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AUTOMATED IN VIVO MEASUREMENT OF
QUASI-STATIC LUNG COMPLIANCE IN THE RAT
Laurent J. La Brie II, MSClinical Engineering Department University of Connecticut Health Center 263 Farmington Ave Farmington, CT 06030 (203) 679-2954 Joseph L. Palladino, PhDJoint Engineering & Computer Science DepartmentTrinity College Hartford, CT 06106 (203)297-2517 |
Joseph D. Bronzino, PhDJoint Biomedical Engineering Program Trinity College / Hartford Graduate Center 175 Windsor St (203) 297-1517 Roger S. Thrall, PhDPulmonary DivisionUniversity of Connecticut Health Center 163 Farmington Ave Farmington, CT 06120-1991 (203) 679-4118 |
Acknowledgment: Funded, in part, by State of Connecticut Apollos Kinsley Yankee Ingenuity Initiative Grant 91k011
For ABSTRACT, INTRODUCTION AND COMPLIANCE MEASUREMENT TECHNIQUES, click here
 |
SYSTEM DESIGN Figure 1 gives an overview of
the main system components.
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| Figure 1.
Schematic diagram of automated lung inflation-deflation valves. The
left-hand valves (L1-L2) provide animal lung inflation air. The
right-hand valves (B1-B3) provide air to the valves in the balloon valve
assembly. The balloon valve assembly (Fig. 2) serves to seal off the
animal airway and ventilator bias flow during a compliance test, and
subsequently to reinitiate ventilation. |
Plethysmograph An air tight plethysmograph
was constructed as described by Koo et al. (1976), with modification to
allow ventilation between compliance tests.
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Transistors
Three transducers were needed for this project to monitor airway pressure, esophageal pressure, and plethysmograph pressure. The airway differential air pressure transducer (Micro Switch,164PC01D37, Freeport, IL) has a linear range of zero to twenty-five centimeters of water pressure. The esophageal differential water pressure transducer (Micro Switch, 163PC01D26) has a range of (25 cm H2O. The negative range is necessary to ascertain the location of the end of the esophageal tube. When its end passes from the abdomen to the area dorsal of the pleural cavity, it crosses the diaphragm, yielding a pressure variation from negative to positive. ASenSym SCX=EB pressure transducer (SenSym , Inc., Sunnyvale, CA) was used to measure the plethysmograph pressure during the test directly related to volume. A high degree of sensitivity (0.02%) is necessary to measure a rat's lung volume change of 10 ml in the plethysmograph with volume of two liters.Ventilator
This system was designed to be used with an ultra-high frequency jet ventilator (Infrasonics, 1010 Ultra-Jet Model 5A San Diego, CA); other ventilator could be substituted. Ultra-high frequency jet ventilation is presently being investigated for use in treating patients with adult respiratory distress syndrome (Grant 1991, Gluck et al. 1993). Humidified bias airflow is provided to prevent tracheal necrosis during ventilation (Carol et al. 1984).Control of Airflow
 Figure 2. Schematic
representation of the balloon valve assembly. Mechanical ventilation is
connected to the animal endotracheal tube port, with humidified air
provided by the bias flow. During a compliance test the balloon valves
seal off the bias flow, and the lungs are inflated and deflated via the
pneumatic network in Figure 1. Ventilation then resumes automatically,
under logic control. |
balloon valve assembly (Hans
Rudolf, Inc. 8250 Kansas City, MO) provides air for both lung inflation
during compliance testing and humidified air during ventilation (See
Figure 2). The ventilator nozzle is inserted in a 4 mm hole in the
balloon valve assembly while the humidified bias flow enters through
the wide-bore ends. The ventilation antechamber, which open to the
lungs, is sealed during a compliance measurement.
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Air/Electronic Control Unit
Compressed air, provided by Thomas Industries, Inc. (Sheboygan, WI) model 1007CM72 air compressor is regulated by a network of solenoid valves under logic control. The compressed air channel bifurcates, one channel supplying the ventilator bias flow balloon valves and the other inflating the lungs of the rat. The air supplied to the balloon valves passes through valve B2. This valve charges a 2 cm3 reservoir and contains a pressure guage and pressure relief valve. The pressure relief valve further protects the valve balloons from rupture due to overinflation. The air then passes through valve and associated reservoir. This associated reservoir is a coiled tygon tube with constricting mechanism that can vary the size of the associated reservoir and thereby controlling balloon pressure. Balloon valve air is released through valve B3.System operation consists of phases 0-6 as follows:
 |
Phase 0: The resting phase of
the machine: Valve B3 is opened as a safety precaution to ensure that
ventilator bias flow balloon valves are deflated. Valve B2 opened to
charge the reservoir balloon valve. Figure 3 shows the logic state of
each valve for each of phases 0-6.
Phase 1: Valve B3 is closed. Valve R1 is opened to release any pressurized air between it and the flow control valve which would cause a transient pressure increase in a sealed airflow channel cavity. Phase 2: This wait state allows valve B3 time to close before valve B1 is opened. |
| Figure 3. Valve timing sequence described in the text. Valves B1-B3 and L1-L2 shown in Figure 1 are either open or closed. The bottom trace depicts animal airway pressure during a compliance test. |
Phase 3: Valve B1 is opened
to provide a bolus of air to the ventilator bias flow balloon valves and
to the calibrated air reservoir.
Phase 4:Valve B1 is closed. |
Phase 6-Lung deflation: Inflation
ends when the transpulmonary pressure attains a preset value (20 cm
H2O).
