[The influence of different respiratory frequencies on hemodynamics in patients on artificial ventilation].

OBJECTIVE

To discuss the influence of different mechanical respiratory frequencies on hemodynamics in patients on mechanical ventilator.

METHODS

Twelve critical patients during weaning from mechanical ventilation with bi-level positive airway pressure (BiPAP) were studied. The ventilatory support was modified by changing the mechanical respiratory frequency only, with inspiratory airway pressure and positive end-expiratory pressure (PEEP) kept constant. Mechanical respiratory frequency was set as follows: 5, 10, 15 and 20 breaths per minute in randomized order. Respiratory mechanics, oxygenation and hemodynamics were determined and recorded 20 minutes after the change in ventilatory parameters.

RESULTS

(1)With the increase in mechanical respiratory rate, the mean pressure (Pmean) increased, minute ventilatory volume in control ventilation (V(Espont)) were also significantly increased, but minute ventilatory volume in spontaneous breaths (V(Espont)) were decreased (all P<0.01). There were no change in the total respiratory frequencies, minute ventilatory volume (V(E)), partial pressure of carbon dioxide in artery (PaCO(2)) and oxygenation index (PaO(2)/FiO(2), all P>0.05). (2)With the decrease in mechanical respiratory frequencies, stroke volume index(SI), cardiac output index(CI), global end-diastolic volume index(GEDVI) and intrathoracic blood volume index (ITBVI )were also increased (all P<0.01), but heart rate (HR), central venous pressure (CVP), mean artery pressure (MAP), systemic vascular resistance index (SVRI) and extravascular lung water index (EVLWI) were relatively stable (all P>0.05). (3)There was significant positive correlation between CI and GEDVI (r=0.569, P<0.01).

CONCLUSION

With an increase in the ratio of spontaneous breaths to control ventilation in patients on mechanical ventilator, cardiac preload is increased, so cardiac output is also increased.