A Taxonomy for Patient-Ventilator Interactions and a Method to Read Ventilator Waveforms

The Anatomy of Ventilator Waveforms

To understand the physiology (determine the load), it is useful to know the determinants of the pressure, volume, and flow waveforms for VC and PC during passive mandatory breaths, as in Figure 2. By passive, we mean that P_mus = 0; and by mandatory, we mean that inspiration is either started or stopped (triggered or cycled) by the ventilator. The waveforms that contain the information on the elastic and resistive loads have the dimensions of pressure. Actually, these are graphical representations of the equation of motion. At any moment in time (horizontal axis of the graphs), the height of the pressure waveform is simply the height of the volume waveform plus the height of the flow waveform (appropriately scaled by E for volume and R for flow) because P_vent = P_E + P_R. Every ventilator that displays pressure, volume, and flow waveforms is plotting the equation of motion (Fig. 2). This knowledge is useful to understand how the resistive and elastic loads manifest in the waveform according to the mode. Here we describe the most common patterns.

Volume Control - Square Waveform

Figure 2 depicts the pressure, flow, and volume waveforms for a VC passive inspiration with a square flow waveform (ie, constant flow). If we consider the resistance and elastance to be constant through the inspiration because the flow and volume are controlled, the resultant pressure waveform will be a manifestation of the patient’s elastance and resistance.

Effects of Resistance.

In VC square waveform (VCsq), the ventilator delivers a constant inspiratory flow. It rises immediately and stops at the end of the inspiratory time. This constant flow from the ventilator first encounters the resistance of the natural and artificial airways. Because inspiration starts with an immediate rise in flow from zero to the set value, and essentially no volume delivered, the airway pressure rises immediately as a manifestation of the resistive load (R × V̇). Once volume starts to be delivered to the alveoli, the airway pressure becomes a manifestation of both elastic and resistive loads. At the end of the inspiration, in the absence of an inspiratory hold, the airway pressure is the result of both the resistive and elastic loads (ie, the end-inspiratory pressure). Figure 3 demonstrates that the resistive load is constant throughout the inspiration (because flow and resistance are constant). If the flow stops (manual or preset inspiratory hold), the resistive load is zero (R × 0 = 0) and only the elastic load remains; this is the classic way taught to evaluate the loads. You now can identify this with or without an inspiratory hold: The higher the resistive load, the higher the initial step up in pressure; and if there is an end-inspiratory pause, the greater the fall from the peak to the plateau pressure.

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Fig. 3.

Effect of changes in P_mus, resistive and elastic loads in volume control. The flow and volume waveforms do not change as these are controlled by the ventilator. Changes in resistive load affect the initial step in P_vent without a change in slope. Changes in elastic load change the slope of pressure rise. Patient effort (P_mus) adds or subtracts to the airway pressure; in this case, patient inspiratory effort subtracts to the airway pressure.

Because the determinants of resistive load are flow and airway resistance, as either flow or resistance increases the resistive load will increase. Consequently, the inspiratory resistive load can also be high by setting high inspiratory flows (often done to decrease inspiratory time). To query if the issue is resistance or high flow, it is useful to evaluate the expiratory flow waveform (see section below) as generally high resistance will manifest both during inspiration and expiration.

Volume Control Descending Ramp Waveform

In VC descending ramp waveform (VCdsc), flow starts at the preset peak value and then decreases in a linear fashion through the inspiratory time. The slope of the ramp depends on the inspiratory time, the set peak flow, and set end-expiratory flow. It is important to remember that not all descending ramp waveforms are the same. On some ventilators, end-expiratory flow is set to a default value of zero, whereas on others it can be set to a percentage of the peak flow (it is unclear why a manufacturer would give this as an option). In some ventilators, the time to reach peak flow may also be modified (called adjustable rise time, with a similarly unclear rationale for this option). For this discussion, we will use examples where inspiration begins at peak flow and ends at zero flow.

Pressure Control Waveform

In PC with a square waveform, the ventilator increases the pressure from PEEP to the set inspiratory pressure. This step increase in pressure is immediate (unless the pressure rise time setting is increased) and lasts for the set inspiratory time. At the end of inspiration, the pressure decreases immediately to the set PEEP. Figure 2 depicts the pressure, flow, and volume waveforms for a PC passive breath.

