Monday, November 22, 2010

The Problem with Adaptive Pressure Control Modes of Ventilation: a Case Study.


Adaptive Pressure Control (APC) is a ventilator modality, which has been applied safely in ICU’s for greater than a decade.The mode delivers a pressure control breath that maintains a ‘target’ operator selected tidal volume (Vt) at the lowest possible pressure independently of changes in pulmonary mechanics (1).

APC is a very popular mode and readily available on various ventilators (fig. 1). What has made this mode so popular is that the practitioner can set a Vt & the flow is variable.

Figure 1: Ventilator Manufacturers names for Adaptive Pressure Control
Unlike traditional volume control ventilation with a fixed flow rate, APC’s variable flow rate provides superior inspiratory flow synchrony in the spontaneous breathing patient.

Even-though APC provides improved patient/ventilator flow synchrony it should be used cautiously and closely monitored in patients with increased inspiratory efforts (e.g. high metabolic rate, sepsis, hypercapnea, pain management/substance abuse withdraw issues). In these patients APC has specific asynchronies

in regards to not enough driving pressure.

Case Study

Overview: 5’-9” male patient who has failed extubation attempts & spontaneous breathing trials multiple times.

Spontaneous breathing trials (SBT) where performed using continuous spontaneous ventilation (CVS), with a PEEP of 5+ cmH2O, and a pressure support of 10 cmH2O was used to off set the imposed work of breathing (WOB) from the artificial airway and Heat Moisture Exchanger (HME).

Figure 2: Shows P0.1 -4.9  indicating increased WOB.

Figure two demonstrates the patients increased WOB during the SBT as indicated by the P0.1 of -4.9.

Figure 3: Compares RSBI & P0.1 trends
Figure three compares the Rapid Shallow Breathing Index (RSBI) to the P0.1, note the RSBI is less than 105 (93) which indicates potential extubation success, however the P0.1 ( -7.8) indicates the patients high WOB, and that the patient would most likely wear out if ventilator support was withdrawn. P0.1 provides the clinician with an additional diagnostic tool.

Rest Phase

After SBT’s the patient was rested in a APC mode to off-load the respiratory muscles. To adequately off-load and rest the respiratory muscles, peak pressures of 15-20 cmH20 is generally needed to provide significant support.

This patient had a high inspiratory drive and during the rest phase APC would wean the driving pressure down, to meet the Vt target.

The figures (4-6) demonstrate the weaning of the driving pressure from a initial pressure of 20 (25 PIP -5 PEEP = 20) to a driving pressure of 8 cmH20. This driving pressure of 8 (during the rest phase) is less then the pressure provided during the SBT (PS of 10 cmH2O). Also notice the WOB increasing when the driving pressure decreases (indicated by the P0.1).

Figure 4:  PIP of 25 & P0.1 of -3.6

Figure 5: PIP of 23

Figure 6: PIP of 13 & P0.1 of -5.0

One has to consider did this patient actually get enough off-loading during the rest phase?

Figure 7: Screen shot of ASV initiation, notice the increased driving pressure & lower P0.1

ASV Initiation

The patient was switched to Adaptive Support Ventilation which is considered “Optimal Control” a modality that uses multiple mathematical models to prevent tachypnea, prevent auto-PEEP, prevent excessive dead space ventilation, prevent high pressures, maintain a preset minimum minute ventilation, & fully ventilate in apnea or low respiratory drive. It also allows the patient to take control if breathing activity is within limits.

Figure 8: Shows 24 hour trending with the cursor scrolled over to measurements before ASV.


Evaluating the trending capabilities of the G5 ventilator we can review the patients respiratory status over a period of 1, 12, or 24 hours. In this twenty-four hour view four parameters were trended (EtCO2, Pinsp, P0.1, & RSBI). The screen is frozen & the arrow is scrolled back when patient was on a APC mode, showing lower driving pressures (10cmH2O) and a higher WOB (P0.1 -6.3).

Figure 9: Shows 24 hour trending with the cursor scrolled over to measurements ~ 15 hours ASV initiation

Reviewing the twenty-four hour trends, one notices that the driving pressure rarely dropped below the mid-twenties. The mathematical model prevented the ventilator from titrating the pressure down which would have resulted in excessive dead-space ventilation, increased WOB, and respiratory muscle fatigue. This review of trends confirms that when the patient was in a APC mode the premature decreasing of driving pressure most likely resulted in further respiratory muscle fatigue  & inappropriate off-loading of respiratory muscles during the patients rest phase.

APC & Sepsis a Grueling Combination

APC: Vigorous Inspiratory Drive 

[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