In a prospective study continuous positive pressure ventilation (CPPV) was compared with the combination of CPPV and high frequency ventilation (CHFV). Forty-seven patients of a surgical intensive care unit who required mechanical ventilation were randomly assigned to a CHFV group (n = 27) or a control group with CPPV (n = 20). Usual hemodynamic and oxygenation variables, intrathoracic blood volume (ITBV) and extravascular lung water (ETV) were assessed before and 6 h after switching to CHFV (CHFV group) or maintaining CPPV (control group). In both groups mean airway pressure (MPaw) was kept constant. The change of the respiratory index (dRI) was used for evaluation of the effect of the ventilation mode. A negative correlation was found between ETV and dRI (r = -0.67), which led to the conclusion that the height of ETV determines the efficacy of CHFV. In a CHFV subgroup with ETV greater than 15 ml/kg (n = 17), CHFV significantly improved PaO2/FiO2 (18.7 to 26.4 kPa), RI (4.44 to 2.99) and intrapulmonary shunt (33.5 to 27.5%) and slightly, but significantly impaired cardiac index (CI, 4.45 to 3.92 l/min.m2), stroke volume index (40.7 to 36.4 ml/m2) and pulmonary vascular resistance index (PVRI, 310 to 366 dyn.s.cm-5.m2), but oxygen delivery (DO2) remained almost unchanged. The CI decrease came about with an increase of PVRI on account of an increased lung volume under CHFV in this group. CHFV induced no significant effects in the subgroup with ETV less than 15 ml/kg (n = 10). There were no differences in any variable during CPPV between the CHFV subgroups and corresponding subgroups of the control group.(ABSTRACT TRUNCATED AT 250 WORDS)