In PC, because pressure is the control variable, we look at the flow and volume waveforms to see the patient’s respiratory system resistance, compliance, and, if present, P_mus effects. In passive conditions, after a step change in pressure (up or down), the flow and volume rise or fall exponentially. The determinants of the rate of exponential change are the respiratory system resistance and compliance. The time constant (τ) is the parameter that explains this (it is derived from the solution of the equation of motion for volume and flow after a step change in P_vent). Understanding the time constant helps us deduce which load (resistive or elastic) is dominating. The formula is simple, (2)where τ is the time constant (in seconds), R is resistance (in cm H₂O/L/s), and C is compliance (in L/cm H₂O). One time constant is the period of time where there is a 63% change in flow or volume (and hence alveolar pressure) in response to a step change in pressure at the airway opening. Suppose the pressure changes from 0 to 10 cm H₂O on inspiration (or 10 to 0 cm H₂O on expiration). At the end of a period of time equal to one time constant (1 × τ), the volume and flow will have changed by 63%. Hence, alveolar pressure will have changed by 6.3 cm H₂O, leaving 3.7 cm H₂O until equilibration). At the end of another time constant (2τ), another 63% (or 0.63 x 3.7 = 2.3 cm H₂O) change will occur and so on (Fig. 4). This change continues, in theory, until infinity. For practical purposes, after a period equal to 5τ, inspiration or expiration is considered complete (< 1% of volume left). Setting inspiratory or expiratory times to at least 3τ is usually acceptable (Table 1).¹⁷

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Fig. 4.

Effect of changes in P_mus, resistive and elastic loads in pressure control. The pressure waveform may demonstrate changes; however, it is being controlled by the ventilator. The time constant (RxC) describes a change of 63% per time constant in flow, volume, and alveolar pressure. The decay of the waveforms is commonly termed exponential decay and is a manifestation of a passive patient. The presence of P_mus will deform the waveforms and will not allow to determine the respiratory loads.

Key Concepts: The flow waveform contains mostly the same information as the volume waveform (including tidal volume, which is the area under the flow-time curve). The time constant is important when examining passive flow waveforms during expiration for VC and for inspiration and expiration with PC. The time constant is the result of a multiplication, so if the compliance or resistance is high, then the τ will be long; and if the compliance or resistance is low, then the time constant will be short. In practice, a short time constant is generally due to low compliance (when has low resistance been an issue?). A long time constant means either the resistance or compliance is high, and most of the time, clinically, the issue is resistance. In general terms, the time constant in a passive patient, intubated, using a heated humidifier with a normal respiratory system is 0.5 s, and thus the time to reach zero flow is approximately 2.5 s.¹⁷ The time constant varies according to the clinical condition (See Table 1). In general, the goal is to recognize extremes: very long or very short τ (Fig. 4).

The reader is advised that there are a few instances when PC is not delivered with a square waveform. In PC using servo targeting (eg, neurally adjusted ventilatory assist [NAVA] and proportional assist ventilation [PAV]) modes, inspiratory pressure is controlled to be proportional to inspiratory effort based on the signal related to P_mus (ie, flow or diaphragm electrical activity). Another reason is that most ventilators allow the user to set the pressure rise time. If rise time is greater than zero, then the pressure rises in a curvilinear fashion to the target, and thus inspiratory volume and flow are not simple exponential equations determined by a single time constant. Nevertheless, the passive expiratory volume and flow waveforms are unaffected.

Expiration

The expiratory phase is normally passive due to the recoil of the lungs and chest wall. The expiratory phase in mechanical ventilation (ie, the period from the start of negative flow to the start of positive flow) is always pressure controlled. That is, during expiration the ventilator controls the pressure (ie, you set the PEEP). Therefore, we look at the flow (and volume) waveforms to see the physiology and patient-ventilator interactions. The driving pressure for passive expiratory flow = ΔP = P_plat, PEEP, for both PC and VC. (Note that ΔP ≠ P_plat, total PEEP because auto-PEEP only exists at end expiration, although auto-PEEP will affect P_plat.) Hence, the same concepts of the time constant apply: a long time constant means high resistance, and a short one means low compliance (Fig. 4).

