Pamela K. Houtz

Effect of blood flow on capillary transit time and oxygenation in excised rabbit lung

We used an isolated perfused lung preparation of the rabbit to study the effect of increasing blood flow on pulmonary capillary transit time by two methods. In one method, capillary transit time was measured from fluorescent dye dilution curves from arterioles and venules of the subpleural microcirculation. Values of transit time were similar to those for the whole lung determined by dividing capillary blood volume by blood flow. Capillary transit times averaged 0.50-0.62 sec at a control blood flow of 80 ml min-1 kg-1 and decreased to 0.14-0.18 sec as blood flow increased to 6 times control. To determine whether the reduced transit time would limit O2 transport, we studied the effect of blood flow on oxygenation. Two isolated rabbit lungs were perfused in series. Blood from one lung deoxygenated by ventilation with a N2-CO2 mixture was oxygenated by the test lung ventilated with air. Ventilation was matched to blood flow. PO2 and PCO2 were measured in blood flowing into and out of the test lung. At all flows, no significant alveolar gas-to-end-capillary blood PO2 gradient (A-aDO2) was measured. The isolated perfused rabbit lung showed no transit time limitation to oxygenation for blood flows that are consistent with heavy exercise in vivo.

Frequency parameters from myographic signals for the evaluation of respiratory muscle activity during an increased ventilatory effort

Reliable estimation of respiratory muscle activity is especially important in patients with obstructive sleep apnea syndrome (OSAS). Changes in power spectral density (PSD) of electromyographic (EMG) and vibromyographic (VMG) signals are related with muscle activity and fatigue. We propose alternative frequency parameters to characterize these signals. They are based on the cumulative PSD and are more robust than classical parameters (mean and central frequencies) in the presence of white random and cardiac noise. A comparative study between OSAS patients and normal subjects during an increased respiratory effort is performed with both types of frequency parameters (FP) calculated for myographic signals from three respiratory muscles: genioglossus, sternomastoid and diaphragm. All the FP are obtained in auto and cross-spectral analysis. Both classical and alternative FP provide interesting information about VMG and EMG signals, respectively, in order to differentiate muscle function between normal subjects and patients.

Cardiac interference in myographic signals from different respiratory muscles and levels of activity

An interesting approach to study pulmonary diseases is the analysis of the respiratory muscle activity by means of electromyographic (EMG) and vibromyographic (VMG) signals. However, both signals are contaminated by cardiac activity reflected in electrocardiographic and cardiac pulse signals, respectively. Adaptive filtering and Singular Value Decomposition techniques were applied to reduce cardiac interference (CI) in signals recorded from three respiratory muscles (genioglossus, sternomastoid and diaphragm) in 19 subjects breathing against progressively increased negative pressure. The parameter Interference Relation (IR) is presented and its reduction with filtering is highly correlated with signal to noise ratio. This correlation indicates that IR is a good index to evaluate the level of interference. The Cl is highest at low levels of ventilation when the respiratory muscles are less active. Furthermore, the level of interference depends on the selected muscle: the most affected muscle is the diaphragm, then sternomastoid, and finally genioglossus. This order is preserved for both EMG and VMG signals. That indicates similar level of CI for signals reflecting electrical and mechanical muscle activity. The reduction of CI by means of the presented filtering techniques is shown by the parameter IR especially in EMG signals.

Airway constriction measured from tantalum bronchograms in conscious mice in response to methacholine

A single-projection X-ray technique showed an increase in functional residual capacity (FRC) in conscious mice in response to aerosolized methacholine (MCh) with little change in airway resistance (Raw) measured using barometric plethysmography (Lai-Fook SJ, Houtz PK, Lai Y-L. J Appl Physiol 104: 521-533, 2008). The increase in FRC presumably prevented airway constriction by offsetting airway contractility. We sought a more direct measure of airway constriction. Anesthetized Balb/c mice were intubated with a 22-G catheter, and tantalum dust was insufflated into the lungs to produce a well-defined bronchogram. After overnight recovery, the conscious mouse was placed in a sealed box, and bronchograms were taken at maximum and minimum points of the box pressure cycle before (control) and after 1-min exposures to 25, 50, and 100 mg/ml MCh aerosol. After overnight recovery, each mouse was studied under both room and body temperature box air conditions to correct for gas compression effects on the control tidal volume (Vt) and to determine Vt and Raw with MCh. Airway diameter (D), FRC, and Vt were measured from the X-ray images. Compared with control, D decreased by 24%, frequency decreased by 35%, FRC increased by 120%, and Raw doubled, to reach limiting values with 100 mg/ml MCh. Vt was unchanged with MCh. The limiting D occurred near zero airway elastic recoil, where the maximal contractility was relatively small. The conscious mouse adapted to MCh by breathing at a higher lung volume and reduced frequency to reach a limit in constriction.

Respiratory and muscular analysis in patients with OSAS at different levels of ventilatory effort

Respiratory muscles activity is especially important in patients with obstructive sleep apnea syndrome (OSAS). A comparative study between patients and normal subjects during two protocols of increased and maintained respiratory efforts is presented. Respiratory pattern is evaluated by means of expired ventilation (V), respiratory frequency (f) and volume. Despite V being similar between groups, f increases more in patients than in normal subjects with effort. Three respiratory muscles are evaluated by means of spectral analysis of myographic signals: genioglossus, sternomastoid and diaphragm. The evolution of frequency parameters during the protocols show differences between groups in the muscular function especially with medium and high levels of effort.