Preload dependency determines the effects of phenylephrine on cardiac output in anaesthetised patients

BACKGROUND: Although phenylephrine is widely used in the operating room to control arterial pressure, its haemodynamic effects remain controversial. OBJECTIVE: We hypothesised that the effect of phenylephrine on cardiac output is affected by preload dependency. DESIGN: A prospective observational study. SETTING: Single-centre, University Hospital of Caen, France. PATIENTS: Fifty ventilated patients undergoing surgery were studied during hypotension before and after administration of phenylephrine. MAIN OUTCOME MEASURES: Cardiac index (CI), stroke volume (SV), corrected flow time, mean arterial pressure, pulse pressure variations (PPV) and systemic vascular resistance index were used to assess effects of changes in preload dependency. RESULTS: Twenty seven (54%) patients were included in the preload-dependent group (PPV ≥ 13%) and 23 (46%) in the preload-independent group (PPV < 13%) before administration of phenylephrine. For the whole cohort, phenylephrine increased mean arterial pressure 58 (±8) mmHg vs. 79 (±13) mmHg; P < 0.0001 and calculated systemic vascular resistance index 2010 (1338; 2481) dyn s cm m vs. 2989 (2155; 3870) dyn s cm m; P < 0.0001. However, CI and SV decreased in the preload-independent group 2.3 (1.9; 3.7) l min m vs. 1.8 (1.5; 2.7) l min m; P < 0.0001 and 65 (44; 81) ml vs. 56 (39; 66) ml; P < 0.0001 for both but not in the preload-dependent group respectively 2.1 (1.8; 3.5) l min m vs. 2.1 (1.8; 3.3) l min m; P = 0.168 and 49 (41; 67) ml vs. 53 (41; 69) ml; P = 0.191. Corrected flow time increased 294 (47) ms vs. 306 (56) ms; P = 0.031, and PPV decreased 17 (15; 19) % vs.12 (14; 16) %; P < 0.0003 only in the PPV at least 13% group. CONCLUSION: The effects of phenylephrine on CI and SV depend on preload. CI and SV decreased in preload-independent patients through increase in afterload, but were unchanged in those preload-dependent through increased venous return.