Saturday, June 4, 2011

SmartCare PS: Concerns with Delayed Cycling

SmartCare/PS® or SmartCare Pressure support (™ Draeger Medical, Telford, PA) is the only automated weaning ventilator mode in the United States that relies entirely on a rule-based expert system[1]. Automated modes may be useful however; the operator needs to realize the limitations before utilizing the mode in all patient populations.

Smartcare is a spontaneous mode of ventilation (CSV), which uses pressure support to augment the inspiratory efforts. The operator is allowed to change the pressurization rate (rise time), however has no control of the “Expiratory Cycling Threshold” which is one of the determinates of how a pressure support breath ends.

A Review of Pressure Support Termination

Due to safety requirements a pressure supported breath is terminated by various ways, most ventilators will terminate the breath based on 3 primary thresholds:

-Flow Deceleration

Flow Deceleration
Ideally a pressure support breath will end based on a preset or operator set flow deceleration. Each vendor has a specific name for this setting (if it can be manipulated) an example is Hamilton Medicals “Expiratory Trigger Sensitivity (ETS)”.

If the flow never decelerates to its cycling threshold (primarily due to leaks) for safety the ventilator will end the pressure supported breath based on a time threshold. The threshold can either be preconfigured or operator selected. A common time threshold is 1.5 seconds for adult patients. 

Another safety feature is the pressure threshold. If the patient generates enough back pressure (attempting to exhale) or sometimes the pressurization rate is set too fast (creating a pressure spike) the pressure supported breath will terminate. A common pressure threshold is ~ 2 cmH2O above the set pressure support value.

As previously mentioned when utilizing Smartcare the operator has no control of the expiratory cycling threshold. The flow deceleration criteria are preset which the operator cannot manipulate. SmartCare’s flow termination criteria is preset at 25%, in other words 25% above baseline or flow has to decelerate 75% of the total peak inspiratory flow rate before the breath ends (e.g. peak inspiratory flow 100 liters per second , breath terminates at 25 liters per second).

Note- on most ICU ventilators a smaller expiratory cycle criteria setting (e.g. 10%, 25%) equates to a longer breath.

This 25% percent expiratory cycle criteria is a common default setting for pressure supported breaths in many ICU ventilators, and is of no concern for a large number of patients. This extended breath may decrease work of breathing and provide optimal comfort for patients recovering from acute lung injury [2].

Conversely, a fixed expiratory cycle criteria of 25% may create delayed cycling in patients with COPD. Prolonged expiratory cycling in these patients may increase work of breathing, intrinsic PEEP (PEEPi), and create trigger asynchronies (ineffective efforts) [2].

Optimal breath termination should couple the patients’ neural timing. Some modes of ventilation offer this, one is Neurally Adjusted Ventilation Assist the other is Proportional Assist Ventilation. Other ventilators allow the operator to select the expiratory cycling threshold; this is also beneficial allowing the operator to titrate the threshold based on the patients’ status or underlining pathology.

SmartCare is an automated weaning platform which may be valuable in specific patient populations. However, the fixed expiratory cycle criteria may increase work of breathing, intrinsic PEEP (PEEPi), create trigger asynchronies (ineffective efforts), and prolong weaning in patients with chronic obstructive pulmonary disease.

[1] Chatburn, R. & Mireles, E. (2011). Closed-loop Control of Mechanical Ventilation: Description and Classification of Targeting Schemes. Respiratory Care. 1 (56).
[2] Richey,S. (2011). Ventilator Graphics: Identifying Patient Ventilator Asynchrony & Optimizing Settings. Chesapeake, VA.

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