Setting too short an I:E ratio could potentially cause circulatory issues, because the thoracic pressure is not allowed to sufficiently drop to allow for refilling of the vascular and cardiac structures. This would cause a decrease in cardiac output overall. Different ventilators have variable flexibility. Some designs may have an adjustable I:E ratio, with 1:2 through 1:4 being the most common available. On simpler models, a preset ratio of 1:2 is a standard acceptable setting.
Types of Ventilators
Pressure limited or pressure cycled: These ventilators deliver a tidal volume based on a preset pressure. Once the preset pressure is reached, inspiration ceases. These ventilators are considered safe because regardless of the size of the patient being ventilated, inspiratory pressure (PIP) will remain constant and, therefore, overly large tidal volumes can be avoided. There is no adjustment that needs to be made between patients as long as a reasonable PIP was selected originally. The tidal volume is in a sense determined by the pressure limit set by the anesthetist. To increase tidal volume, choose a higher pressure setting and vice versa. Usually an alarm will sound if the pressure selected is exceeded (as when a patient spontaneously breathes during IPPV) or if the set pressure is not reached. This is usually the case in a disconnect situation. These ventilators are sensitive to patients with preexisting disease and varying degrees of lung compliance that may cause resistance, so monitoring of CO2 and blood gases can ensure that adequate tidal volumes are being delivered.
Volume cycled: These ventilators have the ability to deliver a tidal volume independent of the pressure achieved in the airways. Most are equipped with safety valves that allow the release of excess pressure in the system. Usually, these are set at 60 cm H2O. These ventilators may have an ascending or descending bellows (Fig. 18.2). Ascending bellows are more helpful in signaling leaks in the system because if there is a leak or disconnect, the bellows will not reach the top of the housing.
Figure 18.2. Air-Shields Ventimeter Ventilator II with ascending bellows. (Photos courtesy of Heather Deering-McPherson.)
Time cycled means that the breaths are given at fixed intervals (chosen as rate), independent of the patient’s respiratory efforts (Davey and Diba 2005). Minute volume is chosen by adjustment of the inspiratory flow control and the rate control. The Hallowell EMC models 2000 and 2002 also have a maximum working pressure limit (MWPL) control. This works somewhat like the pressure limited ventilator in that it allows for a certain maximum airway pressure to be chosen. When this pressure is reached, inspiration is terminated.
Many ventilators in veterinary medicine are a combination of one or more types of ventilatory control, giving greater manipulation of values to the anesthetist. Most commonly a pressure limited/time cycled design has been the basis for veterinary ventilator designs (Fig. 18.3).
Figure 18.3. The reverse of the Hallowell EMC 2000. The rear faceplate is labeled (from R to L). A threaded oxygen port, pressure transducer port (and a provided line and adaptor, which connects to the patient circuit, frequently placed at the site of the reservoir bag or in the expiratory limbs), and driving gas, which correlates the baseplate to the upper bellows housing port (unlabeled but on the R side). The two additional connections on the bellows housing are to the breathing circuit (L) and exhaust system (M). (Photo courtesy of Max S. Hallowell.)
Setup of the Ventilator
Each ventilator will vary to some degree on its functionality in the veterinary patient because many units have been extrapolated from human medicine. In choosing a unit for your practice, the preference is to choose a ventilator that works well in the hands of the veterinary technician, with minimal complication and ease in maintenance, and with the availability of repair parts. Always refer to the manufacturer’s guidance in the proper setup and directions on the use of your particular model.
Power to the ventilator should be connected and the breathing circuit functional. Affix oxygen supply connections to the appropriate connections on both the anesthesia machine and the patient ventilator. Often this requires the use of an oxygen wye. Double-check the connections on your ventilator, which usually are standard to include the breathing system inlet, the exhaust port to a scavenging system, and a driving gas port that may be an independent source from your base unit. Choose a bellows for use in your patient based on approximate tidal volume to be delivered. If you are on the cusp of a smaller- to larger-sized bellow, choose the larger of the two. Although it is not always convenient to read the graduations on the larger bellows housing, it is advantageous to have the larger tidal volume able to be delivered if, for some reason, the patient is dynamic and higher PIP and Tv need to be produced. Before installing, check the bellows for tears, dirt, or hair (Dorsch and Dorsch 1999). The corrugated tubing from the ventilator’s breathing hose terminal should be connected to the anesthesia machine where the rebreathing bag goes.
Depending on your model, decide whether the pop-off valve needs to be closed or a control arm needs to be switched over to change to mechanical ventilations from your anesthesia machine. Most ventilator units will have a low-pressure alarm, a high-pressure alarm, and an oxygen-flow alarm, which alerts the operator to deficiencies in oxygen line pressure. Getting to know your unit the first time you place a patient on the system could be disastrous; therefore, it is advised to do a test run of this setup prior to any patient application until the operator is familiar and proficient with operation. Prior to placing the patient on the ventilator system, a pressure test should be performed, duplicating many of the steps mentioned above in regard to machine connections.
Pressure testing the ventilator and presetting
Just as important as pressure checking the anesthesia machine prior to use, the ventilator should also be pressure tested prior to use each time, especially when the bellows are changed out or the circuit is disconnected. Pressure testing verifies the correct system configuration, monitors the ventilator for consistent cycling, and checks for leaks. It ensures patient safety. To perform a pressure check:
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