Ventilator Graphics: Easy Steps to Success


Vol. 1 • Issue 1 • Page 20
Ventilation

The graphic displays generated by mechanical ventilators are an excellent tool that most respiratory care practitioners have available at bedside. The information offered by ventilator graphics can be used to verify the function of the ventilator, the interaction (or synchrony) of the patient and the ventilator as well as underlying physiology of the patient. However, this valuable information often is underused. Adequate education is necessary to be competent at interpreting ventilator graphics, and respiratory care practitioners (RCPs) often lack the time necessary to look at graphics regularly so they are comfortable interpreting them and discussing, with physicians, the optimizations and adjustments that can be made based on the graphics. Since education and free time are challenges in today’s healthcare atmosphere, there are barriers to consistent use of ventilator graphics. However, a standardized approach offers some easy steps to success.

One of the best ways to be successful with graphics interpretation is to use a step-wise method as described by Nilsestuen.1 This method, shown in Figure 1, has the RCP looking at the four phases of the ventilation cycle in steps:

Step 1-identify and assess trigger;

Step 2-evaluate appropriate breath delivery;

Step 3-assess synchrony of breath termination;

Step 4-determine abnormalities to exhalation.

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Step 1: Trigger

Assessment of the trigger phase is important to assure patient synchrony. Figure 2 displays a variety of trigger scenarios for a patient receiving assist-control ventilation. Problems noted with trigger should focus on the proper function of the ventilator, trigger setting and physiologic reasons the patient may not be able to appropriately trigger a breath.

Step 2: Breath Delivery

Assessment of the breath delivery phase is particularly important for patient work of breathing and comfort. Figure 3 displays a common problem with breath delivery in volume controlled ventilation. Inspiratory flow hunger is shown in the left side of the figure with the characteristic pressure “scoops” and the result after increasing the patient’s inspiratory flow is shown in the right side of the figure.

Step 3: Breath Cycling

Mechanical breaths are generally time, volume or flow cycled. Any of these parameters can cause asynchrony if set distinctly different than the desired respiratory pattern of an actively breathing patient. Figure 4 depicts a common breath cycling asynchrony that can be referred to as “volume hunger.” This problem is more prevalent in patients now because of trends to lower tidal volumes for lung protection and to lighten sedation for earlier extubation. Both of these protective measures are important but they can cause the patient to be asynchronous with the set tidal volume.

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The arrows in Figure 4 show a pressure wave abnormality when the patient continues to pull more volume after the ventilator cycles to exhalation. Depending on the current state of the patient, one of the following may improve synchrony: increase tidal volume by 1 or 2 ml/Kg of ideal body weight, change to a pressure-limited flow-cycled mode such as PSV, slightly increase sedation to make the patient comfortable with the current tidal volume. The remedy to this abnormality depends on the current clinical state and therapy goals using clinical judgment. Allowing the patient to remain with this asynchrony promotes increased work of breathing, anxiety and agitation.

Step 4: Exhalation Phase

Exhalation phase must be monitored for problems impeding the patient while they exhale. Figure 5 displays a patient with COPD. In this figure the air-trapping is present, characterized by expiratory flow throughout the time between mechanical breaths. Compare the exhalation in this figure to the “normal” exhalation phase in Figure 1. Asynchrony also is displayed in this figure. Note the arrows showing patient attempts to trigger the ventilator without success. Air-trapping and asynchrony during exhalation promote patient anxiety, increased work of breathing and failed liberation attempts. Intervention to reduce air-trapping may include optimizing bronchodilators, steroids, minimize inspiratory times, use of spontaneous modes such as PSV and addition of extrinsic PEEP to improve synchrony. The patient with COPD in Figure 5 was switched to PSV and had PEEP increased for synchrony, which allowed the patient to perform an SBT and be successfully extubated.

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Two Interesting Cases

A patient was admitted to the ICU with shortness of breath and respiratory instability even after intubation. The ventilator graphics exhibited an unusual expiratory flow pattern. The patient underwent bronchoscopy and was identified as having severe idiopathic tracheomalacia. It was found that the collapse of the trachea could be stabilized at PEEP of 12 cmH2O until the patient could receive tracheal stenting and then was able to be successfully extubated.

Another patient was intubated for an extended time and was receiving heated humidity. It was noted that there was slight resistance as the suction catheter was advanced. On PSV it was noted that the patient had dyspnea as well as abnormal flow graphics. The inspiratory and expiratory graphics were displaying a characteristic flattened graphic indicative of fixed airflow obstruction. A RescueCath (Omneotec) was used to clean the endotracheal lumen. The graphic after use of the RescueCath showed normalization of the flow graphics, and caregivers noted a reduced work of breathing and ability to reduce PSV level. The patient then was able to tolerate an SBT and be successfully extubated.

Looking Forward

Ventilator graphics can be a valuable tool to evaluate patient physiology, ventilator function and patient-ventilator synchrony. Using a standard approach can be an effective way to improve the use of ventilator graphics, and all staff should be educated on the most common graphics abnormalities and what interventions can benefit the patient when these abnormalities are seen. Knowledge of “normal” graphics coupled with a standard approach to interpretation also can help staff identify uncommon abnormalities. A step-wise approach is recommended through each phase of ventilation: trigger, breath delivery, breath cycling and exhalation. n

John Emberger, BS, RRT-ACCS, FAARC, is respiratory critical care coordinator at Christiana Care Health System, Newark, Del.

Reference

1.Nilsestuen J, Hargett K. Using Ventilator Graphics to Identify Patient-Ventilator Asynchrony. Respir Care 2005;50(2):202-232.

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