp waveform pattern, the inspiratory flow rate is highest at the beginning of inspiration, when patient flow demand is often greatest, and then depreciates to zero flow (Fig. 2.5). Pressure Target With a pressure target, the proximal airway pressure is selected as the independent variable. The ventilator delivers flow to quickly achieve and maintain proximal airway pressure during inspiration. Therefore, flow becomes dependent on proximal airway pressure (target variable), resistance, and alveolar pressure (Fig. 2.6). Pressure-targeted modes naturally produce a decelerating ramp flow waveform. The prior equation can be used to elucidate why: Q = airalv PP R During the inspiratory phase, as air fills the alveoli, alveolar pressure increases. Since proximal airway pressure remains constant during the inspiratory phase of a pressure-targeted breath, and assuming resistance does not significantly change during the breath, flow must decrease as alveolar pressure increases. Therefore, flow will be highest at the beginning of the breath and decrease as the inspiratory phase proceeds. Key Concept #6 • Pressure-targeted modes produce decelerating ramp flow waveforms