Recent advances in the use of high-resolution CT (HRCT) techniques have led to improved tools for the assessment of lung structure and function related to a number of clinically relevant questions, including airway reactivity. We have previously demonstrated the feasibility of electron-beam CT (EBCT) to objectively measure dose-related changes in airway geometry during IV methacholine-induced bronchoconstriction. These changes may influence more peripheral regional lung response to methacho-line, namely lung density, which in turn may allow us to analyze peripheral events beyond the CT spatial resolution of bronchial anatomy. In addition, the temporal aspects of methacholine-induced changes in both airway geometry and lung density have not been well defined. The purpose of the present study was twofold: (1) to use EBCT with HRCT as a noninvasive method to evaluate the time course of changes in airway geometry following a single IV bolus injection of methacholine to pigs, and (2) to assess associated changes in lung density. In order to determine the validity of lung density measurements made from HRCT scans, we also compared CT data reconstructed with “normal” and “sharp” algorithms.
Materials and Methods
Experiments were performed on pigs aged 5 to 7 weeks, with a mean (± SD) weight of 15.1 ± 1.2 kg. They were handled in accordance with national standards, and the protocol was approved by the Children’s Hospital of Philadelphia’s Animal Care and Use Committee.
Three pigs were anesthetized with sodium pentobarbital (30 mg/kg given intraperitoneally) and droperidol (2 mg IV every half hour). Additional anesthesia was administered as necessary judged by altered heart rate from baseline, arterial BP, and compliance with positive-pressure ventilation. After induction of anesthesia, a tracheostomy was performed using a shortened No.
6 endotracheal tube tightly bound in place. Fluid and drugs were administered via a jugular venous line. Animals were placed in a supine position and received ventilation with a programmable ventilator (model CTP-9000; CWE; Ardmore, PA) set for constant flow of room air. All animals were paralyzed IV with pancuronium bromide (100 |xg/kg initially and 75 to 100 |xg/kg supplementary), ensuring complete control over lung volume and apnea during scanning. Heart rate and arterial BP were monitored continuously. Respiratory measurements consisted of pressure at the airway opening (airway pressure [Pao]) and airflow. All signals were amplified using Gould amplifiers, passed through a 12-bit analog to digital converter on an NB-M10-16 h interface board (National Instruments; Austin, TX), sampled at a rate of 150 Hz, and stored on a personal computer running a programmable monitoring software package (Labview; National Instruments). The Pao pressure signals were also displayed and recorded on a Gould X-Y plotter (Gould Electronics; Eastlake, OH).
CT scans were obtained before (baseline) and after bolus IV injection of methacholine to pigs. In previous experiments in which we measured dose-response curves to methacholine in pigs, we determined that a single methacholine dose of 10— mmol/kg resulted in approximately 200% increase in peak Pao. We found also that this dose was tolerated by all animals without significant side effects and was thus used throughout the present study. Methacholine, freshly prepared before each experiment, was administered IV in 3 mL of saline solution followed by a 3-mL saline solution flush. We initiated the first postmethacho-line scan immediately following peak Pao response at approximately 30 s after the IV bolus. The EBCT scan protocol lasted approximately 25 s and was repeated at 2 min and 4 min after methacholine injection. This scan sequence was carried out seven times using three pigs. Each pig was studied on a separate day. Two scans were performed on the first animal, two on the second, and three on the third. There was at least a 1-h interval between the end of a study to the beginning of the next study in individual pigs, allowing all physiologic indexes to return to baseline values.
Tracings of Pao from a representative study shown in Figure 1 serve to demonstrate the protocol. To ensure a standard volume history, the ventilator was programmed to stop ventilation temporarily at end-expiration functional residual capacity (FRC), then inflate the lungs to a Pao of 25 cm H2O for 5 s, and then deflate the lungs to FRC for 5 s for two consecutive breaths after which ventilation was resumed. One minute later, ventilation was suspended at FRC for a baseline scan. Following the scan, ventilation was resumed; 1 min later, methacholine was administered. Ventilation continued uninterrupted for 30 s after injection. At that point, ventilation was stopped at FRC and a first postmethacholine scan was obtained. Ventilation was resumed at the end of scanning, and subsequent scans (without a standard volume history maneuver) were obtained at 2 min and 4 min after methacholine administration.
Figure 1. Tracings of Pao from a representative study. After a standard volume history, HRCT scans were obtained before, and 30 s, 2 min, and 4 min after IV injection of methacholine (MCh).