Special Feature: Patient-Ventilator Dys-Synchrony During Lung Protective Ventilation
Special Feature
Patient-Ventilator Dys-Synchrony During Lung Protective Ventilation
By Dean Hess, PhD, RRT, Respiratory Care, Massachusetts General Hospital, Department of Anesthesiology, Harvard Medical School, Boston, is Associate Editor for Critical Care Alert.
Dr. Hess reports no financial relationship to this field of study.
In 2000, the Acute Respiratory Distress Syndrome Network (ARDSnet) reported an unprecedented low mortality for ARDS patients ventilated with a tidal volume of 6 mL/kg of predicted body weight (and a plateau pressure ≤30 cm H2O) compared to a tidal volume of 12 mL/kg.1 Since then, it has been shown that adoption of a 6 mL/kg tidal volume strategy for ARDS into usual practice results in mortality similar to that of the ARDSnet study.2 Moreover, in patients who do not have ARDS or acute lung injury (ALI) at the time of intubation, tidal volume is an important risk factor for the development of ALI or ARDS during the course of mechanical ventilation.3, 4
Patient-ventilator dys-synchrony occurs when gas delivery from the ventilator does not match the neural output of the respiratory center. One of the clinical observations when implementing a lung protective ventilation strategy in which tidal volume and inspiratory plateau pressure are constrained is the development of patient-ventilator dys-synchrony. For clinicians used to observing a calm patient who is passively ventilated, the emergence of patient-ventilator dys-synchrony is alarming and presents an important impediment to implementation of a lung protective ventilation strategy.5 Although clinicians often associate patient-ventilator dys-synchrony with a low tidal volume setting on the ventilator, it is important to note that dys-synchrony has been recognized for many years—even when high tidal volumes were commonly used. The prevalence of patient-ventilator dys-synchrony with low tidal volumes, compared to high tidal volumes, is unknown.
There are 3 major categories of patient-ventilator dys-synchrony. Trigger dys-synchrony occurs when the effort of the patient fails to adequately trigger the initiation of the inspiratory phase. Flow dys-synchrony occurs when the flow or volume delivered from the ventilator does not meet the demands of the respiratory muscles. During volume-controlled ventilation, for example, flow and volume delivery are fixed. If patient demand increases, airway pressure decreases, the amount of assistance provided by the ventilator decreases, and the imposed work of breathing increases.5 Cycle dys-synchrony occurs when the inspiratory phase of the ventilator does not coincide with the neural inspiratory time of the patient; in other words, the inspiratory phase of the ventilator stops prematurely or it extends into the neural expiratory time of the patient.6
Precise measurement of patient-ventilator dys-synchrony requires use of an esophageal balloon, but this is not practical for usual patient care. Dys-synchrony is clinically identified as use of accessory muscles, active exhalation, tachypnea, tachycardia, diaphoresis, nasal flaring, and other signs of respiratory distress. However, dys-synchrony can be subtle and not easily recognized by clinical examination.7 Patient-ventilator dys-synchrony can also be detected by careful examination of the waveforms of pressure and flow displayed on modern ventilators.8
Patient-ventilator dys-synchrony is common during mechanical ventilation and patient discomfort is also common during mechanical ventilation. The extent to which dys-synchrony and discomfort are related is unknown. Because patient-ventilator dys-synchrony looks uncomfortable to the clinician, we believe that it must feel uncomfortable for the patient. And perhaps it does. However, there has been little study of the relationship between dys-synchrony, discomfort, ICU recall, and long term functional outcomes in patients with ALI and ARDS. One study suggested health related quality of life was similar for survivors of a high versus low tidal volume strategy though the level of dys-synchrony was not reported.9
Improving Patient-Ventilator Synchrony
The following should be considered to improve patient-ventilator synchrony.10, 11 In many cases, selection of ventilator settings to achieve the best synchrony is a matter of trial-and-error and the settings that achieve the best synchrony may vary from time to time.
Respiratory rate: An increase in respiratory rate setting on the ventilator may entrain the patient's breathing pattern to the ventilator. In the ARDSnet study, the respiratory rate was increased as tidal volume was decreased to maintain minute ventilation constant (to a maximum of 35 breaths/min). Even higher respiratory rate settings (and thus a higher minute ventilation) may be required. Presumably this is due to the higher dead space fraction when tidal volume is reduced, requiring a higher minute ventilation to avoid hypercapnia and its resultant effect on respiratory drive. Increasing the respiratory rate setting has been shown to decrease work of breathing and increase patient comfort.12
Auto-PEEP: The triggers on modern ventilators are very sensitive to patient effort. Despite this, trigger dys-synchrony may occur in the presence of auto-PEEP. Accordingly, efforts should use employed to minimize the amount of auto-PEEP. Although concern has been raised regarding the potential for auto-PEEP with the high respiratory rates used in the ARDSnet strategy, this is usually not an issue due to the low tidal volumes employed and the high elastic recoil pressures of patients with ARDS.13
Inspiratory flow: An increase in set inspiratory flow may better meet the flow demand of the patient. Deviation of inspiratory flow from that desired by the patient can greatly affect respiratory comfort.14 However, a higher inspiratory flow also decreases neural inspiratory time, resulting in a greater spontaneous breathing frequency, which can worsen dys-synchrony.15
Inspiratory time: A shorter inspiratory time (the result of a higher inspiratory flow during volume-controlled ventilation) may improve patient-ventilator synchrony. However, if the inspiratory time setting on the ventilator is less than the neural inspiratory time, double-triggering and worsening dys-synchrony may occur.
