BACKGROUND: Pleural effusion is a common finding in critically ill patients and may contribute to circulatory instability and the need for inotropic support. OBJECTIVE: We hypothesised that dobutamine would affect the physiological determinants preload, afterload, contractility and changes of inferior vena cava characteristics during experimental pleural effusion. DESIGN: A randomised, controlled laboratory study. SETTING: Animal laboratory, conducted from March 2013 to May 2013. ANIMALS: Twenty-four Landrace and Yorkshire female piglets (21.3 ±â€Š1.7 kg). INTERVENTION: Twenty piglets were included in the analyses. After inducing bilateral pleural effusion (30 ml kg), the piglets were block randomised to either incremental dobutamine infusion (n = 10) or control (n = 10). MAIN OUTCOME MEASURES: Ultrasonographic measures of left ventricular end-diastolic area, left ventricular afterload, left ventricular fractional area change and inferior vena cava diameter and distensibility were used to assess the basic physiological effect of incremental dobutamine administration during experimental pleural effusion. RESULTS: In the dobutamine group, preload, measured as left ventricular end-diastolic area, decreased from 11.3 ±â€Š2.0 cm after creation of the pleural effusion to 8.1 ±â€Š1.5 cm at a dobutamine infusion rate of 20 µg kg min (P < 0.001). In the same period, central venous pressure and the expiratory diameter of the inferior vena cava decreased from 9 ±â€Š3 to 7 ±â€Š4 mmHg (P < 0.001) and from 1.1 ±â€Š0.2 to 0.9 ±â€Š0.1 cm (P = 0.008), respectively. CONCLUSION: In a porcine model of pleural effusion, dobutamine affected basic haemodynamic determinants substantially by decreasing left ventricular preload. Changes in central venous pressure and inferior vena cava characteristics were minimal, discouraging their use as indices of preload. This study underlines the significance of evaluating basic haemodynamic determinants to avoid inappropriate, potentially harmful treatment.