The waveform below (Figure 1) is from a ventilator with the following settings:

Volume control SIMV, PS 10, PEEP 5. 

The bedside nurse informs you that the patient appears to be "struggling against the ventilator".

a) Given the appearance of the waveform, what is the likeliest cause of the patient's distress?

(10% marks) 

The following waveform (Figure 2) is from a ventilator with the following settings:

Volume control SIMV, PS 10, PEEP 5. 

The bedside nurse informs you that the airway pressures have increased.


b) Given the appearance of the waveform, what is the likeliest cause of the increased airway
pressure and how would you treat it? 

The waveform below (Figure 3) is from a ventilator with the following settings:

Volume control SIMV, PS 10, PEEP 5.

The patient has ARDS and the bedside nurse informs you that the patient appears to be "working hard on the ventilator".


c) Given the appearance of the waveform, what is the likely cause of the increased work of
breathing? 

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College answer

(i)    Patients effort to initiate a breath is not recognised by the ventilator           
 
(ii)    Double triggering- Two breaths occur in less than half mean inspiratory time. Occurs when patient demand outlasts set inspiratory time. 
Treat by increasing tidal volume, increasing inspiratory time, increasing sedation or paralysis 
                                                             
(iii)    Flow starvation. Patients inspiratory demand is greater than that delivered by ventilator resulting in scooped out appearance of pressure waveform.      
 

Discussion

Though the college have removed the images from their official paper, some effort was made by the author to reproduce them faithfully enough to 

a) is a form of dyssynchrony best described as "wasted effort"; the patient is clearly making efforts to breathe but the ventilator is not triggering. This is because the trigger is insufficiently sensitive (or, the patient is significantly weakened).

b) is a case "breath stacking" and gas trapping. There is air hunger: the patient wants a deeper and longer breath, and when the ventilator cycles to expiration the patient takes another breath immediately. The college recommend changing some of the control variables or increasing sedation; however an alternative solution may also be to give the patient more control over their ventilation by changing them to a patient-triggered flow-cycled mode like PSV on the Servo-I.

c) is a case of flow being insufficient to meet demand,; the patient's inspiratory effort as trying to add to the tidal volume, which creates negative pressure - this gives the pressure waveform its "scalloped" appearance.

References

Arnaud W. Thille, MD, and Laurent Brochard, MD. Promoting Patient-Ventilator Synchrony (Clin Pulm Med 2007;14: 350 –359)

Petrof BJ, Legare M, Goldberg P et al. Continuous positive airway pressure reduces work of breathing and dyspnea during weaning form mechanical ventilation in severe chronic obstructive pulmonary disease. Am Rev Respir Dis 1990; 141: 281–9.