Flow waveform: Dys-synchrony may improve with a descending flow waveform in some patients. For the same peak flow, inspiratory time is longer with a descending flow. This may achieve the goal of better synchrony due to higher flow while avoiding double-triggering due to an inspiratory time that is too short.
Pressure-controlled ventilation: Pressure-controlled ventilation achieves the goals of a descending flow waveform and an adjustable inspiratory time independent of flow. Pressure-controlled ventilation may result in better synchrony in some patients.16, 17 Whether this is due to pressure-controlled ventilation per se or the descending flow is unclear. Likely the same effects on patient-ventilator synchrony occur with volume-controlled ventilation using a descending ramp flow waveform.18 A potential limitation of pressure-controlled ventilation in patients with a vigorous inspiratory effort is the possibility that transpulmonary pressure (the principal determinant of ventilator-induced lung injury) may increase due to the negative intrapleural pressure swings. For the same tidal volume and inspiratory flow, work of breathing is likely the same for pressure controlled ventilation and volume-controlled ventilation.19
Pressure rise time: With pressure-controlled ventilation, the clinician can adjust to rate of rise in pressure at the onset of the inspiratory phase. If the pressure rises more quickly, flow is higher at the beginning of inhalation. Rise time adjustment may affect work of breathing and patient comfort.20, 21
Tidal volume: An increase in tidal volume, if accompanied by an increase in alveolar ventilation, decreases respiratory drive by lowering the PaCO2 (chemoreceptor effect) and activating stretch receptors (Hering-Breuer reflex). It is important to note that the ARDSnet protocol allows tidal volume to be increased to 8 mL/kg in the case of patient-ventilator dys-synchrony, provided that the plateau pressure remains ≤30 cm H2O.
Sedation: Although excessive and prolonged sedation is not recommended or appropriate, adequate sedation is necessary during mechanical ventilation, though in selected patients may be difficult to achieve regardless of ventilator settings. Despite the often voiced concern that patients require more sedation when the tidal volume is reduced, this concern has not been born out in studies that assessed sedation requirement with lung protective ventilation. Patient-ventilator dys-synchrony was reported long before lower tidal volume ventilation became acceptable; even with the use of large tidal volumes some patients require large sedative doses to ameliorate ventilator dys-synchrony. That sedative requirements were found to be similar for patients randomized to 6 ml/kg versus 12 ml/kg tidal volumes in two ARDSnet centers supports the conclusion that sedative needs may be largely determined by clinical factors other than ventilation strategy.22, 23 Such factors may include as pain, agitated delirium, metabolic acidosis, drug withdrawal, or agitation from septic encephalopathy.
What's a Clinician To Do?
As clinicians, we should assess synchrony and comfort each time we visit the bedside of a patient undergoing mechanical ventilation.24 If the patient demonstrates signs of dys-synchrony and discomfort, efforts should be undertaken to appropriately adjust the ventilator, and the sedative, analgesic, and other general medical needs of the patient should be addressed. However, given the demonstrated survival benefit of lung protective ventilation strategies, lung protection should remain the priority and should not be abandoned for fear of patient-ventilator dys-synchrony. That the need for lung protection may exist at end-inspiratory airway pressures previously thought to be safe; the tradeoff between lung protection on the one hand, and patient ventilator synchrony, sedative needs, and acid base homeostasis on the other remains a major clinical challenge. More study is needed to determine the prevalence of patient-ventilator dys-synchrony, its mechanism, its impact on important patient outcomes, and the optimal lung-protective strategies to avoid it.
References
- The Acute Respiratory Distress Syndrome Network: Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. N Engl J Med. 2000;342:1301.
- Kallet RH, et al. Crit Care Med. 2005;33:925-929.
- Gajic O, et al. Crit Care Med. 2004;32:1817-1824
- Gajic O, et al. Intensive Care Med. 2005;31:922.
- Kallet RH, et al. Crit Care Med. 2006;34:8-14.
- Kondili E, et al. Br J Anaesth. 2003;91:106-119.
- Parthasarathy S, et al. Am J Respir Crit Care Med. 2000;162:546-552.
- Nilsestuen JO, et al. Respir Care. 2005;50:202-234.
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- Ramnath VR, et al. Clin Chest Med. 2006;27:601-613.
- Hess DR, et al. Crit Care Med. 2006;34:231-233.
- Leung P, et al. Am J Respir Crit Care Med. 1997;155:1940-1948.
- Hough CL, et al. Crit Care Med. 2005;33:527-532.
- Manning HL, et al. Am J Respir Crit Care Med. 1995;151:751-757.
- Tobin MJ, et al. Am J Respir Crit Care Med. 2001;163:1059-1063.
- Cinnella G, et al. Am J Respir Crit Care Med. 1996;153:1025-1033.
- MacIntyre NR, et al. Crit Care Med. 1997;25:1671-1677.
- Davis K Jr, et al. J Trauma. 1996;41:808-814.
- Chiumello D, et al. Eur Respir J. 2002;20:925-933.
- Chiumello D, et al. Eur Respir J. 2001;18:107.
- Chiumello D, et al. Crit Care Med. 2003;31:2604-2610.
- Cheng IW, et al. Crit Care Med. 2005;33:63-70.
- Kahn JM, et al. Crit Care Med. 2005;33:766-771.
- Hansen-Flaschen JH. Respir Care. 2000;45:1460-1464.
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