Response to Varying the Rate of Administration of a Fluid Challenge

Administration of intravenous fluids is a key step in the management of the post-operative patient. Fluid management has been identified as one area that can affect rates of post-operative complications and outcomes. It is important to give the right amount of fluid, enough to optimise cardiovascular function, but not so much that we flood the cells. To guide this, we use a fluid challenge, administration of a small amount of fluid to test the cardiovascular response. At present there is significant inter-user variability in the method of administration of a fluid challenge. To ensure accuracy and reliability in assessing the response to a fluid challenge, the optimal method of administration needs to be determined. The aim of this study is to investigate the optimal rate to give a fluid challenge over. Patients will be randomised to receive a fluid challenge over either 5 or 20 minutes. Both of these rates are within the currently accepted range. All patients will receive a dose of 4ml/kg of intravenous fluid. Previous work has shown this to be the optimal dose to reliably stress the system. Measurement of the mean pressure in the cardiovascular system (mean systemic filling pressure or Pmsf) during the administration of a fluid challenge will be used to assess which rate of administration effectively challenges the cardiovascular system. Pmsf is measured using a pneumatic tourniquet inflated for sixty seconds, above an arterial line. The invasive arterial pressure is observed to then determine Pmsf. Cardiac output will also be monitored during and after administration of the fluid challenge. Since the microcirculation may remain impaired despite stabilisation of the macrocirculation, we will also observe this at baseline, on completion of the fluid challenge, and at 5 minute intervals for 15 minutes. This is observed using a handheld camera placed under the tongue. The hypothesis is that administration of a fluid challenge at a faster rate, over 5 minutes as op