Ventilator graphics: interpretation and optimization of mechanical ventilation 

Monitoring of ventilator graphics is a crucial facet of care in mechanically ventilated patients.  The information gleaned from interpretation of graphics offers pertinent information regarding the delivery of ventilation and patient-ventilator interaction. Ventilatory management based on information derived from the graphics display enables the prevention of potential complications, and thereby, improves clinical outcomes (1). A thorough knowledge of ventilator graphics may thus be analogous to the interpretation of electrocardiogram and arterial pressure waveforms to evaluate cardiac physiology. 

The first step towards evaluating ventilator graphics is to understand the basic mechanisms of delivery of mechanical breaths. A mechanical breath is delivered based upon three main variables. These include (a) how a breath is initiated, (b) the set target, and (c) how the breath is ceased (2). 

Trigger 

The cue for the ventilator to commence inspiration is called the trigger. If the breath is fully controlled with no patient effort, the pre-set time acts as the trigger to commence inspiration. For instance, with a set rate of 15 breaths/min, the ventilator triggers inspiration every 4 seconds.  In partial assist modes, the ventilator must sense a patient breath and synchronize with it; this constitutes a patient-triggered breath. There are two types of patient-triggered breaths, pressure and flow-triggered. In the pressure-triggered setting, the machine senses a pre-set drop in circuit pressure to commence inspiration. In flow trigger, a continuous flow occurs through the circuit during the expiratory phase, known as the bias flow. The flow is measured at the expiratory limb of the circuit. When the patient inspires, the bias flow is diverted to the patient; the machine senses a drop in flow in the expiratory limb, thus triggering inspiration.  

Target 

Once inspiration is triggered, the breath continues until a pre-set target is achieved. The common target variables include pressure, volume, or flow. Thus in pressure-controlled modes, a target pressure is set; in volume-controlled modes, tidal volume is set as the target. 

Cycle

The cessation of the inspiratory phase and commencement of expiration occurs by cycling. In other words, cycling is the signal for the ventilator to cease inspiration. Cycling to expiration may be set to a target tidal volume, as in volume-controlled ventilation. In the pressure support mode, inspiratory support is ceased when the inspiratory flow drops below a pre-set fraction of the peak inspiratory flow. In time cycling, the inspiratory phase is pre-set to a specific duration; time cycling is used in pressure-controlled ventilation. Pressure cycling involves the use of a target pressure to cycle off to expiration; this is used mainly as a safety measure to prevent excessive build-up of pressure in the system. 

Scalars

How do we represent breaths in a graphic form? Scalars represent respiratory parameters plotted against time. Thus, we can plot time along the x-axis and pressure, flow, or volume on the y-axis. Pressure and flow are measured directly, while the volume is calculated. Close examination of the scalars enables the clinician to analyze the ventilatory pattern and make adjustments as appropriate. 

Volume-time scalar 

The volume-time scalar constitutes a graphical representation of the volume of gas delivered over time. The graphic has an ascending limb during inspiration and a descending limb representing expiration (3). Both limbs look similar, but are in opposite directions. If the expiratory limb plateaus off and does not return to baseline, it indicates a leak from the circuit or the presence of auto PEEP (4) (Fig 1). 

Figure 1. The volume-time scalar. The inspiratory and expiratory limbs are similar, but in opposite directions. The expiratory limb not returning to baseline indicates leak or auto PEEP 

Flow-time scalar 

Flow of gas during the inspiratory and the expiratory phase is represented by the flow-time scalar. The upward deflection represents the inspiratory flow and the downward deflection, the expiratory flow. The shape of the inspiratory limb depends on the type of flow. Two flow patterns are commonly employed. In the decelerating type, an initial high flow-rate is followed by a tapering phase, with a typical pattern on the scalar. This flow pattern is employed in pressure controlled and pressure support modes. Constant flow is used in volume controlled modes and appears as a “flat” inspiratory phase on the flow-time scalar (Fig 2). The type of flow determines the peak and mean airway pressures. With a decelerating flow, the peak airway pressure is lower and the mean airway pressure higher compared to constant flows (5). 

Figure 2. The flow-time scalar. Flow may be constant (left) or decelerating (right)

The expiratory limb of the flow-time scalar enables identification of obstruction to gas flow. In the presence of obstructive airways disease, the peak expiratory flow is lower and return of flow to baseline is more prolonged. Air trapping and auto PEEP may be evident on the flow-time scalar. Normally, the expiratory limb returns to baseline before commencement of the next inspiration. However, if auto PEEP is present, the expiratory limb does not return to baseline before onset of the next inspiration (Fig 3).  The peak expiratory flow typically increases with a decrease in the lung compliance (6). 

Figure 3. Flow-time scalar showing air trapping with auto PEEP. The expiratory limb does not return to baseline before the next inspiration begins

Pressure-time scalar 

Pressure is plotted against time in the pressure-time scalar. The shape of the graphic varies depending on the mode of ventilation. In pressure controlled mode, a constant pressure is delivered, resulting in a square waveform. In volume controlled mode, the flow is held constant; the pressure waveform rises to a peak and drops down (6) (Fig 4).

