Ventilator Graphics

Updated: Dec 31, 2015
  • Author: Shakeel Amanullah, MD; Chief Editor: Zab Mosenifar, MD, FACP, FCCP  more...
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Overview

Overview

Understanding ventilator graphics is an integral part of adequately treating patients on mechanical ventilators.

Just as pulmonary functions tests are used to better understand 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 encountered from the oropharynx that occurs with routine pulmonary functions tests as the endotracheal tube bypasses the oropharynx.

Patient-ventilator asynchrony is common and may be seen in up to 25% of patients within 24 hours of initiating mechanical ventilation. [1] This article will enable the reader to understand ventilator waveforms and to identify and correct patient-ventilator asynchrony.

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

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Graphical Displays

Graphical displays are common in the intensive care setting. [2, 3] The following are examples of graphical displays:

  • Ventilator waveforms
  • Arterial waveforms
  • Venous waveforms (central venous pressure)
  • 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.

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 analyzing the waveforms in a systematic fashion, akin to reading ECG tracings, is critical when analyzing these waveforms. Just as changing the gain and paper speed can help to 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 4 components helps to better identify and correct asynchronies:

  • Trigger phase
  • Inspiratory phase
  • Cycle phase and
  • Expiratory phase
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Trigger Phase

The mechanical breath may be initiated by the patient (patient trigger) or as a function of time (time trigger). The 2 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 has been demonstrated in earlier studies, [4] 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. [5, 6]

The trigger phase can be divided 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 the 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 not appropriately set. Inappropriate rise times can also affect the time for the pressure to rise to the peak airway pressure, as is described in 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, the IDT can be expressed by the following equation: IDT = ITT + 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 overtly sensitive settings that can lead to autotriggering and insensitive settings that can lead to missed triggering.

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 such that any noise in the system (eg, cardiac oscillations) may trigger a breath
  • High cardiac-output states, valvular heart disease, [7, 8] and cardiomegaly

Mechanical ventilation can have a negative impact on the patient's respiratory drive, as has been shown by Kondili et al. [9] 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). [10] External PEEP in this setting reduces the work of breathing needed to trigger the ventilator. [11, 12, 13] The most effective method for eliminating ineffective asynchrony in this setting is to reduce the level of ventilator support. [14] This study [14] also demonstrated that the application of external PEEP reduced but did not eliminate ineffective triggering.

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

The following clinical conditions may predispose to ineffective triggering:

  • Increasing pressure support - 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 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 is the delivery of 2 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. [1]

Double triggering occurs when the patient’s ventilatory demand is high and the inspiratory time set on the ventilator is too short. That is, double triggering occurs 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 triggers a second ventilator cycle and thus occurs more commonly in modes with fixed inspiratory flow times, such assist-control ventilation. [1]

Double triggering also occurs more commonly in patients whose PaO2/FiO2 (fraction of inspired oxygen) ratio is lower and whose peak inspiratory pressure is higher than in patients without this asynchrony. This situation is commonly seen in patients with acute lung injury or acute respiratory distress syndrome.

Thus, as patients with acute lung injury or acute respiratory distress syndrome are optimally ventilated, double triggering can deliver volumes much higher than the intended volume prescription and thus can potentially result in worse outcomes. [15]

Increasing the inspiratory time or increasing the tidal volumes may help with double triggering. If the patient has a variable respiratory drive such that setting a flow on a fixed mode of flow delivery is not adequate, changing to a variable flow (eg, pressure-control ventilation) or a dual-control mode may be helpful. Sedation adjustments may need to be made 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, pulmonary embolus) when making these adjustments.

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Inspiratory Phase

During the inspiratory phase, the presence of inappropriate flow and patterns can be identified by close inspection of the flow and pressure graphics. Flow may be inadequate or excessive, both of which may contribute to patient-ventilator asynchrony. Importantly, 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 3 forms:

  • Fixed flow (eg, assisted-controlled ventilation, synchronized intermittent mandatory ventilation)
  • Variable flow (eg, pressure-control ventilation)
  • 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, flow pattern, or a combination of the 2. Common patterns of fixed-flow delivery include constant flow, descending ramp, or 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

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

Excessive flow

Excessive flow can be identified on the pressure waveform by the presence of acute "take off" 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 patient’s inspiratory effort may give the illusion of excessive flows; however, close inspection of the ascending limb of the pressure curve shows a slow take off of the ascending limb, as opposed to an acute take off 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, the descending-ramp flow pattern or variable flow associated with pressure-control ventilation has been shown to be preferable. [16, 17] However, care should be taken when 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 pressure-control ventilation, the flow is variable. The flow depends on various variables such as respiratory system compliance, set target pressure, and patient effort. [18] 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 how a primary asynchrony may lead to multiple secondary asynchronies.

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Cycle Phase

The cycling between inspiration and expiration in assist-control ventilation (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 f The pressure, volume, and flow to time waveforms for assist-control ventilation.

During assist-control ventilation, prolongation of the inspiratory breath into neural breath termination and expiration can lead to cycle asynchrony. Careful analysis of the pressure and flow curve reveals the presence of a spike at the terminal part of the pressure waveform and may have 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 volume curve.

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

Overdistension of the lung with large tidal volumes may also show a similar pattern in the pressure-time curve and pressure-volume curve, and differentiation from neural synchrony can be confusing. Adjusting the tidal volume or Inspiratory time can help differentiating between the 2.

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 as a result of the following:

  • 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 but 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 has been shown in a study by Tokioka et al. [19]

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Expiratory Phase

Shortened expiatory time may lead to auto-PEEP (see the image below). This can be seen in clinical conditions resulting in slow expiratory time constants, such as chronic obstructive pulmonary disease. 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–posit The flow to time waveform demonstrating auto–positive end-expiratory pressure (auto-PEEP).

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

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