Study Objective: To study the effects of circuit leak development over time and response during volume and pressure controlled ventilation using low flow in human patient simulator and to examine the minimum fresh gas flow needed to compensate for such a leak. Design/Setting: Prospective study using a patient Simulation Lab at Wayne State University. Measurements: A human patient simulator was endotracheally intubated. The endotracheal tube (ETT) was connected to the Datex-Ohmeda AS/3 Anesthesia machine. The tidal volume was set to 500ml in the volume controlled trial and the pressure to 6cm H2O in the pressure controlled trial. A hole was created in each experiment placed 10 cm after the inspiratory valve. Leaks were simulated from holes using 4 different needle diameters: 25, 21, 18 and 16G. A series of data were collected using fresh gas flow at 4 different flow rates (0.5, 1, 1.5 and 2 liters.min-1). Data was measured at different time points (baseline, 1, 3 and 5 minutes) in the series of simulated leaking breathing circuits. Results: Leak alarms were only detected with 16G hole at 5 minutes in the volume control mode versus leaks at 3 minutes with 16G hole and at 5 minutes with 18G hole in the pressure control mode. Conclusion: When a very low flow of 0.5 L/min is used, volume control is safer than pressure control modes.