being delivered to the cavity of a patient 48 as will be discussed in greater detail below gases are allowed to travel downwardly through the second tube 84 but are not allowed to travel from the upper portion 82 of the second chamber 80 through tube 84 as discussed above the second chamber 80 includes a second column of fluid 98 which is 2 cm high in the preferred embodiment of the invention the second column of fluid 98 acts to prevent gas present in the upper portion 82 of the second chamber 80 from entering the first end 86 of the second tube 84 in the event that the pressure in the upper portion 82 of the second chamber 80 is greater than the pressure in the patient 39 s cavity the pressure in the upper portion 82 of the second chamber may be greater than the pressure in the patient 39 s cavity when the system 10 is disconnected from a high vacuum source in this event the pressure and the upper portion 82 of the second chamber 80 will rise to atmospheric pressure if fluid is not present in the second chamber 80 this atmospheric pressure can be transmitted to the patient 39 s cavity causing the patient 39 s lung to collapse this is prevented by the presence of a second column of fluid 98 and the second chamber 80 when the pressure in the upper portion 82 of the chamber 80 is greater than the pressure in a patient 39 s cavity each exhalation of the patient will cause the fluid in the second chamber 80 to be drawn up into.