The respiratory system has 2 time constants, inspiratory and expiratory, as compliance, and especially resistance, is usually not the same during inspiration and expiration. However, in the majority of cases, both move in the same direction. That is, if you see features of decreased compliance or increased resistance in the inspiratory waveform, you are likely to see them in the expiratory waveform too. We use the expiratory flow waveform to confirm the inspiratory waveform findings or when there is P_mus in inspiration. Rarely will you see them to be discordant, and if that is the case, look for something causing variable obstruction, circuit issues, or ventilator settings. For example, a normal resistive load during inspiration and high resistive load during expiration may point to an issue with the expiratory valve or expiratory filter.

The output from step 2 is to define what the dominant load (resistive or elastic) is or there is P_mus. This allows to move to the next step understanding the ventilator mode and the patient physiology.

Trigger

The trigger event (start of mechanical inflation) is assessed in terms of synchrony (timing) with the start of the patient’s inspiratory effort. It can occur early (before the patient signal), on time (synchrony), or late (a clinically important delay). There are 2 other conditions that are not related to timing but rather to ventilator function: false trigger, where a non-P_mus signal triggers an inspiration; and a failed trigger, where P_mus fails to trigger inspiration.

Normal (Synchrony)

The ventilator responds to a patient trigger effort with a delay that is clinically unimportant. Most ventilators today have a very short trigger delay (ie, period from the start of trigger effort to the start of flow delivery, sometimes defined as the delay from pressure drop below PEEP to return to PEEP).¹⁸ This is the most common trigger interaction seen (Figure 5). Normal trigger is defined by absence of any other abnormalities described below, as it is impractical to measure trigger delay at the bedside.

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Fig. 5.

Classification of trigger patient-ventilator interactions. Mode: PC-CMVs. P_vent: airway pressure waveform displayed by ventilator. P_mus: patient-generated pressure waveform, simulated, overlay to demonstrate effect across waveforms. Vertical white dotted lines are for reference of the start of the P_mus. Normal trigger: minimal drop in pressure with immediate pressurization. Late trigger: note flow crossing baseline and a prolonged drop in pressure below baseline. Early trigger: machine-triggered breath followed by evidence of patient effort (rise in flow above baseline). False trigger: patient-triggered breaths; however, no evidence of P_mus, in this case triggered by circuit leak. Failed trigger: P_mus does not trigger a mechanical breath. P_mus is manifested as flow waveform moving toward baseline and a concomitant drop in airway pressure. PC = pressure control, CMV = continuous mandatory ventilation, s = set point targeting.

Late Trigger

The ventilator responds to a patient trigger effort with a delay that may have important clinically implications. In current modern ventilators, when trigger sensitivity is set appropriately, the trigger lag is < 100 ms.¹⁸ This mainly occurs with inappropriate sensitivity settings (making it too hard to trigger). In general, flow triggering results in less effort (trigger work) for the patient than pressure triggering.¹⁸ Late trigger is recognized when there is evidence of P_mus (either a drop in baseline pressure or a rise in flow above baseline) well before the start of inspiratory flow from the ventilator. Most current ventilators will indicate patient triggering by pressure or flow by a color change in the trigger phase of the corresponding waveform.

Another trigger observation, which is not necessarily related to the trigger being late, but manifests similarly, is when the pressurization rate of the ventilator after trigger is not fast enough. This situation occurs when the ventilator pressurization rate can’t match the patient effort or, more commonly, an operator sets inappropriately long inspiratory rise times (Figure 5). Both these situations, late trigger and delayed pressurization, will cause a prolonged trigger phase and increased work of breathing early in inspiration.^18,19 For simplicity, we lump them both under the same classification.

Early Trigger

A machine-triggered inspiration precedes the patient trigger effort. The key finding is the start of inspiratory flow followed by evidence of P_mus, which may or may not trigger another breath (Figure 5). The patient effort may occur any time during inspiration or early during expiration. This was initially described²⁰ as reverse trigger, meaning that inspiratory flow from the ventilator somehow stimulated an inspiratory effort from the patient, the reverse of the normal situation. It seems there are different causes and variants, and a fair amount of work on its physiology has been described, but the phenomenon is still being clarified.^21-23 We use the term early trigger, as it describes the event in terms of the signals, not the physiology, and thus it avoids changes in the taxonomy in the future. It is likely that many causes for early trigger will be found.

