Ventilator Graphics

Understanding ventilator graphics is an integral part of adequately treating patients who are on mechanical ventilation. ^{[1, 2, 3]}

Just as pulmonary function tests are an important part of understanding the lung pathophysiology in nonmechanically ventilated patients, ventilator graphics are an important part of understanding the pathophysiology in mechanically ventilated patients. Ventilator graphics have the added advantage of not producing the noise from the oropharynx that occurs with routine pulmonary function tests, because the endotracheal tube bypasses the oropharynx.

Patient-ventilator asynchrony is common and may be seen in as many as 25% of patients within 24 hours after the initiation of mechanical ventilation. ^{[4, 5, 6]} This article will enable the reader to understand ventilator waveforms and to identify and correct patient-ventilator asynchrony. ^{[7, 8]}

The following Medscape Drugs & Diseases articles may be of interest:

• Barotrauma and Mechanical Ventilation
• Mechanical Ventilation
• Noninvasive Ventilation

Graphical displays are common in the intensive care setting. ^{[9, 10]} The following are examples:

• Ventilator waveforms
• Arterial waveforms
• Venous waveforms (central venous pressure [CVP])
• Intracranial pressure waveforms
• Intra-aortic balloon pressure waveforms

One of the many advantages of using graphical displays is that they enable the analyst to better comprehend the behavior of the system being monitored. This is extremely important, especially during mechanical ventilation, because patient-ventilator asynchrony can be better understood and treated. ^[11]

Also important is understanding that more than one asynchrony can be in play and that one form of asynchrony (primary) may subsequently lead to multiple other (secondary) asynchronies.

Thus, carefully assessing the waveforms in a systematic fashion, akin to reading electrocardiographic (ECG) tracings, is critical in the analysis of these waveforms. Just as changing the gain and paper speed can help identify abnormalities in an ECG tracing, changing one or both of the axis scales may be important in identifying abnormalities that would otherwise be missed.

Dividing a mechanical breath into the following four phases is helpful for identifying and correcting asynchronies:

• Trigger phase
• Inspiratory phase
• Cycle phase
• Expiratory phase

The mechanical breath may be initiated by the patient (patient trigger) or as a function of time (time trigger). The two common types of triggering available are pressure and flow.

In order to overcome the considerable effort that could be spent initiating a breath during a pressure trigger, as was demonstrated in earlier studies, ^[12] flow triggering was introduced. With this mode, the ventilator is triggered when a patient’s effort creates a difference between the inspiratory and expiratory base flow in the circuit. However, with the newer-generation ventilators, these modes are comparable. ^{[13, 14, 15]}

The trigger phase can be subdivided into the following components:

• Trigger pressure (TP) is the pressure that must be attained by patient effort to trigger the mechanical breath
• Inspiratory trigger time (ITT) is the time elapsed from the patient’s effort to reach the TP set on the mechanical pressure; thus, in patients with a low respiratory drive, this time could be prolonged and vice versa
• Rise time to baseline pressure (RTBP) is the time elapsed from the mechanical breath trigger to attain the baseline pressure (trough airway pressure or positive end-expiratory pressure [PEEP], if set); the patient does not receive any support until the circuit is pressurized to the baseline pressure, and considerable work could be spent during this time if settings are not appropriate; inappropriate rise times can also affect the time for the pressure to rise to the peak airway pressure (see the section on flow asynchrony below)
• Inspiratory delay time (IDT) is the total time elapsed from the initial patient effort to the pressurization of the circuit to baseline pressure; in other words, IDT can be expressed as the sum of ITT and RTBP

Trigger-phase asynchrony

Trigger asynchrony can occur with any mode of mechanical ventilation. Common trigger problems include autotriggering, missed triggering, and double-triggering. Appropriate valve sensitivity settings are required to avoid overly sensitive settings that can lead to autotriggering and insufficiently sensitive settings that can lead to missed triggering.

Autotriggering

Autotriggering is a breath delivered by the ventilator in the absence of patient effort. Autotriggering may be caused by fluid in the circuit, circuit leaks, chest tube leaks, or vibration of the ventilator tubing (as might occur during insufflations and exsufflations of the lungs with poor compliance). Autotriggering can also occur in the following clinical settings:

• Low respiratory rate, low respiratory drive, and apnea testing - Allow for low flow in the circuit so that any noise in the system (eg, cardiac oscillations) may trigger a breath
• High cardiac-output states, valvular heart disease, ^{[16, 17]} and cardiomegaly

Missed triggering

Mechanical ventilation can have a negative impact on the patient's respiratory drive, as shown by Kondili et al. ^[18] Thus, increasing ventilatory support can be associated with ineffective triggering.

