Monday, January 10, 2011

The "ASV Target Point": Adjusting the %MinVol setting During Adaptive Support Ventilation

After a few years of implementing Adaptive Support Ventilation (ASV), I still receive questions from staff members and physicians regarding the ideal %MinVol target setting when initially setting-up ASV. I usually respond by asking, “do you want to wean or rest the patient”?

First, if the goal is to wean the patient start at a %MinVol setting of 100%. Monitor the patient’s spontaneous effort, and trend the following ventilator parameters: Peak Inspiratory Pressure (PIP), Rapid Shallow Breathing Index (RSBI), and airway occlusion pressure at 0.1 second (P0.1). These trends allow the practitioner to assess the patients’ work of breathing (WOB), in comparison to various levels of driving pressure/pressure support.

If the goal is to rest the patient or decrease WOB, precisely setting the %MinVol may be more time consuming.

At some facilities ASV is used primarily as a weaning modality were patients’ are transitioned to ASV from a conventional ventilator mode (e.g. VC-CMV, PC-CMV). If this is the standard then merely use the measured exhaled minute ventilation from the traditional settings and use this as your %MinVol target. This should adequately off-load the Inspiratory muscles.

Conversely, ASV is becoming the standard mode of ventilation at many institutions, primarily due to the modes specific safety rules (1). Unlike transitioning from a conventional ventilator mode were the practitioner already has a minute ventilation trend, a patient that is placed on ASV from the start needs additional assessment.

The operator manual recommends with initially setting up ASV to set the %MinVol at 100%. You can additionally, add 10% to this if using a Heat Moisture Exchange, and an additional 20% if the patient has a uncompensated metabolic acidosis (Radfords nanogram).

Dr. Wu and colleagues observed in their sampled patients that targeting a %MinVol of 100% is insufficient in decreasing WOB in patients with respiratory failure (2).

From their observations they recommend setting the %MinVol based on what they term as the “ASV Target Point”. The ASV target point is determined when:

1. All baseline breaths were spontaneous.

2. Then 1-to-5 mandatory breaths per minute appear.

3. Measured minute ventilation (VE) remains unchanged however; WOB decreases (as evidence by P0.1 changes).

Basically, increase the %MinVol setting by 10% every five minutes until 1-to5- mandatory breaths appear, and trend P0.1 for comparisons.

Titrating the %MinVol based on the ASV Target point is easy to comprehend and one doesn’t have to wait on additional test (e.g. temperature, ABG’s, CBC, blood culures, etc.) to optimally target a percent minute volume.

%MinVol Setting on a Hamilton G5 ventilator.

1. ASV is classified as using “Optimal Control” a ventilator mode that uses multiple mathematical models to prevent tachypnea, prevent auto-PEEP, prevent excessive dead space ventilation, and prevent high pressures.

2. Wu,C.P. et. Al. (2010). Correlation Between the %MinVol Setting and Work of Breathing During Adaptive Support Ventilation in Patients with Respiratory Failure. Respiratory Care. 55 (3).


Adaptive Support Ventilation in the Morbidly Obese

ASV & ABGS: Adjusting %Min Vol Based on Arterial Blood Gases