False Trigger

Inspiration is triggered by a non-P_mus signal (Figure 5). These (pressure or flow) signals are generally caused by secretions or fluid in the circuit/endotracheal tube/airway, cardiac oscillations, etc. False trigger can also be caused by a leak in the patient circuit. This can be challenging to recognize. Some clues are the absence of evidence of P_mus during inspiration, presence of apparently patient-triggered breaths in heavily sedated or paralyzed patients, or high-frequency oscillations in the flow waveform. Continuous capnography can provide a clue, as it will demonstrate oscillations in the expiratory waveform tracing (Figure 6, panel B). There are 2 techniques to help diagnose this. First is to assess P_mus in the patient. Observe the neck muscles for evidence of effort, look at the abdomen, and place your hand on the patient. The second is to do an end-expiratory pause and observe for patient effort and for negative deflections in the airway pressure, a manifestation of P_mus.

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Fig. 6.

Capnography as an aid in recognizing asynchrony. In panel A, the capnography demonstrates a deformation during exhalation consistent with a failed trigger. The waveform moves toward baseline, demonstrating an inspiratory effort. The vertical dashed line demonstrates the timing matching the deformations in pressure, flow, and CO₂ waveforms. In panel B, images from a patient who had false trigger due to cardiac oscillations transmitting to the airway. Notice capnogram demonstrates oscillations with progressive decrease of the CO₂ level; this corresponds to oscillations in the flow waveform (gain increased to demonstrate the oscillations). The vertical dashed lines demonstrate the correlation electrocardiogram (ECG), capnogram and flow waveforms.

Failed Trigger

The patient effort fails to trigger inspiration. This is recognized in the expiratory flow waveform as patient-generated deflections toward the baseline that do not reach zero, a requirement for either flow or pressure triggering. It can also be recognized in waveform capnography as a downward deflection in the phase 3 expiratory plateau, often referred to as a “curare cleft” (Figure 6A). For flow triggering, the inspiratory effort must generate a positive flow higher than the sensitivity threshold (eg, 2 L/min). For pressure triggering, there must be enough positive flow to withdraw enough volume from the patient circuit to drop airway pressure below the trigger threshold (eg, 3 cm H₂O). Failed trigger efforts may occur any time during the expiratory phase. Although there are many causes, the most concerning is auto-PEEP or air trapping, so we assess for other features of a high resistive load. (Note that ΔP_mus during the trigger effort must exceed auto-PEEP for flow to cross zero, and this is why muscular weakness or high auto-PEEP leads to failed trigger efforts.) Other causes are over assistance (such as too large of volume delivered leading to auto-PEEP), excess sedation, and neuromuscular weakness. Naturally, a trigger threshold set too high (ie, sensitivity too low) can cause a failed trigger effort even though the effort is normal. In the waveform, high trigger threshold manifests differently from auto-PEEP, as the flow will cross zero but it will not reach the trigger threshold (Figure 5).

Inspiration

During the inspiratory phase (ie, the period from the start of positive flow to the start of negative flow), the patient-ventilation interaction is characterized by the relationship of work performed by the ventilator and the patient (Figure 7).

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Fig. 7.

Relation between patient effort and ventilator-delivered pressure according to mode of mechanical ventilation. P_vent = pressure delivered by the ventilator. P_mus = pressure generated by patient respiratory effort, PAV = proportional assist ventilation, NAVA = neurally adjusted ventilatory assist, PRVC = pressure-regulated volume control. Representative sample of modes and mode names.

Work Shifting

In passive ventilation (P_mus = 0), the ventilator does all the work. In the simplest case, PC, where P_vent is held constant, work is simply the product of P_vent and V_T. On the other hand, when ventilatory assistance is zero (eg, CPAP), then all the work is done by the patient (ie, P_mus generates the V_T). When P_vent and P_mus are active together, some portion of the total work is done by the ventilator and some by the patient. We call this situation work shifting because some portion of the total work has shifted from the ventilator (passive case) to the patient (active case). A work shifting index could be used to quantify and characterize the relation.³ Work shifting can occur in any phase of inspiration, as it will depend when the P_mus is active, the ventilator settings (mode, inspiratory time, trigger sensitivity, cycle threshold), and patient-ventilator interaction.