The application of external PEEP has been shown to decrease ineffective triggering in patients with high auto-PEEP (see the image below). ^[19] External PEEP in this setting reduces the work of breathing needed to trigger the ventilator. ^{[20, 21, 22]}  Chao et al found that the most effective method for eliminating ineffective asynchrony in this setting was to reduce the level of ventilator support. ^[23] They also demonstrated that the application of external PEEP reduced but did not eliminate ineffective triggering.

The flow to time waveform demonstrating auto–positive end-expiratory pressure (auto-PEEP).

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The following clinical conditions may predispose to ineffective triggering:

• Increasing pressure support - This may be associated with a reduction in respiratory drive and ineffective triggering
• High tidal volumes
• Alkaline pH and increased bicarbonate levels
• Chronic obstructive pulmonary disease (COPD) and dynamic hyperinflation

The use of in-line nebulizers may lead to ineffective triggering during flow triggering because of the interference with base flow. Thus, use of pressure-trigger settings during nebulizer treatment or the use of ultrasonic nebulizers may avoid this problem.

Double-triggering

Double-triggering is the delivery of two consecutive ventilator cycles separated by a very short expiratory time, with the first cycle being patient-triggered. Double-triggering is commonly encountered in mechanically ventilated patients. ^[6]

Double-triggering occurs when the patient’s ventilatory demand is high and the inspiratory time set on the ventilator is too short—that is, when the ventilator inspiratory time is shorter than the patient’s inspiratory time. The patient’s effort is not completed at the end of the first ventilator cycle, and this triggers a second ventilator cycle. Thus, double-triggering occurs more commonly in modes with fixed inspiratory flow times, such as assist-control ventilation. ^[6]

Double-triggering also occurs more commonly in patients whose ratio of arterial oxygen tension (PaO₂​) to fraction of inspired oxygen (FiO₂) ratio is lower and whose peak inspiratory pressure (PIP) is higher than those of patients without this asynchrony. This situation is commonly seen in patients with acute lung injury or acute respiratory distress syndrome (ARDS). Thus, as such patients are optimally ventilated, double-triggering can deliver volumes much higher than the intended volume prescription, potentially resulting in worse outcomes. ^[24]

Increasing the inspiratory time or the tidal volumes may help with double-triggering. If the patient has a variable respiratory drive, so that setting a flow on a fixed mode of flow delivery is not adequate, changing to a variable flow (eg, pressure-controlled ventilation [PCV]) or a dual-control mode may be helpful. Sedation adjustments may be necessary if all these measures fail. If the patient’s ventilatory need is high or has suddenly changed, it is important to determine the cause of this change (eg, stroke or pulmonary embolism) when making these adjustments.

During the inspiratory phase, the presence of inappropriate flow and patterns can be identified through close inspection of the flow and pressure graphics. Flow may be inadequate or excessive, and either state may contribute to patient-ventilator asynchrony. It is important to realize that inappropriate flow rates and patterns may lead to a number of secondary asynchronies. For example, inappropriate flow may reduce the expiratory time, leading to auto-PEEP, which may lead to ineffective triggering.

Flow can be delivered in the following three forms:

• Fixed flow (eg, assist-control ventilation [ACV] or synchronized intermittent mandatory ventilation [SIMV])
• Variable flow (eg, PCV)
• Combined fixed and variable flows (dual modes, eg, volume-assured pressure support and pressure augmentation)

Evaluation of the inspiratory-phase asynchrony begins with identifying whether the flow is fixed or variable.

Fixed flow and asynchrony

Fixed-flow asynchrony can be related to the flow rate, the flow pattern, or a combination of the two. Common patterns of fixed-flow delivery include the following:

• Constant flow
• Descending ramp
• Sinusoidal patterns

Flow rate–related asynchrony

Adequate flow is represented by a smooth, rounded initial part of the pressure graphic with a plateau on the latter half of the pressure curve.

Inadequate flow can result in a significant increase in work of breathing. Inspection of the pressure and flow graphics show a "scooped-out" appearance of the pressure waveform and may also show increased flow in the direction of the flow if the patient’s effort draws in air through the demand valve.

Inadequate flow rates may cause undue prolongation of the inspiratory time, leading to shortened expiratory times, which, in turn, may lead to auto-PEEP and ineffective triggering.

