Sunday, April 8, 2012

The Worst Modes of Mechanical Ventilation

IMV one of the worst ventilator modes in regards to patient comfort. 

Many will argue the best mode of mechanical ventilation and prefer the mode that is most commonly used in their patient populations. Strict believers of the ARDS net protocol will favor VC-CMV to ensure lower tidal volume delivery. Pediatric centers will insist on Pressure targeted modes to limit high pressures, and trauma centers may favor biphasic modes of ventilation to obtain higher mean airway pressures.

However, some institutions use the worst modes of ventilation that directly impede on patient comfort. Promoting patient comfort is one of the main goals of mechanical ventilation [1] and not providing comfort may lead to over sedation, over ventilation, ventilator induced diaphragmatic dysfunction, and increased length of stay.

So what are the worst modes of ventilation specifically in regards to patient comfort?

The two worst modes of ventilation are:

1.    Volume Controlled-Intermittent Mandatory Ventilation (VC-IMV). This mode (as far as I know) is on every ICU ventilator that is commercially available.

2.    Synchronized Intermittent Mandatory Ventilation with Pressure Limited Ventilation. This mode is specific to Draeger ventilators (a.k.a. Pmax).

So why are they, the worst modes of ventilation in regards to patient comfort?

First, let’s evaluate the modes in regards to patient ventilator asynchrony.

There are 10 documented patient ventilator asynchronies [2, 3] and each of the two targeting schemes [4] is associated with all of the classified asynchronies. No other modality is associated with this many asynchronies [5].

Second, let’s evaluate the modes in relation to promoting patient comfort [1]:

1.    Maximize trigger/cycle synchrony- both modes are associated with all trigger & cycle asynchronies. Additionally, the operator has to adjust both of these target values.

2.    Minimize auto-PEEP- both modes are associated with auto-PEEP.

3.    Maximize flow synchrony- both modes are associated with flow asynchrony.

4.    Coordinate mandatory & spontaneous breaths- both modes are IMV and allow for both mandatory & spontaneous breaths, however diaphragmatic monitoring has shown that the diaphragm continues to contract during the mandatory breaths. This leading to increased work and asynchrony.

5.    Optimize work demand versus work delivered- both modes targeting schemes do not allow for this.
6.    Minimize inappropriate shifting of work from the ventilator to the patient- both modes targeting schemes do not allow for this.
So when evaluating the modalities based on these considerations the operator should never utilize these modes on a spontaneous breathing patient. Better yet not use these modes on any patient.

No mode of ventilation is a panacea and it remains arguable what the ideal mode of ventilation may be. Conversely, it is easy to identify the worst modes of ventilation in regards to patient comfort and the operator should refrain from using them.

1.    Chatburn, R. & Mireles-Cabodevila, E. (2011). Closed-LoopControl of Mechanical Ventilation: Descritpion & Classification of TargetingSchemes. Respiratory Care. 56 (1): 85-102.
2.    10 documented patient ventilator asynchronies: Auto-triggering, trigger delay, ineffective efforts, double triggering, flow mismatch, driving pressure, pressurization rates, pre-mature cycle, delayed cycle, & PEEPi.
4.    Targeting Scheme: “feedback control system used by a mechanical ventilator to deliver a specific ventilatory pattern: a key component of a ventilation mode” [1].
5.    The next highest number of associated patient ventilator asynchronies is eight (8) out of the ten.