The pattern of work shifting is affected by the mode and targeting scheme (Figure 7).²⁴ In modes using VC or that use adaptive targeting schemes, the relationship is inverse (ie, as the patient does more work, the ventilator does less work, and total work remains constant). In PC modes that use set-point targeting, the relationship is such that the work output of the ventilator per liter of tidal volume stays the same as the patient work increases, although the total work increases due to the larger tidal volume. For modes that use a servo-targeting scheme, work output of the ventilator increases as the work output of the patient increases. Be aware that when work shifting becomes extreme (ie, high ventilatory drive due to hypoxemia or metabolic acidosis), this can result in either diaphragm or lung injury (ie, tidal volume overdose), and no mode or mode setting will ameliorate it. Sedation and paralysis may be required.

Volume Control With Set-Point Targeting.

During VC with set-point targeting, the operator sets the tidal volume, the peak flow, and sometimes the flow waveform. If the patient is generating inspiratory effort, the magnitude of the flow delivered by the ventilator in relation to the patient’s flow demand will affect the pressure waveform (Figure 8). The inspiratory pressure waveform will move toward the baseline as P_mus increases. Remember, in the equation of motion, P_mus and P_vent are on the same side of the equation; as the patient increases P_mus, the P_vent will have to decrease to maintain the equality with the other side of the equation, representing the pressure to deliver the tidal volume (elastic load, P_E) and the pressure to deliver the flow (resistive load, P_R) as shown in Figure 8.

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Fig. 8.

Work shifting in volume control (VC) square flow waveform. Mode: VC-CMVs. P_mus: graphic representation of patient-generated pressure. Passive breath: no P_mus. Work shifting: The pressure waveform is deformed toward baseline due to the presence of P_mus. It does not cross the baseline. Interrupted line demonstrates where the pressure waveform would be if P_mus was passive. The ventilator is still doing some amount of work on patient. It may be clinically appropriate. Severe work shifting: The pressure waveform is deformed due to the presence of P_mus; the pressure crosses the baseline (PEEP). Under this circumstance, the patient is doing work against the ventilator. This is never clinically appropriate; it needs immediate clinician attention. Note: red P_mus line was overlaid by hand onto a ventilator screen image, and P_mus is shown inverted for clarity. CMV = continuous mandatory ventilation.

The total work of inspiration stays constant because volume and flow are unaffected by P_mus (ie, the patient can maximize effort but won’t get more volume), and the total pressure (P_vent + P_mus) stays constant; hence, work, a function of pressure and volume, is unchanged. P_vent (and ventilator work) decreases in exact proportion to the increase in P_mus (and patient work). Work shifting occurs whenever the P_vent decreases in presence of P_mus. However, as long as the pressure remains above baseline (ie, PEEP), the ventilator is still performing work on the patient, and the flow delivered is still above the flow requested by the patient. In extreme cases, when the patient generates high levels of P_mus, the P_vent decreases below baseline (severe work shifting). In this case, the patient is actually doing work on the ventilator system. This extreme, commonly called flow starvation, is most often seen during VC ventilation with set-point targeting. How much work shifting is appropriate depends on the patient evaluation and clinician judgment and an area we need further research. Severe work shifting (also known as flow starvation) is never appropriate.

Pressure Control With Set-Point Targeting.

During PC with set-point targeting, the operator sets the inspiratory pressure and either the inspiratory time (CMV and intermittent mandatory ventilation [IMV]) or the flow cycle threshold (continuous spontaneous ventilation [CSV]). On some ventilators, the operator can also set the rise time, which determines the time required to reach the target pressure and affects both peak flow and tidal volume. Ideally, the ventilator should be able to achieve the target inspiratory pressure regardless of any P_mus. In practice, ventilators vary in how well they can achieve this. The presence of P_mus is identified by deformations in the flow waveform (Figure 4 and 9). Inspiratory P_mus will increase volume and flow. Because the total driving pressure (P_vent + P_mus), volume, and flow all increase, the total work increases. Furthermore, the proportion of the total work the patient does increases because P_mus increases relative to P_vent. In the presence of effort, the clinician can adjust the inspiratory pressure target up or down according to the level of ventilatory support desired. If set too low, the patient is assuming the majority of the work; and if the P_mus is large enough, airway pressure may even drop below baseline. Monitoring the patient’s ventilatory effort is helpful in this case to adjust the inspiratory support.

