Respirator

Positive pressure ventilators are attached to a tube leading directly into the trachea or windpipe. These machines then force air into the lungs at sufficient force to expand the chest and lungs. The most sophisticated positive pressure respirators have an alarm system to sound if the device fails, gas blenders to infuse more than one gas into the lungs, pop-off valves to relieve pressure if the machine begins to build gas pressures to undesirable levels, humidifiers to moisturize the gas or nebulizers to infuse a medication into the gas stream, gas sampling ports, and thermometers.

Positive pressure respirators are pressure cycled or pressure limited, time cycled, volume cycled, or a combination of these.

Pressure cycled or pressure limited respirators force gas into the patient's lungs until a preset pressure is reached. A valve in the machine closes off the gas stream and the patient exhales. These machines now are used only in cases of drug overdose or with comatose patients whose lungs are easy to ventilate. With this type of respirator the preset pressure is not always delivered. Changes in airway resistance can influence the pressure detected by the machine so the gas may be cut off at what the machine detects as the set pressure when in fact the gas entering the lungs is far below the desired level. The postoperative patient who may have improved lung mechanics because of muscle relaxants given for surgery may become overventilated because resistance to the infusion is lower and the preset pressure is not attained until more than the desired level of gas has been delivered. Bronchial spasms also may influence the amount of gas reaching the lungs. The spasmodic bronchi will reduce in diameter and increase the resistance to the pump, so the preset pressure is detected at too low a level.

Volume cycled machines deliver a preset volume of gas into the lungs without regard for pressure. These machines are capable of delivering gas at high pressure, so they can overcome respiratory system resistance such as stiff lungs to administer the needed oxygen. They are used often in critical care situations.

Time cycled machines, as the name implies, deliver gas for a set time, shut off to allow the patient to exhale, then deliver again for the set time. Pressure and flow of the gas may vary over the time, depending upon patient characteristics, but these factors are not considered with time cycled machines.

Any of these positive pressure machines now can be controlled by computer and the volume, time, or pressure reset from breath to breath, according to need.

A unique type of positive pressure apparatus is designed to deliver very rapid, shallow breaths over a short time. Some are designed to deliver 60-100 breaths per minute, others 100-400 breaths, and a very high frequency oscillator is available to deliver very small tidal volumes of gas at the rate of 900-3,000 breaths a minute. These small volumes provide oxygenation at lower positive pressures. This may be important in that it reduces cardiac depression and does not interfere with blood return to the heart. Also, the patient requires less sedation.