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Is your mechanically ventilated patient getting enough oxygen?

Lian, Jin Xiong BSN, RN, CNS

doi: 10.1097/01.CCN.0000396667.84519.3d
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Flow dyssynchrony occurs when the inspiratory flow delivered by a ventilator doesn't match the patient's needs. Although flow dyssynchrony is rare in pressure target ventilation, by examining the patient's ventilator waveforms, you can recognize the problem and respond appropriately to improve your patient's oxygenation.

Jin Xiong Lian is a clinical nurse specialist in the ICU at Concord Repatriation General Hospital, a principal teaching hospital for the University of Sydney in Australia.

Find out how to use waveforms to identify and correct flow dyssynchrony on pressure target ventilation.

Figure

Figure

Flow dyssynchrony occurs when the inspiratory flow delivered by a ventilator doesn't match the patient's needs. On the patient's ventilator waveform, flow dyssynchrony appears as a concavity on the inspiratory limb of a pressure-time curve (Figure 1) or pressure-volume (PV) loop (Figure 2).1–7

A "figure eight" on a PV loop also indicates flow dyssynchrony (Figure 3).

Figure 1

Figure 1

Figure 2

Figure 2

Figure 3

Figure 3

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Getting the right settings

Most volume control ventilators require a healthcare provider to set peak inspiratory flow rate, tidal volume, and other parameters. If the peak flow rate is set below the patient's demand, flow dyssynchrony occurs. To manage this problem, the clinician can increase the set peak inspiratory flow rate or switch to pressure target ventilation (pressure control or pressure support). However, increasing the peak inspiratory flow rate may elevate peak inspiratory pressure (PIP) and cause patient discomfort. Monitor your patient's airway pressure and comfort if a high flow rate is used.2–8

The peak inspiratory flow rate doesn't need to be set for pressure target ventilation because the flow delivery is automatically adjusted to match the patient's requirement (as long as the inspiratory flow rise time is set properly). Therefore, flow dyssynchrony rarely occurs in pressure target ventilation.1,3,5–10 However, if inspiratory rise time isn't set properly, dyssynchrony can occur, especially when a patient demands excessive inspiratory flow or has extreme lung mechanics, such as high airway resistance, or uncharacteristically low or high lung compliance.6–8

Inspiratory flow rise time is the time that the ventilator needs to achieve the target pressure at the beginning of the inspiration, and is determined by the initial speed of the inspiratory flow delivered by the ventilator. On the other hand, a patient's flow demand is controlled by his or her neural drive. Inspiratory flow rise time should be set at a level that meets the patient's flow demand and lung impedance (that is, airway resistance and lung compliance).1,3,6–13

A short rise time (also called a fast rising) involves delivering a high speed of inspiratory flow at the onset of the inspiration, which often reduces the work of breathing and improves patient comfort. However, if the rise time is too fast or the patient has high lung impedance (high airway pressure and/or low lung compliance), the short rise time may cause pressure "overshoot" (Figure 4A) and turbulent gas flow, which increase PIP, lead to premature cycling, and cause patient discomfort.

Figure 4

Figure 4

On the other hand, when the rise time is set too slow or the patient demands high inspiratory flow or has low lung impedance (low airway resistance and high lung compliance), flow dyssynchrony can occur (Figure 4B).1,6–8,10–13

Adjusting inspiratory rise time can alleviate dyssynchrony and reduce the work of breathing for patients on pressure target ventilation.6–14 However, the PIP and patient comfort need to be monitored closely when changing rise time.

The optimal rise time varies between patients and may change as the patient's condition changes. Rise time is influenced by the target pressure, lung impedance, and the patient's respiratory drive. Most modern ventilators allow the healthcare provider to adjust the rise time to reduce the work of breathing and improve patient-ventilator interaction. By observing pressure-time and flow-time curves simultaneously, and taking into account the patient's condition, a clinician can determine an optimal rise time setting.6–14

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Using waveforms to set rise time

The configuration of the pressure-time curve for pressure target breaths is affected by the target pressure, inspiratory rise time, lung mechanics, and the patient's respiratory drive. Generally, an appropriately set rise time that meets the patient's requirement will generate a square waveform (Figure 4C) with a smooth rise and even plateau. This pressure–time curve has no concavity or a spike at the beginning of inspiration.1,6–13

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Patient-ventilator interaction

For a spontaneously breathing patient, mechanical ventilation involves two systems: the patient's respiratory system and the ventilator pump. The patient's respiratory system is mainly controlled by respiratory drive and respiratory muscle activity, although it can be influenced by some clinical interventions.3,15,16 For instance, if the patient's hyperventilation or respiratory alkalosis is caused by pain, analgesics can be administered.