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Fig. 9.

Work shifting in pressure control. Mode: PC-CMVs. P_mus: graphic representation of patient generated pressure. Work shifting: The pressure waveform is deformed toward baseline due to the presence of P_mus. It does not cross the baseline. Interrupted line demonstrates where the pressure waveform would be if P_mus was passive. Ventilators are unable to perfectly control the P_vent, thus the deformations. Green interrupted line demonstrates the passive flow waveform. The presence of P_mus will modify the flow waveform. In inspiration, flow will move away from baseline. Works shifting may be clinically appropriate. Notice the increase in volume as a manifestation of the added P_vent to P_mus. Note: red P_mus line was overlaid by hand onto a ventilator screen image, and P_mus is shown inverted for clarity. PC = pressure control. CMV = continuous mandatory ventilation.

Pressure Control With Adaptive Targeting.

In PC with adaptive targeting (eg, PC-CMVa, PC-CSVa, PC-IMVa,a), the operator sets a target tidal volume, and the ventilator software automatically adjusts the inspiratory pressure between breaths to achieve the target. As P_mus increases, the P_vent will decrease (Figure 10) in an attempt to maintain the V_T at target.¹⁵ The relationship of work is similar to VC with set-point targeting; the difference is that in PC with adaptive targeting the total work is not constant because the tidal volume can be larger than the set target value on any given breath. The P_vent can only decrease so far (ie, PEEP or a little above depending on the design of the ventilator).¹⁵ In patients with high enough P_mus, the patient may be breathing at PEEP level with little assistance from the ventilator and larger V_T than target. Some degree of work shifting may be clinically acceptable. In severe cases, this is manifested by very low P_vent (at baseline) and consistently larger tidal volumes, all generated by the patient. In some cases, the P_vent will be below baseline. Severe work shifting in adaptive targeting should be addressed promptly by the clinical team.

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Fig. 10.

Work shifting in adaptive targeting schemes. Mode: PC-CMVa. Interrupted blue line demonstrates the target tidal volume. In left panel, volume is below target; the ventilator increases inspiratory pressure gradually to reach target tidal volume. In right panel, the patient effort (P_mus) leads to tidal volume above target; the ventilator gradually decreases inspiratory pressure in an aim to decrease delivered tidal volume. However, the patient effort generates larger tidal volume. The spike in pressure at the end of the breath is a manifestation of late cycle. PC = pressure control. CMV = continuous mandatory ventilation.

Cycle

Cycle (start of expiration) is assessed in terms of synchrony (timing) with the end of the patient’s inspiratory effort (ie, patient signal or P_mus). It can occur early (before the patient signal), on time (synchrony), or late (a clinically important delay) (Figure 11). Similar to triggering, there are 2 other conditions that are not related to timing but rather to ventilator function: false cycle, where a nonpatient signal cycles inspiration (eg, pressure alarm); and a failed cycle, where patient cycle signal fails to cycle the inspiration (eg, runaway phenomena). However, these manifest as early or late cycle; thus, we classified them as a cause rather than a separate patient-ventilator discordance.

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Fig. 11.

Classification of cycle patient-ventilator interactions. Mode: PC-CMVs. P_vent: airway pressure waveform displayed by ventilator. P_mus: patient-generated pressure waveform, simulated, overlay to demonstrate effect across waveforms. Normal cycle: patient-triggered; the effort was small; the flow decays passively to zero flow with no evidence of inspiratory or expiratory effort. Late cycle: Green dotted line demonstrates end of patient breath; flow reaches baseline, and there is an increase of airway pressure due to relaxation of inspiratory muscles against a close valve (zero flow). Red dotted line demonstrates point where ventilator cycles. Early cycle: the machine cycles breath (red dotted line), expiratory flow with evidence of inspiratory patient effort (flow moves toward baseline). Green dotted overlay to demonstrate passive flow waveform as a reference. Note: red P_mus line was overlaid by hand onto a ventilator screen image, and P_mus is shown inverted for clarity. PC = pressure control, CMV = continuous mandatory ventilation.

