A spontaneously breathing, intubated patient has to overcome the resistances imposed by the tracheal tube, the ventilator and its circuit. This is usually called ‘additional work of breathing’. Pressure support ventilation has been widely used to compensate for additional work of breathing. Pressure support ventilation is not ideal, however, because it applies a constant positive pressure throughout inspiration . Recently, two alternative ventilatory modes have appeared: automatic tube compensation and amplified spontaneous pattern. Automatic tube compensation  was designed solely for the purpose of compensating for additional work of breathing. It applies an inspiratory airway pressure that varies almost instantaneously according to the pressure drop generated by the patient across the tracheal tube. Amplified spontaneous ventilation  is another spontaneous breathing support implemented in the VectorαXXI ventilator. It includes a respiratory interface which is connected directly to the tracheal tube. In this device a flow generator, inspiratory and expiratory valves, a pneumotachograph, connections for pressure measurement and activation of the trigger are incorporated. Neither volume nor pressure is preset but there is an amplification factor (between 1 and 10) of the patient's spontaneous flow. The patient's flow and the proportional change of the applied inspiratory flow as a function of the resistance provided by the tracheal tube are continuously calculated. We have compared these three modes with respect to ability to compensate for additional work of breathing and breathing pattern.
With Hospital Ethics Committee approval, we studied 30 postoperative patients (age 65 ± 8 yr) during ventilator weaning in the post-surgical intensive care unit (ICU). Inclusion criteria were scheduled major surgery (cardiac, orthopaedic, general or neurosurgery), requiring mechanical ventilation and absence of prior pulmonary disease.
An Edgar tracheal tube (Rusch, USA) that allows measurement of tracheal pressure was used. On admission to the ICU, controlled mechanical ventilation was initiated and the patients were randomized to one of three groups: pressure support ventilation (Evita 4; Dräger, Germany; n = 10), automatic tube compensation (Evita 4; Dräger, Germany; n = 10) or amplified spontaneous pattern (VectoraXXI; Temel, Spain; n = 10). Controlled ventilation was maintained until standard weaning criteria were achieved. During weaning, pressure support ventilation was used initially in all three groups. The mode of each group was selected when the following characteristics were present: pressure support ventilation ≤15 cmH2O, minute volume <10 L min−1, respiratory rate <20 min−1, PaO2: FiO2 ratio >27, PaCO2 between 4.7 and 6 kPa and absence of metabolic disorders. Four support levels were studied in each group decreasing progressively: pressure support ventilation 15, 10, 5 and 0 cmH2O; automatic tube compensation 100%, 60%, 20% and 0%; and amplified spontaneous pattern 8, 6, 4 and 1. The final step before extubation was T-piece ventilation, which was used as control level in all groups. At minimum support levels (pressure support ventilation, 0; automatic tube compensation, 0; and amplified spontaneous pattern, 1) the patients have to overcome the entire resistance load of the ventilatory system and we considered this to constitute the total additional work of breathing. At each support level, including T-piece breathing, the patients were allowed to equilibrate for 15 min before measurements. End-expiratory pressure (0 cmH2O) and inspired oxygen fraction (40%) were similar in all three groups. Trigger sensitivity was set at minimum in all groups (0.3 L min−1 in Evita 4, and −0.5 cmH2O in VectorαXXI).
Breathing pattern and additional work of breathing were obtained using the Bicore CP-100 pulmonary monitor (Bicore Monitoring Systems, Irvine, CA, USA). Additional work of breathing was calculated as ∫ [Positive end-expiratory pressure (PEEP) - Pt] × dV as previously reported .
Kruskal-Wallis, Fiedman and Wilcoxon tests were used to analyse differences within each group and between groups. The Spearman correlation was used to study the relationship between respiratory rate, tidal volume and the level of assistance using each ventilatory mode. P < 0.05 was taken as the level of statistical significance.
Breathing pattern (Fig. 1)
During pressure support ventilation, we found that increments in support level were associated with increases in tidal volume and reductions in respiratory rate. At pressure support ventilation of 5 and 10 cmH2O, the minute volume was constant with no change in PaCO2, as expected. However, we found an increased minute volume at pressure support ventilation of 15. Changes in inspiratory flow or inspiratory time may cause variations in breathing pattern during mechanically assisted ventilation . This has not been studied during pressure support ventilation but the observed rise in minute volume at pressure support ventilation of 15 may have been caused by this.
With automatic tube compensation we found an increasing respiratory rate and a decreasing tidal volume with decreasing support level resulting in a constant minute volume. This physiological compensation mechanism tends to minimize energy expenditure for ventilation when additional work of breathing increases.
Amplified spontaneous pattern fulfils the characteristics of a partial ventilatory support in which a constant minute volume is maintained independent of the support level.
Additional work of breathing (Fig. 2)
With pressure support ventilation, additional work of breathing diminished with progressive increments of support until it practically disappeared at pressure support ventilation of 15 which is consistent with a previous report .
At automatic tube compensation 100% (maximal) we only found a 46% compensation for the additional work of breathing. This is in contrast to a previous clinical study where the additional work of breathing was almost eliminated with automatic tube compensation 100% . Obviously, the degrees of compensation do not correspond to those selected on the ventilator. Two recent studies of automatic tube compensation in commercial ventilators conclude that this mode does not compensate fully for additional work of breathing [6,7] and that the accuracy of the automatic tube compensation algorithms should be addressed.
With the exception of amplified spontaneous pattern 1, the amplified spontaneous pattern compensated for additional work of breathing independently of the amplification level chosen.
In all three modes, the additional work of breathing during spontaneous breathing without support (pressure support ventilation, 0; automatic tube compensation, 0; and amplified spontaneous pattern, 1) was not significantly different from T-piece ventilation. This indicates that the circuits (or respiratory interface) and demand valves in these ventilators offer negligible resistance and do not affect total additional work of breathing.
In conclusion, all three spontaneous breathing modalities tested met the characteristics of partial ventilatory support maintaining minute ventilation independent of the level of support. However, the additional work of breathing was reduced to a clinically significant degree only with pressure support ventilation (≥10 cmH2O) and amplified spontaneous pattern (≥4 amplification units). The conclusions obtained in our study can be applied to routine postoperative patients without lung injury and further investigations would be needed to establish these ventilatory modes in patients with pulmonary disease.
J. L. Jover
F. J. Belda
*Department of Anesthesiology and Postsurgical Intensive Care, Hospital Clínico Universitario, Valencia, Spain
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