Minute ventilation stabilization during all pressure-control / support mechanical ventilation modes.

The main goal of our prospective randomized study was comparing compare the effectiveness of ventilation control method "Automatic proportional minute ventilation (APMV) "versus manually set pressure control ventilation modes in relationship to lung mechanics and gas exchange. 80 patients undergoing coronary artery bypass grafting (CABG) were randomized into 2 groups. 40 patients in the first group No.1 (APMV group) were ventilated with pressure control (PCV) or pressure support ventilation (PSV) mode with APMV control. The other 40 patients (control group No.2) were ventilated with synchronized intermittent mandatory ventilation (SIMV-p) or pressure control modes (PCV) without APMV. Ventilation control with APMV was able to maintain minute ventilation more precisely in comparison with manual control (p<0.01), similarly deviations of ETCO(2) were significantly lower (p<0.01). The number of manual corrections of ventilation settings was significantly lower when APMV was used (p<0.01). The differences in lung mechanics and hemodynamics were not statistically significant. Ventilation using APMV is more precise in maintaining minute ventilation and gas exchange compared with manual settings. It required less staff intervention, while respiratory system mechanics and hemodynamics are comparable. APMV showed as effective and safe method applicable on top of all pressure control ventilation modes.

Relationship between dynamic expiratory time constant tau(edyn) and parameters of breathing cycle in pressure support ventilation mode.

Study of the relationship between ventilation parameters: monitored expiratory time constant - tau(edyn) and breathing - trigger frequency (f(trig)) and time of breathing cycle (T(cy)) are main goals of this article. Parameters were analyzed during last 4+/-2 h before weaning from ventilation in 66 patients ventilated in pressure support mode (PSV). We have found out, that there exist mathematical relationships, observed during adequate gas exchange, yet not described. Monitored parameters are represented by tau(edyn), f(trig) and T(cy). The analysis showed close negative correlation between T(cy) and f(trig) (R(2)=0.903). This implies that each increasing of tau(edyn) causes decreasing of f(trig) and vice versa. The calculation of regression equation between tau(edyn) and T(cy) outlined that T(cy) = 5.2625 * tau(edyn) + 0.1242 (R(2)=0.85). Regulation of respiratory cycles by the respiratory center in the brain is probably based on evaluation of tau(edyn) as the tau(edyn) probably represents a regulatory element and T(cy) regulated element. It can be assumed, that respiratory center can optimize the work of breathing in order to minimize energy in system patient + ventilator. The unique relationship, described above could be useful in clinical practice for development of new ventilation modes.