Normal (Synchrony)

Inspiration ends within a clinically acceptable time near the P_mus peak (ie, P_mus increases as the diaphragm contracts and decreases as the diaphragm relaxes). In VC, we observe the inspiratory pressure waveform and the expiratory flow waveform. For PC, this is recognized by observing the flow waveform. There should be no evidence early or late cycle. Sometimes a sharp rise in end-inspiratory pressure can be a ventilator artifact.²⁵

Late Cycle

Inspiration ends with a clinically important delay after the P_mus peak. Another way to state this is that the patient’s neural inspiratory time is shorter than the inspiratory time imposed by the ventilator. It can also be observed with spontaneous breaths if the flow cycle threshold is set too low or when patient effort ceases but it fails to cycle inspiration. The primary example is the “runaway” phenomena in the mode called PAV, where the ventilator continues the assistance in spite of patient terminating inspiratory effort due to the ventilator’s inaccurate estimation of lung mechanics.

Late cycling must be assessed according to the clinical context. For example, if a patient makes only a short trigger effort and inspiration is time or volume cycled, then by definition this is late cycling. It may be perfectly acceptable from both a patient safety and comfort point of view. However, late cycling becomes relevant when there is evidence of expiratory effort before the cycle event. In VC, the pressure waveform increases abruptly at end inspiration, indicating expiratory effort or respiratory muscle relaxation. The same can sometimes be seen with PC, and in addition, the expiratory effort will cause a downward deflection in the flow waveform, possibly even crossing into negative flow before the ventilator cycles inspiration (Figure 11).²⁶

Early Cycle

Inspiration ends within clinically important time before the P_mus peak. The cycle event occurs before the patient effort ceases. Another way to say this is that the patient’s neural inspiratory time is longer than the inspiratory time imposed by the ventilator (Figure 11). For both VC and PC, this is recognized in the expiratory flow waveform as a distortion of the peak expiratory flow and disruption of the normally smooth exponential flow decay of passive expiration. Early cycle is a common cause of multiple triggering (also known as double trigger). With PC, in some patients with very prolonged time constants (eg, COPD, asthma), flow may still be positive at the end of the inspiratory time. However, if there is no evidence of inspiratory effort during the early expiratory phase, this is not a synchrony problem.

Early cycle can also occur if the ventilator cycles inspiration by a non-patient signal. This is typically recognized as an unusually short inspiratory time; for example, a patient with very low compliance and a rapid rise time in pressure support mode. This causes a high peak flow followed by rapid decay due to a short time constant, thereby too rapidly reaching the flow cycle threshold. It can also occur if ventilator safety features, such as a pressure limit or a spontaneous tidal volume limit, are reached.

Expiration

During expiration (ie, the period from the start of negative flow to the start of positive flow), the patient-ventilation interaction is not characterized by timing (synchrony) but by work. Normal expiration is passive. During expiration, the ventilator controls the pressure with set-point targeting (ie, the target value is PEEP). In a passive patient, we expect to see a smooth exponential decay of the expiratory flow and volume waveforms. This allows us to observe the predominant physiology and the effects of trigger and cycle discordances. The expiratory phase can demonstrate, as described above, other patient-ventilator interactions (early cycle and failed trigger); however, the one that is proper to the expiratory phase is expiratory work.

Expiratory Work

Patient expiratory effort (ie, negative P_mus) will deform the flow waveform in a negative direction (away from baseline). Expiratory work may be normal, as when exercising or coughing, but it may also indicate the presence of high resistive load (eg, COPD), acidosis, or anxiety (Fig. 4).

Waveform Patterns

Ventilator waveform patterns are important to recognize as they may have clinical implications. A waveform pattern is a sequence of or stereotypical waveforms that may have a clinical consequence and are due to a patient-ventilator interaction. Many reports describe these patterns as a specific patient-ventilator interaction; however, as described below, they can have several etiologies. We approach these separately from the standardized waveform analysis. Patterns should trigger further evaluation.

Multiple Trigger

Multiple trigger is characterized by 2 or more ventilator breaths delivered in close succession without complete expiration between them. Many ventilators have a trigger window immediately after inspiration where another breath can’t be triggered; this leads to a brief but consistent pause between mechanical inspirations. Terminology for this phenomenon varies in the literature. It is sometimes called double triggering, double cycling, clusters, or breath-stacking. All these terms have issues with meaning or accuracy (see related supplementary material at http://rc.rcjournal.com). Although double trigger is the most common presentation, there are instances where multiple breaths are triggered, and thus the term multiple. Multiple trigger is a pattern that has at least 3 causes (Figure 12): early cycle, early trigger, and false trigger.²⁷ Its presence has been associated with poor outcomes,²⁸ the main concern being, in VC modes, overdosing of tidal volume by breath-stacking leading to ventilator-induced lung injury (this is less so in PC, as V_T is dependent on R and C).

