Effects of sternotomy on heart-lung interaction in patients undergoing cardiac surgery receiving pressure-controlled mechanical ventilation.

BACKGROUND: The key concept underlying the dynamic indexes of preload dependence is the physiological heart-lung interaction. During sternotomy this interaction undergoes various changes, some of which remain unclear. Our primary aim was to investigate how the interaction changes during sternotomy by evaluating pulse pressure variations (PPV) with the chest closed and after sternotomy in patients ventilated using the pressure-controlled mode.

METHODS: We prospectively studied 25 patients undergoing coronary artery bypass grafting (CABG) receiving pressure-controlled ventilation. Standard hemodynamic data, PPV and tidal volume delivered were recorded before and after sternotomy, and, with the chest open, before and after positive end-expiratory pressure (PEEP) was applied and inspiratory pressure was increased.

RESULTS: Sternotomy left all variables statistically unchanged from values before thoracotomy although in the subgroup of patients with a PPV > 8% (56%) sternotomy significantly reduced PPV (from 14.4 +/- 5.2% to 8.9 +/- 4.5%). With the chest open, when PEEP was applied at 5 cm H(2)O, tidal volume decreased (from 643 +/- 83 to 587 +/- 104 ml) and stroke volume decreased (from 77 +/- 17 to 72 +/- 15 ml) but PPV remained unchanged. When PEEP was discontinued and inspiratory pressure was increased by 5 cm H(2)O, tidal volume increased (from 643 +/- 83 to 814 +/- 89 ml) and PPV increased (from 8.2 +/- 3.9% to 12.3 +/- 6.8%) but stroke volume remained unchanged.

CONCLUSIONS: In patients ventilated in the pressure-controlled mode, except those with a pre-sternotomy PPV > 8% (fluid responders), sternotomy leaves standard hemodynamic data and PPV unchanged. When the chest wall is open, cyclic changes (tidal volume) but not continuous changes (PEEP) in intrathoracic pressure directly influence PPV.