 

Figure 4. The pressure-time scalar. Pressure may be constant throughout the cycle as in pressure-controlled ventilation or rise to a peak and drop as in volume-controlled ventilation

The peak pressure generated includes 3 components: (a) the pressure generated due to airway resistance, (b) the pressure related to overcoming the lung elastance, and (c) the total PEEP (6). If the breath is held in inspiration, airway resistance-related pressure does not come into play. Thus, during an inspiratory hold maneuver, the pressure recorded, the plateau pressure (Pplat), is entirely dependent on lung compliance (compliance = 1/elastance). The stiffer the lung (poor compliance), the higher the Pplat (7). The peak pressure (Ppeak) also rises with the rise in Pplat. Targeting a Pplat of ≤ 30 cm H₂O as part of a lung-protective strategy was shown to improve outcomes, including mortality in patients with acute respiratory distress syndrome (ARDS) (8). 

What happens when the airway resistance is high? The Ppeak rises; however, if the lung compliance is normal, the Pplat does not increase. Thus, the greater the difference between the Ppeak and the Pplat, the higher the resistance to gas flow (Fig 5) (6). 

Figure 5. The pressure waveform with inspiratory hold during volume-controlled ventilation. The pressure rises to a peak (Ppeak) followed by a plateau level (Pplat).  The peak-to-plateau pressure difference rises with increasing airway resistance. When the lung compliance is poor, both the Ppeak and the Pplat rise.  

Stress index

The stress index is an important parameter that can be discerned from the pressure-time scalar. It may be calculated by the ventilator software; however, visual analysis provides useful information (9). The stress index is based on the assumption that during constant flow ventilation, the rate of change of airway pressure is directly correlated to the rate of change of compliance. On eyeballing the pressure-time scalar, a constant slope suggests normal lung compliance (stress index of 1). A progressive decrease in the upward slope of the curve indicates tidal recruitment of the lung (stress index <1). An rise in the slope suggests hyperinflation (stress index >1) (Fig 6) (6). The titration of PEEP and tidal volume based on optimizing the stress index may be an effective method of lung recruitment.  

Figure 6. The pressure-time scalar showing stress index. The green line denotes normal lung compliance; the red line indicates overdistension; the blue line indicates tidal recruitment 

Loops

There are two types of loops: pressure plotted against volume and flow against volume. There is an inspiratory and expiratory phase in each loop that enable evaluation of respiratory mechanics. 

Pressure-volume loop 

In the pressure-volume loop, pressure is plotted along the x-axis and volume along the y-axis. The inspiratory limb of the loop begins on the left side of the time axis. The inspiratory limb rises gradually to begin with, representing gas flow into areas of low compliance. This is followed by a point wherein the slope begins to rise steeply, indicating recruitment and increase in compliance. This point is referred to as the lower inflection point. As the curve rises further, it flattens out again, at a point discernible as the upper inflection point. The upper inflection point indicates that the lung is maximally recruited and further rise in pressure results in minimal increase in the volume. The curve may become “beaked” if the pressure continues to rise, indicating overdistension (Fig 7). Setting the PEEP level to just above the lower inflection point in patients with ARDS has been suggested as part of a lung-protective ventilation strategy (10); however, in practice, it is often difficult to clearly identify inflection points, limiting the efficacy of this strategy in most situations. 

Figure 7. Pressure-volume loop with a “beaked” appearance indicating overdistension 

Flow-volume loop

Volume is plotted along the x-axis and flow along the y-axis in the flow-volume loop. The inspiratory phase is represented above and the expiratory phase below the x-axis. The inspiratory phase commences at the intercept of the x– and y-axes. The flow rises during the inspiratory phase and reaches a peak with a corresponding increase in the volume. After it reaches a peak, the flow begins to drop, but the volume continues to increase until completion of the inspiratory phase. The curve changes direction to below the x-axis during the expiratory phase. The expiratory flow increases, reaches its maximal level, and then drops; the volume decreases continuously until both volume and flow reach zero representing complete emptying of the inspired gas. The inspiratory phase of the curve (above the x-axis) is square-shaped with a constant flow mode (volume controlled) in contrast to a descending pattern with pressure controlled ventilation (decelerating flow). 

A drop in the peak expiratory flow rate may indicate obstructive airways disease. Lower flows are evident at each level of volume. This results in the expiratory limb assuming a “scooped out” appearance. Air trapping is evident if the expiratory limb stops short of reaching zero on the y-axis (6) (Fig 8). 

Figure 8. The flow-volume loop. The inspiratory phase is above and the expiratory phase below the x-axis. Left: normal flow-volume loop. Right: Air trapping with the expiratory limb stopping short of reaching zero on the y-axis with a “scooped out” appearance

If there is gas leak from the circuit, the expiratory flow abruptly drops to zero, but the volume does not (Fig 9). 

