Abstract & Commentary
The Vexing Problems of Lung-Protective Ventilation: Asynchrony, Work of Breathing, and Sedation
By Richard H. Kallet, MS, RRT, FAARC, FCCM
Director of Quality Assurance, Respiratory Care Services, San Francisco General Hospital
Mr. Kallet reports no financial relationships relevant to this field of study.
SYNOPSIS: This single-center, prospective, observational study examined the effects of clinician adjustments in ventilator settings or sedation/analgesic dosing on "breath stacking" (an inspiratory effort that causes the ventilator to deliver two consecutive breaths without an intervening expiratory phase). Although both strategies significantly reduced the incidence of breath stacking, ventilator adjustments were markedly superior.
- SOURCE: Chanques G, et al. Impact of ventilator adjustment and sedation-analgesia practices on severe asynchrony in patients ventilated in assist-control mode. Crit Care Med 2013;41:2177-2187.
Thirty mechanically ventilated patients with acute respiratory failure were enrolled in the trial. Eligible patients had evidence of "severe asynchrony" as measured by the Asynchrony Index (AI) and defined as > 10% of all inspiratory efforts resulting in breath stacking. Continuous evaluation of ventilator waveforms, as well as sedation and pain levels, were monitored in 30-minute periods before and after 50 clinician-directed decisions were made on how to treat breath stacking. Patients were uniformly managed with volume assist-control ventilation at a tidal volume (Vt) of 6 mL/kg predicted body weight and a set peak inspiratory flow rate of 60 L/min (0.5 sec inspiratory time) and trigger sensitivity of -2 cm H2O. A sedation/analgesic protocol initially targeted a RASS (Richmond Agitation and Sedation Scale) score of 0 to -2. Pain was assessed during daily sedation interruptions and treated with opiate boluses and a 50% infusion rate increase. Likewise, bolus sedation was used for RASS scores > +2.
Sedation and analgesic adjustments were highly effective in reducing agitation and pain and coincided with a significant reduction in AI from 41% to 27%. In contrast, increasing the ventilator inspiratory time to 1 second (by adding an end-inspiratory pause) or changing to pressure support ventilation greatly reduced the AI score from 38% to 2%. However, use of pressure support ventilation resulted in a significant increase in Vt.
COMMENTARY
The puzzle of patient-ventilator asynchrony goes back to the 1960s when it was observed that setting a physiologic Vt of 7 mL/kg in conscious, critically ill patients frequently produced subjective complaints of dyspnea ("inadequate chest expansion").1 Thus, patient perception of "volume starvation" was largely responsible for the practice of using a generous Vt (10-15 mL/kg). Since the mid-1980s, numerous studies have investigated patient-ventilator interfacing and its effect primarily on patient work of breathing (WOB). This research has both enriched our understanding of patient-ventilator synchrony and resulted in substantial improvements in ventilator technology.
Four important aspects to this discussion are: 1) patient Vt demand and inspiratory flow represent the effects of global inspiratory muscle shortening and the velocity of muscular contraction; 2) when the corresponding ventilator-set Vt and flow are below demand, an additional workload is imposed on the patient’s respiratory muscles; 3) neural-mechanical dissociation (the primary cause of dyspnea) occurs when effort and muscle tension are both disproportionate to and out of phase with chest excursion (the response to which is a reflexive increase in respiratory drive); and 4) neural inspiratory off-switch occurs at the Vt "targeted" by the patient. Therefore, mismatching in Vt, inspiratory flow, and inspiratory time between patient demand and ventilator settings causes increased patient WOB and "asynchrony." In addition, emotions and bodily sensations are expressed partly through changes in the breathing pattern, so that asynchrony is exacerbated by pain, discomfort, anxiety, and delirium. Therefore, sedation and analgesia play an immense role in promoting synchrony.
Restricting Vt while providing adequate inspiratory flow (e.g., > 60 L/min) in an effort to limit imposed WOB necessitates an abnormally brief inspiratory time. This in itself potentiates breath stacking. Unfortunately, increasing the pause time can increase patient effort and induce acute negative-pressure pulmonary edema because continued vigorous inspiratory efforts occur against an occluded circuit.2 Pressure support ventilation may alleviate breath stacking but inadvertently may promote ventilator-induced lung injury from poor Vt control. Moreover, during lung-protective ventilation, pressure modes are not necessarily superior to volume modes with a high flow rate at a comparable Vt.3
Asynchrony accompanying lung-protective ventilation may not have a satisfactory solution. However, a practical tool for balancing the competing problems of excessive Vt delivery, excessive WOB, and excessive sedation is simply to utilize a brief trial of continuous positive airway pressure. This allows clinicians to assess both the magnitude and variation in Vt and flow rate generated by patients, and thereby determine how far these deviate from clinician goals. Also, intermittently executing an expiratory hold (prior to an inspiratory effort) allows an estimate of inspiratory muscle pressure. This can be used to assess patient effort in response to therapeutic interventions. By using this simple assessment technique, clinicians can make clear decisions that balance the competing priorities of lung-protection, WOB, and sedation that often are at odds with one another during mechanical ventilation.
EDITOR's COMMENTARY
David J. Pierson, MD
Patient-ventilator asynchrony is a huge problem in respiratory care, often constituting a practical barrier to the implementation of lung-protective ventilation.4 Its potential adverse effects are numerous and important:5
• Ineffective ventilation
• Hypoxemia
• Lung over-distension
• Dynamic hyperinflation
• Increased work of breathing
• Patient discomfort
• "Fighting the ventilator"
• Distress for family members and others at the
bedside
• Conflict among team members
• Excessive administration of sedatives and
neuromuscular blocking agents
• Respiratory muscle dysfunction
• Confusion with respect to readiness for weaning
• Prolongation of mechanical ventilation
• Neuromuscular complications of prolonged
immobility.
Breath stacking, also known as double-triggering, is an especially troublesome manifestation of asynchrony in patients on lung-protective ventilation using volume-targeted, assist-control ventilation. The latter remains the most widely used ventilation mode in the world, and knowing how to adjust it properly to minimize asynchrony is important.6 As the study by Chanques et al shows, and Kallet emphasizes in the above discussion, increasing sedation is much less effective in reducing patient-ventilator asynchrony than adjusting the ventilator settings when breath stacking occurs.
REFERENCES
1. Pontoppidan H, et al. Acute respiratory failure in the adult.
N Engl J Med 1972;287:799-806.
2. Kallet RH, et al. Exacerbation of acute pulmonary edema during assisted mechanical ventilation using a low-tidal volume, lung-protective strategy. Chest 1999;116:1826-1832.
3. Kallet RH, et al. Work of breathing during lung-protective ventilation in patients with acute lung injury and acute respiratory distress syndrome: A comparison between volume and pressure-regulated breathing modes. Respir Care 2005;50:1623-1631.
4. Patient-Ventilator Interaction. Respir Care 2011;56:13-102, 56:127-228.
5. Pierson DJ. Patient-ventilator interaction: Conference summary. Respir Care 2011;56:214-228.
6. MacIntyre NR. Patient-ventilator interactions: Optimizing conventional ventilation modes. Respir Care 2011;56:73-81; discussion, 81-84.
