SedationObservational study of patient-ventilator asynchrony and relationship to sedation level☆
Introduction
Patient-ventilator asynchrony is a frequently encountered problem in mechanically ventilated patients. Thille et al [1] found that 24% of patients experienced asynchrony in at least 10% of their breaths, with ineffective triggering and double triggering being the most common asynchronies. Indeed, ineffective triggering is especially common in patients with chronic obstructive pulmonary disease (COPD) occurring in up to 80% of patients [2], [3], [4]. Patient-ventilator asynchrony has also been found to be associated with longer duration of mechanical ventilation and lower rates of successful weaning [1], [5].
A number of mechanisms for poor patient-ventilator interactions have been identified including abnormal respiratory mechanics and ventilator factors [1], [5], [6], [7]. Clinicians cite facilitation of mechanical ventilation and promotion of patient-ventilator synchrony as among the most common reasons for administration of sedation during the course of mechanical ventilation [8], [9]. However, little evidence exists to support this practice. In a study of 8 patients undergoing mechanical ventilation, Grasso et al [10] found increasing sedation depth resulted in incremental decreases in inspiratory muscle effort. Extending these findings, it is possible that increasing sedation depth results in progressively lower maximal inspiratory flow and weaker muscle effort, clinically manifesting as ineffective triggering. However, because these patients appear calm, the asynchrony may not be diagnosed by clinicians and may remain unrecognized. Paradoxically, patient agitation due to asynchrony is often treated with medications that cause respiratory depression that may then lead to ineffective triggering. To our knowledge, other investigators have not accounted for sedation when analyzing the patient-ventilator interaction.
We conducted a pilot study to determine the frequency of asynchrony in medical intensive care unit patients and to evaluate the relationship between asynchrony and sedation level. We postulated that deeper levels of sedation were associated with more frequent asynchrony, particularly ineffective triggering.
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Inclusion and exclusion criteria
All patients undergoing invasive mechanical ventilation in the medical intensive care unit at Virginia Commonwealth University Medical Center (Richmond, Va) were eligible for study participation unless they met exclusion criteria. Exclusion criteria were age less than 18 years, positive end-expiratory pressure (PEEP) of at least 9 cm H2O, partial pressure of oxygen divided by fraction of inspired oxygen of less than 150, ventilation through a tracheotomy, and inability to initiate breaths
Statistical analysis
Mixed model repeated measures analysis of variance was used to test the relationship between sedation level (RASS, CAM-ICU, and wakefulness) and the proportion of asynchronous breaths. Mixed model repeated measures analysis of variance allows for multiple observations per patient. Other variables entered in the model were diagnosis of COPD, arterial partial pressure of carbon dioxide, trigger method (pressure vs flow trigger), mode of mechanical ventilation, plateau pressure, amount of pressure
Results
Twenty medical patients underwent 35 observations. Table 1 lists the demographics of the patients. Five patients had COPD and underwent 9 observations. Mechanical ventilation modes were synchronized intermittent mandatory ventilation with pressure support (19 observations), pressure support alone (15 observations), and pressure control (1 observation). Trigger method was flow triggering on 22 occasions (2.6 ± 0.7 L/min) and pressure triggering on 13 occasions (2.1 ± 0.2 cm H2O). Positive
Discussion
The main finding of our pilot study is that (i) asynchrony is common, with ineffective triggering being the most common asynchrony, and that (ii) sedation level affects patient-ventilator interactions, with deeper sedation level associated with more ineffective triggering. In particular, comatose patients, patients who are not awake and those who are more deeply sedated have significantly higher rates of ineffective triggering compared to noncomatose patients who are awake, alert, and
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The study was supported with the following grants: NIH K23 GM068842 and NIH M01 RR00065; Novametrix (Wallingford, Conn) provided equipment and supplies at no cost. The granting institution is the NIH located in Bathesda, Maryland.