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Fig. 12.

Causes of multiple trigger. A. Early trigger, the pattern is mandatory (machine triggered) followed by evidence of Pmus, and then a patient triggered breath. B. False trigger, no evidence of P_mus and triggering of a breath immediately after exhalation. C–D. Early cycle: evidence of P_mus through first breath leading to triggering of next breath. Note: red P_mus line was overlaid by hand onto a ventilator screen image, and P_mus is shown inverted for clarity.

Tidal Volume Discrepancies

The volume waveform contains mostly the same information as the flow waveform; however, there is one area where it conveys specific information. Knowledge of how the ventilator displays volume is key. The volume waveform is derived from the flow measurements for most ventilators; volume is the integral of flow, which means that the area under the flow curve over the inspiratory flow time equals the tidal volume. At the beginning of each breath (patient or machine triggered), the ventilator resets the volume waveform to zero so that the inspiratory tidal volume displayed is accurate. This means that differences in inhaled and exhaled tidal volumes manifest with a pattern where the volume waveform has a sharp drop (reset) prior to the next breath, similar to a square root sign (Figure 13). This pattern has at least 4 causes: (1) leak from the circuit, airway, or lung; (2) active exhalation during inspiration (some ventilators do not account for V_T exhaled during inspiration)²⁶; (3) air trapping, the patient has not been able to exhale the inhaled V_T, and another breath is triggered; and (4) flow sensor malfunction.

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Fig. 13.

There are many etiologies that exist for inspiratory and expiratory discrepancy. This example is due to a leak (around endotracheal tube) causing false trigger. The ventilator flow trigger detects inspiratory flow (due to the leak) and triggers a breath. Note the characteristic “volumen reset” for the new breath leading to the “square root sign.”

There are multiple other artifacts and perhaps other patterns. This manuscript does not intend to be an exhaustive review of all possible patient-ventilator interactions. Instead, we offer a standard nomenclature and an organized method to read waveforms from which we are sure much will be added, researched, and improved.

Applying the Method

We have created a tool to aid the clinician in systematically reading waveforms (Table 4). This method also allows one to summarize the patient state in a single sentence (eg, The patient is on PC-CMVa, with a high elastic load and has early triggers). It is common that more than one discordance is found in a given tracing. Some discordances are associated with others (eg, early trigger is commonly followed by early cycle and work shifting), in which case we only mention the first discordance (ie, early trigger). Following this method will allow the clinician to decide if that patient-ventilator interaction matches the goals of ventilation and if it does not to determine the changes in settings or modes needed to achieve the goals.

In our practice, we emphasize that it is easier to decide what to do when you know what the goal is. There are only 3 goals of mechanical ventilation (safety, comfort, and liberation).⁷ They are not mutually exclusive, but one must choose which is primary at the given time. For example, in a patient with ARDS in the first days, the main goal is safety. We want to ensure the settings and interactions lead to lung-protective ventilation and ensure gas exchange. Comfort is important, but we would not choose a mode that favors comfort over lung protection. As the patient recovers, the clinician is likely to be attempting liberation; yet if the patient is not ready, comfort (improving synchrony and work of breathing) in order to minimize sedation would be the leading goal. Safety (ie, preventing lung injury and ensuring gas exchange) is still important, but modes that serve comfort and yet maintain safety should be preferred.⁷

This also helps put the interpretation of the waveform in context, as the changes done to the ventilator should be in line with the goal of mechanical ventilation. For example, a patient with extreme work shifting, in the setting of safety as a goal, will guide the clinician toward correcting the cause (eg, sedation, neuromuscular blockade, correction of metabolic acidosis).

A word of caution to the reader, patient-ventilator interactions occur breath by breath; and as such, they will change with patient condition, level of awareness, interventions, etc. Many interactions will be temporary, harmless, and may be irrelevant clinically (eg, mild work shifting); others may be harmful, especially if frequent (eg, multiple trigger in VC). Not every interaction requires an intervention.