The results are in, and the correct answer was Ineffective Efforts
Ineffective efforts are one of the most common forms of patient-ventilator dyssynchrony. They occur when the patient is unable to trigger the ventilator during the expiratory phase of ventilation.
Visual inspection of the expiratory waveform is often sufficient to detect ineffective efforts. There is a often a drop in pressure below baseline (PEEP) with a concurrent decrease in expiratory flow. A truly diagnostic approach is to view the esophageal pressure (Pes) waveform (fig 1), or electrical activity of the diaphragm (EaDi) to see inspiratory effort occurring without the ventilator responding with support. However, with or without Pes or EaDi, it is also important to observe the patient for efforts, including examination of abdominal activity.
Figure 1 showing the esophageal tracing along with pressure and flow. Red arrows indicate patient effort.
The most common cause of ineffective efforts is in fact over assistance by the ventilator. Many clinicians may take the approach of adjusting ventilator sensitivity, but this would not correct the problem. With flow trigger, the patient would still be required to inspire above a flow of 0 l/min, you cannot set a flow trigger below 0. For pressure trigger, because there is still a significant amount of volume in the lung, generating a significant amount of pressure is also difficult. In fact, making the ventilator too sensitive can introduce the risk of auto-cycling.
The correct approach to correcting ineffective efforts is to address the issue of over assistance. In general, over assistance results in patients getting more tidal volume delivered than they are demanding, and the total inspiratory time of the ventilator exceeds the patients own neural inspiratory time. The first approach should be to lower the pressure support level, particularly if the exhaled tidal volume is > 7 ml/kg of predicted body weight. This will help be avoiding over assistance, and prevent the patient from getting more than their neural demand. Many of these patient are breathing 25 or more breaths per minutes, and they are not usually demanding excessive volume (6.3 ml/kg of predicted body weight is physiologically normal!) A second approach can be to shorten the cycling criteria of the breath. In Pressure Support ventilation this setting is called by different names (insp cycle %, Esens%), but whatever the name, it should be a ‘%’. This % setting is a percentage of the peak inspiratory flow. To shorten the breath you must consider that the peak inspiratory flow is “100%” and by increasing the % cycle criteria, you get closer to 100% and therefore cycle the breath off sooner (fig 2). This will help by shortening the amount of time the breath is delivered to the patient with the goal of being more synchronous with their own neurological inspiratory time. In the case of ineffective efforts the patients neural inspiratory time is usually shorter than what the ventilator is delivering.
When correcting ineffective efforts, an increase in respiratory rate measured by the ventilator is not an indication of a worsening condition, it is simply unmasking the missed efforts! In the example below, the level of support is decreased to correct ineffective efforts. The result was tidal volume close to 7 ml/kg predicted body weight, and because the missed breaths were now being captured, the respiratory rate was higher as read by the ventilator. The reason the breath attempts were not being detected prevously was because ventilators currently only register breaths that cause delivery of a postive pressure breath. Any effort made during the expiratory phase that does no trigger a breath is not counted.
Ineffective efforts are common and waveforms should be routinely monitored to detect them. Although this can occur with any patient being over supported, patients with COPD and high respiratory system compliance are very likely to exhibit this type of dyssynchrony, and likely pressure support as well as cycle % would need to be adjusted more precisely in attempt to allow pressure delivery by the ventilator to be shorter and more similar to the patients own neural inspiratory time. Additionally, if your patient is receiving close to 6 ml/kg of predicted body weight, the ventilator cycle % is set appropriately, and the patient has difficulty triggering every breath (not just every other breath), consider causes of auto-PEEP and ways to address them (another blog post topic!).
Here is an excellent article describing the techniques used to decrease ineffective efforts:
Thille, Arnaud W., Belen Cabello, Fabrice Galia, Aissam Lyazidi, and Laurent Brochard. 2008. “Reduction of Patient-Ventilator Asynchrony by Reducing Tidal Volume during Pressure-Support Ventilation.” Intensive Care Medicine 34 (8): 1477–86. Pubmed