ASV in Mixed ICU

Utilization of an Automatic Mode of Ventilation (ASV) in a Mixed ICU population: Prospective Observational Study

By J-M. Arnal^a M.D., C. Nafati^a M.D., M. Wysocki^b M.D.,
Y-S. Donati^a M.D., I. Granier^a M.D., J. Durand-Gasselin^a M.D.

Introduction

Adaptive Support Ventilation (ASV) is an automatic mode of mechanical ventilation that has been proven to simplify postoperative respiratory management^{References 1 & 2} and to improve patient-ventilator interaction.^Reference 3 Prior to this study, there was no data available on routine ASV utilization in a large population of mixed intensive-care patients.

Method

This prospective observational study reports the use of ASV as the primary mode of ventilation in an 11-bed mixed ICU, over a 7-month period. While the clinician sets minute volume (as a percentage of the "ideal" minute volume: %MinVol), the ASV algorithm determines tidal volume and respiratory frequency based on respiratory mechanics, in such a way as to minimize the work of breathing.^Reference 4 Moreover, ASV encourages spontaneous breathing activity, providing full or partial ventilatory support. It is therefore well suited to use in the initiation, maintenance or weaning phase of mechanical ventilation. Contra-indications for ASV were noninvasive ventilation, bronchopleural fistula and Cheynes-Stocke breathing. The clinicians were allowed to switch to another mode of ventilation if the optimal tidal volume was not achieved despite a plateau pressure above 30 cmH₂O or in the case of patient-ventilator asynchrony. Data (Table 1) were recorded on a day-by-day basis (6 a.m. daily) and analyzed each day of invasive ventilation. Results are given with mean ± SD.

Results

Over the study period, 322 patients were admitted to the intensive care unit, amounting to 2144 days of hospitalization. The mean IGS II was 46. There were 1506 days of ventilation (70%) with 1349 days of invasive ventilation (89%).

ASV was used in 98% of invasive ventilation-days (Figure 1) including the weaning period. The %MinVol set was between 116% and 137%.

Figure 1: Modes of ventilatory support used. VAC: volume control, PS: pressure support, DOM: homecare ventilator.

Figure 2: Indications for mechanical ventilation.

Indications for mechanical ventilation are detailed in Figure 2.

Breathing pattern, mechanics and gas exchange based on the underlying lung disease (assessed by the physician in charge) are given in Table 1. Mean duration of ventilation and stay in intensive care are respectively 6,6 and 7,6 days. ICU mortality rate was 30% (predicted mortality 37%). No side effects were reported with the use of ASV.

	Normal lung	Obstructive lung disease	ARDS	Restrictive lung disease	Acute lung injury


%ASV	116 ±25	119 ±29 NS	131 ±25**	137 ±31**	130 ±29**
PEEP (cmH₂O)	22 ±6	24 ±7**	30 ±6**	27 ±6.7**	25 ±6**
Vt (ml)	516 ±131	585 ±113**	453± 112**	386 ±91**	503 ±109 NS
Ftot (C/mn)	17 ±5	16 ±6*	20 ±6**	23 ±6**	18 ±6**
Fspont (c/mn)	9 ±10	9 ±10 NS	6 ±10**	13 ±13 NS	9 ±11 NS
I:E	0.50 ±0.17	0.41 ±0.15**	0.62 ±0.27**	0.54 ±0.24 NS	0.48 ±0.17 NS
RCe (s)	0.78 ±0.28	1.13 ±0.70**	0.55 ±0.21**	0.4 ±0.15**	0.70 ±0.22
Cstat (ml/cmH2O)	46 ±23	56 ±25**	30 ±14**	22 ±10**	41 ±20**
Rins (cmH2O.s/l)	16 ±7	16 ±10 NS	17 ±7 NS	14 ±9 NS	16 ±7 NS
pH	7.40 ±0.07	7.37 ±0.09**	7.29 ±0.14**	7.37 ±0.11 NS	7.38 ±0.08**
PaO2/FiO2	330 ±113	248 ±115**	140 ±48**	267 ±106**	215 ±61**
PaCO2 (mmHg)	40 ±7	44 ±11**	49 ±9**	45 ±9**	42 ±7**
Vt/IBW (ml/Kg)	8.3 ±1.3	9.3 ±2.1**	6.8 ±1.2**	7.0 ±1.1**	8.1 ±1.2 NS

Table 1: Breathing pattern, mechanics and gas exchange based on the underlying lung disease.
* p < 0.01; ** p < 0.001 between diseased lung and normal lung (using Mann-Whitney test).
% ASV: percentage of ideal minute ventilation, Vt: tidal volume, Ftot: total respiratory frequency, Fspont: spontaneous respiratory frequency, I/E: inspiratory expiratory ratio, RCe: expiratory time constant, Cstat: static compliance, Rins: inspiratory resistance.

Conclusions

The present prospective observational study found that the automatic mode of ventilation — ASV — was used in 98% of invasiveventilation days, with patients suffering from very different types of underlying disease. There was only very occasional need to switch to an alternative mode of ventilation.

Although breathing patterns varied, depending on the underlying lung diseases, ASV consistently and automatically selected protective ventilation with low tidal volume for ARDS patients.