Hyperventilation Strategies for Optimized ANEclear Performance
Joseph Orr, PhD Assistant Director Department of Biomedical Engineering
Successful application of the ANEclear requires hyperventilation during emergence. Hyperventilation means substantially higher ventilation than the patient received during the maintenance period of the anesthetic. For example, if minute volume during maintenance was 5 L/min, we would expect 10-15 L/minute during emergence. However, hyperventilation achieved with high respiration rates and small tidal volumes is not productive; tidal volume must be maintained during emergence for successful outcomes. Adequate tidal volume is required for ANEclear performance and safety. The labeling requires a minimum of at least 500 ml. The ideal volume is equal to the patient’s own anatomic dead space, plus the dead space added by the other apparatus in the breathing circuit (filter, sampling adaptor etc.) and the dead space of the ANEclear . This means that tidal volumes of between 700 and 1000 ml are ideal. Very large tidal volumes are not indicated in some patients. Remember, if the patient cannot safely tolerate tidal volumes of at least 500 ml during emergence, the ANEclear should not be used.
While no one has intentionally delivered small tidal volumes during emergence with the ANEclear , some users inadvertently set the ventilator in ways that lead to smaller tidal volumes during emergence. An all too common scenario occurs when the set respiration rate is increased during emergence and the delivered tidal volume decreases such that Minute Ventilation actually falls.
There are two main reasons why the tidal volume can shrink when the respiratory rate increases: inadequate inspiratory flow rate and exceeding the pressure limit. Inadequate flow rate is corrected by simply increasing the set inspiratory flow rate. Pressure limit problems are usually resolved by increasing the set pressure limit. It is often the case that both the inspiratory flow rate and the pressure limit are left at whatever setting was used during the previous case and no attention is given to either setting. Re-adjusting these settings prior to emergence will help ensure adequate ventilation during emergence leading to rapid emergence when using the ANEclear.
Inadequate Inspiratory Flow Problem
Pressure Limit Problem
On some newer electronic ventilators, raising the respiration rate tells the ventilator to automatically raise the flow rate during inspiration. This means that when you double the respiration rate, you tell the ventilator to double the gas flow rate during inspiration. This causes the back pressure to rise causing the pressure to exceed the limit and therefore causing the delivered tidal volume to drop.
As an example, consider a situation in which the respiration rate is set at 10 breaths per minute during maintenance and the added back pressure during each inspiration is 8 cm H2O. If the breath rate is increased to 20 breaths per minute during emergence, then the back pressure might increase from 8 to 32 cm. H2O. Adding 32 cm H2O of back pressure to the actual pressure in the lungs of about 15 cm H2O will exceed a normal peak pressure limit set at 30 cm H2O and therefore only a small volume will be delivered to the lungs.
The simple solution to this problem is to simply raise the set peak pressure limit on the ventilator. This will allow the full tidal volume to be delivered regardless of the back pressure induced by increased flow rate. Note that on some machines, the high pressure limit is set by adjusting an alarm limit. This means that adjusting the high pressure alarm limit automatically adjusts the maximum pressure that the ventilator will deliver.
The peak inspiratory pressure limit is provided as a safety mechanism to prevent “barotraumas” or damage to the lungs due to over-inflation. Most clinicians understand that the actual pressure in the lungs is much less than the pressure seen by their ventilator and displayed on their monitor screens and that increasing the pressure limit does not mean that the lungs will experience the set pressure limit. However, there is a limit beyond which the pressure limit can not be increased.
If the pressure limit is already at the maximum safe level, an alternative approach is to increase the time allotted to inspiration during each breath by adjusting the I:E ratio setting on the ventilator. The I:E ratio is the ratio of inspiration time relative to expiration time for each breath. For example, if the I:E ratio is set at 1:3, then for every one second of inspiration time, there will be three seconds of expiration. Decreasing the I:E ratio gives more time for inspiration and therefore lowers the flow rate needed to deliver the set tidal volume. Lower flow rates mean less back pressure so more volume can be delivered to the lungs without reaching the peak pressure limit.
As an example, consider a case in which the set respiration rate is 10 breaths per minute and the I:E ratio is 1:3 during maintenance. This means that inspiration lasts 1.5 seconds and expiration lasts for 4.5 seconds for each breath. If the breath rate is doubled during emergence, then the time allowed for each inspiration drops to 0.75 seconds, and the ventilator would need to double the flow rate to deliver the entire breath in the shortened time. The doubled flow rate willcause an increase in back pressure causing the peak pressure limit to be reached and the tidal volume to be decreased. If the I:E ratio is adjusted lower, to 1:1, then the time allowed for inspiration remains at 1.5 seconds and the ventilator does not have to increase flow rate even though the respiration rate has doubled. In this manner, the respiratory rate can be doubled and the peak pressure does not increase.
Another method of raising the delivered volume is to increase the respiratory rate less dramatically. For example, doubling the respiration rate, without changing the I:E ratio might cause a fourfold increase in back pressure; however, a 50% increase in respiration rate might only raise the back pressure twofold. This means that if respiratory rate is high and tidal volume has fallen, it may be wise to decrease the respiration rate until the tidal volume has been restored to its set value. One strategy is to decrease the respiratory rate in small steps (1-2 breaths per minute) until the set tidal volume is delivered.
So the first strategy is to increase the peak pressure limit. If the pressure limit is already at the highest safe level, then adjust the I:E ratio down. Both of these adjustments can be done prior to emergence. If neither of these strategies is available, decrease respiration rate in steps of 1-2 breaths/minute until the set tidal volume is achieved.
Two Tidal Volumes
Watch The Bellows
Minute Volume Delay
A DRY RUN
In this case, the emergence was slower than expected. The set tidal volume was large and the delivered tidal volume was sufficient prior to emergence when the respiratory rate was 11 breaths per minute. When the respiratory rate was increased to 22, the pressure limit was reached during inspiration and the tidal volume dropped. In this case, the minute volume during emergence was less than half the maintenance minute volume. Because the small tidal volume was about the same as the patient’s dead volume (volume in the trachea that does not participate in gas exchange) there was virtually no gas exchange in the lungs. Had this problem been recognized, any of the three strategies suggested earlier could have corrected this situation. Either raising the peak pressure limit, adjusting the I:E ratio to 1:1 or stepping down the respiration rate would have caused higher tidal volumes and a faster emergence.