Impact of Circuit Configuration on Oxygen Delivery on a Homecare Ventilator: A Bench Model

Abstract

Background:

Homecare ventilators are used across an array of patients and diseases. Patient characteristics often determine best circuit configuration. We aimed to evaluate FIO₂ delivery of two commonly used circuits with a low flow O₂ source.

Methods:

An Astral 150 ventilator in PS mode with two 22mm circuits was tested: dual limb active flow circuit (AFC) vs. single limb passive leak circuit (PLC) (Figure 1). Flow was measured using PowerLab with pneumotachometers placed at the inspiratory limb and y-piece (AFC) and proximal to leak port and at the y-piece (PLC). An ASL 5000 simulated an actively breathing 5 kg infant with BPD: C = 2 mL/cm H₂O, R_in = 70 < R_out = 90 cm H₂O/L/s, URC = 0.2 L, frequency = 40 breaths/min, sinusoidal pattern. O₂ was bled in at 1 L/min increments from 1-6 L/min and FiO₂ measured at the y-piece. Ten consecutive breaths for each run were analyzed. A two-way ANOVA evaluated the interaction of circuit type and O₂ flow on FIO₂ and two-sample t-tests compared FIO₂ and flow between circuits.

Results:

A difference in FiO₂ as a function of O₂ flow and circuit type was observed, with significant interaction between the two (P < .001). Mean FIO₂ was lower at each O₂ flow using the PLC compared to the AFC (Table 1). When comparing O₂ delivery for both circuits with similar FIO₂ (1 L/min = 0.32, AFC and 4 L/min = 0.33, PLC) the L/min O₂/peak inspiratory flow (PIF) ratio from the inspiratory limb (AFC) or proximal to the leak port (PLC) was of similar magnitude (0.073 vs 0.083, respectively). This was the result of higher PIF proximal to the leak port (48.1 ± 0.1 L/min) vs. the inspiratory limb of the AFC (13.6 ± 0.1 L/min)(P < .001). The difference in PIF between the inspiratory limb and y-piece of the AFC was 0.62 ± 0.31 L/min vs. 37.5 ± 0.46 L/min (P < .001) between proximal to the leak port and the y-piece for the PLC.

Conclusions:

An active lung model highlights considerations that must be made when choosing the appropriate circuit for patients. Higher FIO₂ can be achieved at lower O₂ flow when using an AFC vs. PLC because of the high flow required through the proximal portion of the PLC to compensate for intentional leak. While there was a statistical difference between L/min O₂/PIF in the inspiratory limb (AFC) and proximal to the leak port (PLC), the ratios were of similar magnitude. Higher flow to compensate leak allows for delivery of prescribed settings; however, ability to achieve target FIO₂ may be compromised. Feasibility and sustainability of high O₂ flow must be weighed to optimize support, comfort, and safety.

Average FiO2 at each oxygen flow level for each given circuit configuration.

OXYGEN FLOW INPUT (L/MIN)	MEAN FIO2 ACTIVE FLOW CIRCUIT	MEAN FIO2 PASSIVE LEAK CIRCUIT
1	0.32	0.26
2	0.42	0.27
3	0.51	0.30
4	0.61	0.33
5	0.75	0.38
6	0.91	0.43

Figure 1. Dual limb active flow circuit pictured on top and single limb passive leak pictured on bottom with respective configuration of two pneumotachometers. Arrows indicate pneumotachometer placement.