- Design: mechanical, electrical, software, safety
ABBU was designed to replace manual ventilation of a bag valve resuscitator when a conventional ventilator device is unavailable (Fig. 1A-B). ABBU features include VT (200-800 mL), RR (10-40 bpm), TI (0.5-1.5 s), and adjustable patient-initiated breath sensing (-1 to -20 cm H2O). ABBU can use low flow oxygen (5-15 L/min) from widely available sources (e.g. concentrators, hospital wall-source, tanks, and liquid oxygen reservoirs).
ABBU senses the patient inspiratory effort below a software-calculated pressure threshold to trigger a breath. Patient-ABBU synchrony is facilitated by clinician titration of the triggering threshold. Auto-cycling can be reduced by increasing the triggering threshold (e.g. more negative). Conversely, ineffective triggering is reduced by decreasing the sensing threshold (e.g. less negative). Patients unable to trigger breaths (e.g., weakness, neuromuscular blocker-induced paralysis, central apneas) receive mandatory breaths at the set VT, RR, and TI. Total RR will be determined by the patient-triggered rate and the set rate.
ABBU provides visual and auditory alarms for circuit blockage, air-leaks, low pressure (e.g. disconnection), high airway pressure (50-70 cm H2O), motor, and electric failure. The audible power loss alarm has a backup battery. A high visibility enclosure facilitates rapid troubleshooting of the circuit and motor-bag interface (Fig. 1B). If ABBU fails, clinicians can quickly open the enclosure to access the bag and provide manual ventilation (Fig. 1C). This capability is a key safety feature of the ABBU design.
Figure 1D shows the breathing circuit components used in animal experiments. The patient exhalation valve (CPR-2 bag, Mercury Medical, Clearwater, FL) includes a manual adjustable PEEP valve. The resuscitator bag (adult Ambu® Spur® II bag, AMBU Inc., Columbia, MD) is centered in a cradle and secured on both ends by an elastic cord inside the unit. The bag PEEP valve (Ambu Disposable PEEP Valve, 0-20 cm H2O size) is set to 0 cm H2O, and PEEP is adjusted manually on a second PEEP valve interfaced to the patient exhalation valve. Two pressure sensing lines (3/16” ID, 22mm OD AirLife connector) are used for circuit pressure monitoring and breath triggering assist. An FiO2 analyzer (MaxO2+AE, Maxtec, Salt Lake City, UT) was interfaced into the breathing circuit for all animal experiments.
A Michigan adult dual-lung simulator (Model 1600, Michigan Instruments, USA) and Ventilator Validation System (VVK100-SYS, BIOPAC Systems, Goleta, CA) was used to validate ventilator parameters. ABBU was tested at compliances of 20, 40 and 70 mL/cm H2O with resistances 5-50 cm H2O/(L/s). VT of 200, 400, 600 and 800 mL were tested across a range of compliances and resistance levels. For performance experiments, RR was set at 15 bpm and the PEEP was set at 15 cm H2O. Twenty breath cycles were collected for each measurement and processed to validate measures of VT, pressure, TI, RR, and confirm PEEP value.
Durability and VT stability of different brand resuscitation bags: AMBU (SPUR II, Ambu, Columbia, MD), HUDSON (RCI 5387, Teleflex, Morrisville, NC), MEDLINE (CPRM1116, Medline, Northfield, IL), Mercury (CPR-2, Mercury Medical, Clearwater, Florida) were evaluated on mechanical test lungs (SmartLung 2000, IMT Analytics, Buchs, Switzerland) at maximum RR (50 bpm) and TI of 0.5s continuously over 7 days. Cardone electric motors (Model 85-3024, Cardone Industries, Ontario, CA) were operated continuously for > 30 days to assess durability.
Animal procedures were approved by the Institutional Animal Care and Use Committee at the University of Texas Health Science Center at San Antonio. Studies were performed on 6 healthy female pigs (Berkshire, 112±5.8lb). Pigs were sedated via Telazol (4–8 mg/kg IM), Xylazine (1–2.2 mg/kg IM), and 3–4% Isoflurane, followed by endotracheal intubation and maintenance on 0.5–3% Isoflurane. Body temperature was kept in the normal range (38-39oC) by heated pad. Arterial pulse pressure was monitored by a micromanometer pressure sensor in the descending thoracic aorta. After collection of baseline blood samples, ABBU was connected to the proximal end of the endotracheal tube by a 90-degree adapter plugged into the breathing circuit, which included the FiO2 analyzer and side-stream ETCO2 analyzer (Fig.1D).
ABBU settings were changed in accordance with the experimental protocol. TI was kept constant (1s) during the entire experiment. Baseline testing was performed on healthy lungs, followed by testing on saline injured lungs. Neuromuscular paralysis was used as needed (vecuronium, IV, 0.1-0.2 mg/kg). Heart rate (HR), blood pressure (BP) and body temperature (rectal) were monitored continuously. Pigs were euthanized (Euthasol, IV, 100 mg/kg) following completion of experiments (6-8 hours).
Saline lung lavage was performed as previously described (19). In brief, warmed saline (30 mL/kg) was poured into the lungs through a funnel. As arterial pressure fell below 50 mm Hg, lavage fluid was drained passively. The animal was reconnected to ABBU with an O2 flow rate of 15 L/m and RR was adjusted to maintain arterial pH>7.25. Lavages were repeated until partial pressure of oxygen (PaO2) was < 100 mm Hg [13.3 kPa] for 30 min.
Arterial blood samples were analyzed by CG4+ cartridges (iSTAT analyzer, Abbott, IL, USA). Blood gas responses for different VT, RR and PEEP were compared with their respective baselines for normal and lung injury model. Parameters: FiO2 (%), HR (bpm), RR (bpm), ETCO2 (mm Hg), SpO2 (%), and BP (mm Hg), were recorded concurrent with blood sample collection.
Data in graphs is shown as mean ± SE. Two-tail T test and one-way ANOVA were used for all comparisons. A value of p < 0.05 was considered statistically significant. Pearson Correlation Coefficient (PCC) was computed to test correlation between two variables.