The ventilator pump is operated by a bedside clinician. Ideally, the initiation of mechanical inspiration should be easily triggered by a minor patient effort, without the ventilator being so sensitive that it causes autotriggering. The amount and speed of flow delivered by the ventilator should match the patient's requirement and lung mechanics (control variable). These involve setting up the peak inspiratory flow rate and tidal volume for volume control ventilation, and inspiratory pressure and rise time for pressure target ventilation. Switching from inspiratory to expiratory flow should coincide with the end of neural inspiration. With pressure support ventilation, clinicians can adjust the expiratory sensitivity to minimize cycle dyssynchrony.2,17 Patient-ventilator interaction can be improved by adjusting ventilator settings and managing some patient-related factors, but it requires skill, knowledge, and experience.3,10,15–22

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Determining the cause of dyssynchrony

Patients with ventilator dyssynchrony may be distressed, but at the early stage of dyssynchrony are more likely to be quiet.23,24 As a result, patient-ventilator dyssynchrony is frequently unrecognized, although it's common.3,19,25 By inspecting ventilator waveforms, clinicians often can identify patient-ventilator dyssynchrony despite the absence of clinical signs and symptoms. Most importantly, clinicians often can determine the cause of dyssynchrony and fine-tune ventilator settings to resolve it.1,2,11–13,17,21,24

For example, Figure 5 shows pressure-time and flow-time curves in a patient on pressure support ventilation who has dyssynchrony. The waveforms indicate ineffective triggering caused by intrinsic positive end-expiratory pressure (PEEPi).2,19

Figure 5

Figure 5

PEEPi is caused by high ventilatory demands or incomplete exhalation due to insufficient expiratory time, high airway resistance, or decreased elastic recoil of alveoli. Airway resistance can be reduced by clearing airway secretions, using a larger endotracheal or tracheostomy tube, or administering bronchodilators. These interventions can lower or eliminate PEEPi. Another way to reduce PEEPi is to shorten inspiratory time, which prolongs expiratory time and can reduce the frequency of ineffective triggering.1,2,12,17–19

Remember to monitor the patient's condition closely after ventilator settings are changed. Obtain and review arterial blood gas analyses regularly, in addition to inspecting ventilator waveforms.

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A case report

A 72-year-old man had severe acute respiratory distress syndrome (ARDS) secondary to pneumonia. The patient experienced severe hypoxemia and profound hypercapnia, resulting in a high respiratory drive.

The patient is on pressure control ventilation (assist/control mode), and his PV loop shows a "figure eight" configuration when inspiratory rise time is at 50%, suggesting flow dyssynchrony. The plateau of the pressure-time curve isn't smooth, indicating patient-ventilator dyssynchrony. His PIP is 47 cm H2O and mean airway pressure is 32 cm H2O. Although shortening the inspiratory rise time may eliminate the "figure eight" and produce a square pressure-time curve, these interventions may intensify the existing high airway pressure and cause pressure overshoot.

The patient was switched from pressure control to pressure support ventilation, PEEP and FiO2 were reduced, and the inspiratory rise time was set at 70%, resolving the flow dyssynchrony.

This patient had high lung impedance due to ARDS, so the most likely causes for his flow dyssynchrony were excessive inspiratory flow demand caused by severe hypoxemia and hypercapnia and high lung impedance. Despite the flow dyssynchrony, the patient showed no signs of respiratory distress or increased work of breathing. Mild hypoxemia and permissive hypercapnia are allowed for a short period of time in patients with refractory hypoxemia and severe hypercapnia.26–29 Optimizing patient-ventilator interaction reduces metabolic effort and work of breathing, thereby reducing oxygen consumption and carbon dioxide production and increasing tolerance with mechanical ventilation.30

At the same time, as healthcare providers work to optimize patient-ventilator interaction, they also must use lung-protective strategies.2,10,24

No gold standard ventilator settings suit a patient at all times. The interaction between the patient and the ventilator is dynamic, changing at different stages of illness. But understanding when to change ventilator settings can help enhance the effectiveness of mechanical ventilation and may shorten the duration of mechanical ventilation.2,3,12,17,30 Using ventilator waveforms can help you identify problems so that the patient can receive appropriate care.

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