Figure 9. The expiratory flow on the flow-volume loop abruptly drops to zero, indicating circuit leak

Asynchrony

The patient-ventilator interaction may become asynchronous in several situations (11). Asynchrony may lead to increased requirement for sedation or the use of muscle paralysis, increase in the work of breathing, ventilation-perfusion mismatch, and dynamic hyperinflation. Furthermore, adverse clinical outcomes may occur, including delayed weaning, increased duration of mechanical ventilation, an increase in the length of stay, and higher mortality (12). Several types of asynchronous breaths may occur. 

 Failed trigger 

The patient may initiate an inspiratory effort, but the ventilator fails to sense and support the breath. A deflection from the baseline appears on the flow-time graphic with a decrease in airway pressure, but the ventilator does not deliver a breath. Failure to trigger may require adjustment of the trigger sensitivity on the ventilator. Another common cause for failed trigger is the presence of auto PEEP. Auto PEEP may occur due to incomplete expiration in patients with obstructive airways disease. In this case, the patient needs to overcome the auto PEEP level with the inspiratory effort before a ventilator breath may be triggered. The addition of extrinsic PEEP may overcome this phenomenon. Besides, increase in the expiratory time and treatment with bronchodilators may help overcome this problem (13). 

Double trigger 

If the inspiratory time on the ventilator is too short, premature cycling to the expiratory phase may occur, even as the patient continues to inspire. This results in the triggering of a second breath, with an expiratory phase in between (Fig 10). Double triggering may be corrected by re-setting the inspiratory time or by increasing the tidal volume (14).

 

Figure 10. Double triggering. A second breath is triggered as the patient continues to inspire during the expiratory phase of the ventilator 

Reverse trigger 

Reverse triggering usually occurs in patients who are deeply sedated. A ventilator breath leads to stimulation of the diaphragm; this is sensed by the ventilator as patient effort, resulting in the delivery of a second breath (15). Reverse triggering appears similar to double triggering on ventilator graphics. 

Auto-trigger

The ventilator may trigger a breath in the absence of a patient effort, leading to the phenomenon of auto-triggering (16). Auto-triggering may occur due to cardiogenic oscillations of the chest wall, hiccup, inappropriate trigger sensitivity setting , water droplets in the circuit, or a leak in the circuit. 

Flow starvation 

The flow setting may be disproportionately low compared with the patient effort with constant flow modes, including volume controlled ventilation. The incongruent flow setting is evident on the pressure-time scalar as a concavity or dip in the waveform (Fig 11). Flow starvation may be corrected by increasing the set flow or by switching to a pressure controlled mode. 

Figure 11. Flow starvation appears as a concavity or dip in the pressure-time scalar 

Delayed cycling 

The inspiratory time on the ventilator may be set too long. In this case, the patient begins to exhale even as the ventilator continues to be in the inspiratory phase. This becomes evident as an additional upward deflection on the pressure time scalar; besides a brief period of zero flow is seen on the flow-time scalar (Fig 12). This may be corrected by shortening the inspiratory time on the ventilator appropriately or by adjustment of the flow rate. 

Figure 12. Delayed cycling. The inspiratory time is set too long on the ventilator. This leads to the patient beginning to exhale during the inspiratory phase of the ventilator. Note the upward deflection on the pressure time scalar and a period of zero flow on the flow-time scalar

Summary 

Modern ventilators display ventilator graphics. The interpretation of graphics, akin to electrocardiography for cardiac evaluation, offers instantaneous information regarding ventilation function and interaction with patient efforts. A systematic, step-wise approach to evaluation of ventilator graphics enables optimization of mechanical ventilation. In particular, ventilator graphics provide real-time information regarding air trapping and auto PEEP, increase in airway resistance and compliance, and patient-ventilator asynchrony. Clinicians who provide care to mechanically ventilated patients must be well-versed with the interpretation of graphics and resort to expeditious corrective action as appropriate. 

References

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10.       Amato MB, Barbas CS, Medeiros DM, Magaldi RB, Schettino GP, Lorenzi-Filho G, et al. Effect of a protective-ventilation strategy on mortality in the acute respiratory distress syndrome. N Engl J Med. 1998 Feb 5;338(6):347–54. 

11.       Holanda MA, Vasconcelos R dos S, Ferreira JC, Pinheiro BV. Patient-ventilator asynchrony. J Bras Pneumol. 2018;44(4):321–33. 

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15.       Telias I, Beitler JR. Reverse Triggering, the Rhythm Dyssynchrony: Potential Implications for Lung and Diaphragm Protection. Am J Respir Crit Care Med. 2021 Jan 1;203(1):5–6. 

16.       Sassoon CS. Triggering of the Ventilator in Patient-Ventilator Interactions. Respir Care. 2011 Jan 1;56(1):39–51. 

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