During SIMV, evaluation of the flow rate during the pressure-supported breath may help with setting of the flow rate during the mandatory breath. (See the images below.)

The pressure, volume, and flow to time waveforms for synchronized intermittent mandatory ventilation (SIMV).

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The pressure, volume, and flow to time waveforms for synchronized intermittent mandatory ventilation (SIMV) with pressure-support ventilation.

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Excessive flow can be identified on the pressure waveform by the presence of acute "takeoff" of the ascending limp of the pressure curve along with a pressure spike at the beginning of the curve. Sometimes, the presence of a continued strong inspiratory effort by the patient may give the illusion of excessive flow; however, close inspection of the ascending limb of the pressure curve would show a slow takeoff of the ascending limb, as opposed to the acute takeoff that would be expected with excessive flow rates.

Flow pattern–related asynchrony

Certain patterns of flow are used in certain clinical situations. For example, in patients with chronic obstructive pulmonary disease (COPD), the descending-ramp flow pattern or variable flow associated with PCV has been shown to be preferable. ^{[25, 26]} However, care should be taken in switching between patterns, especially when prolonged expiratory times are required, because flow-pattern changes could be accompanied by a prolonging of inspiratory times with shortened expiratory times, resulting in auto-PEEP.

Variable flow and asynchrony

During PCV, the flow is variable. The flow depends on several variables, such as respiratory system compliance, set target pressure, and patient effort. ^[27] When the rise time is adequate, the representative pressure waveform has a rounded front end and a plateau body.

Excessive rise time appears as a pressure overshoot at the front end of the pressure-time waveform, and inadequate rise time appears as a concave beginning to the pressure-time waveform. Rapid rise times may be associated with premature breath termination and may lead to double-triggering if the patient’s effort is sufficient to trigger a mechanical breath.

Inadequate rise time, on the other hand, may lead to prolongation of inspiration, leading to neural asynchrony, and insufficient expiratory time, leading to auto-PEEP and even triggering asynchrony. This, again, is an example of how a primary asynchrony may lead to multiple secondary asynchronies.

The cycling between inspiration and expiration in ACV (see the image below) is a function of the preset inspiratory time and tidal volume. During pressure-control/pressure-support breathing, the cycling between inspiration and expiration is brought about by a drop in the flow rate and the breath cycles when the flow reaches a percentage of the peak flow. Other secondary cycling characteristics also are present as a safety precaution if the inspiratory time is unduly prolonged.

The pressure, volume, and flow to time waveforms for assist-control ventilation.

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During ACV, prolongation of the inspiratory breath into neural breath termination and expiration can lead to cycle asynchrony. Careful analysis of the pressure and flow curves reveals the presence of a spike at the terminal part of the pressure waveform and may show a "zero" flow in the flow waveform (eg, pressure-regulated volume control; see the image below) or a sudden decrease in the flow. This spike may also be identified in the pressure and volume curves.

The pressure, volume, and flow to time waveforms for pressure-regulated volume-controlled ventilation.

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Overdistention of the lung with large tidal volumes may also show a similar pattern in the pressure-time curve and the pressure-volume curve, and differentiation from neural synchrony can be confusing. Adjusting the tidal volume or inspiratory time can facilitate differentiation between these two states.

Termination of insufflations during pressure-control/pressure-support ventilation is usually a function of the peak inspiratory flow or attainment of the peak pressure. Thus, premature termination may occur in the following ways:

• Excessive rise time results in an initial pressure overshoot with the resultant termination of the breath because the target pressure is exceeded
• Excessive rise time may also cause early breath termination because the cycling threshold, which is usually a percentage of the peak flow (eg, 25% of the peak flow in the Siemens 300C; this percentage varies between ventilator manufacturers and can easily be changed), is reached early, and the breath cycles to expiration

The effects of varying rise time on breath cycling have been evaluated in a study by Tokioka et al. ^[28]

Shortened expiratory time may lead to auto-PEEP (see the image below). This can be seen in clinical conditions resulting in slow expiratory time constants (eg, COPD). In addition, as noted in the previous sections, it can result from other primary asynchronies and, by itself, can result in secondary asynchronies (eg, trigger asynchrony).

The flow to time waveform demonstrating auto–positive end-expiratory pressure (auto-PEEP).

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Managing auto-PEEP includes correction of any asynchronies, as is described in previous sections, and treatment of the underlying clinical problems.