Within the last few years in the United States ventilator manufactures have made this mode available on their newer generation anesthesia machines. Manufactures highly recommend APC during surgical procedures in which positioning or insufflation of the abdomen creates dramatic changes in pulmonary mechanics (e.g. laparoscopic, thoroscopic, prostatectomy).
Before utilizing this modality anesthesia practitioners should be very familiar with the modes operational algorithms and limitations, which I will address below.
Operational Algorithm
Defining APC mode- one set-point (pressure) of the ventilator is automatically adjusted over several breaths to maintain another set-point (target Vt), as patients lung mechanics changes.
Thus, the ventilator adapts to the need for changing the set-point[1].
Operation- upon initiation the ventilator delivers a volume-control breath with a constant flow wave form, used to calculate dynamic compliance. Compliance is then used to determine the pressure required for the next breath to achieve the target Vt (note- 'target' pressure setting = Vt/compliance). Pressure is titrated 2-3 cmH2O each breath to maintain a target Vt during changes in respiratory mechanics.
- What happens if the target Vt cannot be reached?
-The ventilator will automatically increase the driving pressure until the target Vt is reached. This is fine if the operator sets the high pressure limit appropriately, conversely, it can be detrimental if the pressure keeps increasing to an over distending level causing sheering or baro-trauma.
- What is the latency of Vt adaptation during changes in lung mechanics?
-The four following pictures represent dynamic changes in lung compliance during pneumoperitoneum & deflation of pneumoperitoneum.
- These dynamic compliance changes have a direct impact on actual Vt delivery & the time it takes to achieve the 'set' target Vt.
Another reference which looks at Vt consistency and latency of Vt adaptation is "Adaptive Pressure Control: Tidal Volume Variance during Simulated Bariatric Laparoscopic Surgery". This is an abstract I wrote for the ASA 2009 conference.
- Can APC give the clinician a false since of security?
-During rapid changes in patient compliance the Vt can vary significantly in regards to delivered Vt from the set Vt.
-During prolonged latency the lungs are exposed to over-distension or a de-recruited state.
-If the operator is unaware of these clinical situations which rapidly change lung compliance, the actual delivered Vt may lead to atelectasis, lung de-recruitment, and/or volume-trauma.
Summary
APC titrates driving pressure to achieve a targeted (operator pre-set) tidal volume. These 2-3 cmH2O titrations between breaths may create delay times during rapid compliance changes resulting in dynamic hyper-inflation or lung de-recruitment. Replacing the anesthesia provider of clinical decision making can be detrimental during rapid changes in lung mechanics.
References
[2], [3]
[1] Chatburn, R. (2007). Classification of Ventilator Modes: Update and Proposal for Implementation. Respiratory Care. 52 92): 311-312.
2] Jaecklin, T et. Al (2007). Volume-Targeted Modes of Modern Neonatal Ventilators: How Stable is the Delivered Tidal Volume? Intensive Care Medicine. 33 (2).
[3] Marini, J et. Al (1989). Determinants and limits of Pressure-Preset Ventilation: a Mathematical Model of Pressure Control. Journal of Applied Physiology. 67 (3): 1081-1092.
