Always select the smallest gauge peripheral catheter that will accommodate the prescribed therapy and patient need. Peripheral catheters larger than 20G are more likely to cause phlebitis. Do not use peripheral veins for continuous infusions of vesicants, parenteral nutrition or infusates with an osmolarity of 900 mOsm/L.¹ External catheter diameters, length and water flow rates are variable among each brand of catheter, with the dimensions and flow rates listed below being approximate. Fluid flow rates in actual patient use, are influenced by the type and viscosity of fluid, fluid temperature, height of the container and the use of needleless connectors.² There are numerous factors that could also affect fluid flow rates, once the catheter is inserted into a patient.