Valve R2 is closed to stop lung inflation. Valve B2 is closed to end
re-pressurization of the inflation of the inflation bolus volume. The
lungs now slowly deflate as air passively escapes through a 18guage hole
in the lung inflation tubing. Deflation periods over 10-12 seconds
minimizes pressure drop along the airway and therefore yields accurate
pressure volume curves (Koo et al. 196).
SYSTEM CALIBRATION
Calibration of the Airway Transducer
With the airway transducer open to atmospheric pressure, the output voltage is adjusted to zero. The airway transducer is then calibrated against a known applied constant pressure ranging from 0-25 cm H2O.
Calibration of the Esophageal Transducer
The water-filled esophageal tube is placed at the same horizontal level as the transducer membrane, and transducer output voltage is adjusted to zero. The transducer gain is then calibrated by raising the esophageal tube to a known, fixed height from 0-10 cm.
Calibration of the Volume Transducer
With the rat in position and the plethysmograph chamber sealed, the user adjusts the output voltage of the volume transducer to zero. The volume transducer is calibrated by applying a known, fixed volume of air from 0-10 ml into the plethysmograph chamber.
OPERATION
The designed equipment is fully automated for volume-pressure curve generation after initial calibration. When the equipment receives a start signal, the sequence previously described is executed. All output voltages are measured during testing using an x-y recorder (Houston Industries Omnigraphic 2000 Houston, TX)
RESULTS AND DISCUSSION
The system was initially tested on a latex balloon yielding a hysteresis compliance curve with quasi-static compliance calculated as 0.4 ml/cm H2O, the same order of magnitude as that of rat lung measured in our lab.
Animal Experiments
Adult male, pathogen-free, Fischer 244 inbred rats, obtained from Charles River Laboratories (Worthington, MA), at an average weight of 200 g, were used in this study. They were free of pulmonary disease and housed in isolation from other laboratory animals. After anesthesia with ketamine hydrochloride (75 mg/kg) and rompun (10 mg/kg) (Bristol Laboratories, NY) given intraperitoneally, each rat was tracheostemized and positioned in the plethysmograph. The same procedures were conducted for both manual and automated compliance measurements. Manual measurements were made by a trained lab technician. |
A total of seventeen tests
were run on there rats, yielding pressure-volume curves like that in
Figure 4. Mean lung compliance and standard deviation was
0.54±0.07 ml/cm H2O.This compares favorably
with mean values based on manual curve performed in our laboratory, 0.52
± 0.1 ml/cm H2O, in the rat (Grunze et al.
1988) and values reported by Koo and colleagues (1976) in the hamster.
Deflation curves for the automated system were smooth and ascertaining
the maximum slope was facilitated. The system was found to yield accurate, reproducible compliance measurements in the rat. Its ease of use obviates the need for highly trained lab personnel to perform these tests. An automated quasi-static lung compliance system for use with the rat was developed. With a larger air flow controller and plethysmograph, it may be adapted for use on medium- sized animals such as dogs and cats. Future modifications and adaptations may allow for its application in clinical medicine for certain types of human lung diseases. |
| Figure 4. Volume-pressure curves measured on the rat using the automated (A) and manual (B) methods of lung inflation and deflation. The curves are in close agreement both in magnitude and in shape. |
LITERATURE CITED
Carlo WA, Chatburn RL, Martin RJ, et al. Decrease in airway pressure during high-frequency jet ventilation in infants with respiratory distress syndrome. J Pediatr 1984;104;101-7.Cilley RE, Wang JY, Coran AH. Lung injury produced by moderate lung overinflation in rats. J Ped Surg 1883;28(3):488-93.
Cotes JE. Lung Function: Assessment and Application in Medicine. Cambridge, MA: Blackwell Scientific Publications 1993.
Gluck EH, Heard S, Patel C, Mohr J, Kalkins J. Use of ultrahigh frequency ventilation in patients with ARDS. Chest 1993:103:1413-20.
Grant EJ. Adaptation of ultra-high frequency jet ventilation and its physiological effects on the rat. M.S. Thesis 1991. Hartford Graduate Center. Hartford, CT.
Grunze MF, Parkinson D, Sulavik SB, Thrall RS. Effect of corticosteroids on lung volume-pressure curves in bleomycin-induced lung injury in the rat. Experimental Lung Research 1988;14:183-95.
Koo KW, Leith DE, Sherter CB, Snider GL. Respiratory mechanics in normal hamsters. J Appl Physiol 1976;46:29-42.
Petty C. Research Techniques in the Rat. Springfield, IL: Charles C. Thomas 1982.
Snider GL, Sherter CB, Koo KW, Karlinsky JB, Hayes JA, Franzblau C. Respiratory mechanics in hamsters following treatment with endotracheal elastase or collagenase. J Appl Physiol : Respirat Environ Exercise Physiol 1977;42:206-15.
Thrall RS, Phan SH, McCormick JR, Ward PA. The development of bleomycin-induced pulmonary fibrosis in neutrophil-depleted and complement-depleted rats. Am J Pathol 1981;76-81.
Thrall RS, Swendsen CL, Shannon TH, Kennedy CA, Frederick DS, Grunze MF, Sulavik SB. Correlation of changes in pulmonary surfactant phospholipids with compliance in bleomycin-induced pulmonary fibrosis in the rat. Am Rev Respir Dis 1987;114-8.
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