Exhaled flow monitoring can detect bronchial flap-valve obstruction in a mechanical lung model

Characteristics of unilateral main bronchus obstruction and differentiation from chronic obstructive pulmonary disease by spirometry

Background

Pattern of flow-volume (F-V) loop in unilateral main bronchus obstruction (UMBO) is under-represented and sometimes misinterpreted as chronic obstructive pulmonary disease (COPD).

Methods

A cross-sectional study was performed among patients with UMBO and COPD confirmed by bronchoscopy, radiographic imaging and spirometry from 2006 to 2019. Data were extracted from electronic medical records. Spirometry data and flow-volume curves (F-V curves) were analyzed. Expiratory F-V curve was classified as monophasic or biphasic according to the absence or presence of breakpoint separating two distinct slopes. Propensity score method was used to reduce the selection bias, and logistic analysis in combination with decision tree approach was performed to explore the differences among groups.

Results

Fifty-six patients with UMBO, 121 individuals with COPD and 68 healthy subjects were included. Typical biphasic expiratory F-V curve was observed in 57.1% in UMBO group, especially of grade II (stenosis was 51–90%), and in 46.3% in COPD group, while biphasic inspiratory curve presented in 7.1% of UMBO, and none in COPD. In UMBO, breakpoints tended to appear gradually and smoothly between MEF₇₅ and MEF50, whereas in COPD they often occurred abruptly and rigidly, ahead of MEF₇₅.

Conclusions

The characteristics of F-V curve, apart from biphasic pattern, the location and configuration of breakpoint in expiratory curve, seemed to be important features of UMBO, which might help to differentiate them from COPD. More data is needed to validate these findings.

Changes in the flow-volume curve according to the degree of stenosis in patients with unilateral main bronchial stenosis

Objectives

The shape of the flow-volume (F-V) curve is known to change to showing a prominent plateau as stenosis progresses in patients with tracheal stenosis. However, no study has evaluated changes in the F-V curve according to the degree of bronchial stenosis in patients with unilateral main bronchial stenosis.

Methods

We performed an analysis of F-V curves in 29 patients with unilateral bronchial stenosis with the aid of a graphic digitizer between January 2005 and December 2011.

Results

The primary diseases causing unilateral main bronchial stenosis were endobronchial tuberculosis (86%), followed by benign bronchial tumor (10%), and carcinoid (3%). All unilateral main bronchial stenoses were classified into one of five grades (I, ≤25%; II, 26%-50%; III, 51%-75%; IV, 76%-90%; V, >90% to near-complete obstruction without ipsilateral lung collapse). A monophasic F-V curve was observed in patients with grade I stenosis and biphasic curves were observed for grade II-IV stenosis. Both monophasic (81%) and biphasic shapes (18%) were observed in grade V stenosis. After standardization of the biphasic shape of the F-V curve, the breakpoints of the biphasic curve moved in the direction of high volume (x-axis) and low flow (y-axis) according to the progression of stenosis.

Conclusion

In unilateral bronchial stenosis, a biphasic F-V curve appeared when bronchial stenosis was >25% and disappeared when obstruction was near complete. In addition, the breakpoint moved in the direction of high volume and low flow with the progression of stenosis.

Non-steady state monitoring by respiratory gas exchange

Traditionally, the study of CO2 and O2 kinetics in the body has been mostly confined to equilibrium conditions. However, the peri-anesthesia period and the critical care arena often involve conditions of non-steady state. The detection and explanation of CO2 kinetics during non-steady state pathophysiology have required the development of new methodologies, including the CO2 expirogram, average alveolar expired PCO2, and CO2 volume exhaled per breath. Several clinically relevant examples of non-steady state CO2 kinetics perturbations are examined, including abrupt decrease in cardiac output, application of positive end-expiratory pressure during mechanical ventilation, and occurrence of pulmonary embolism. The lesser known area of non-steady state O2 kinetics is introduced, including the measurement of pulmonary O2 uptake per breath. Future directions include the study of the respiratory quotient per breath, where the anaerobic threshold during anesthesia is identified by increasing respiratory quotient.

Can capnography detect bronchial flap-valve expiratory obstruction?

OBJECTIVE

We have previously shown in a mechanical lung model [1] that bronchial flap-valve expiratory obstruction results in sequential lung expiration, best detected by prolonged and low magnitude tracheal expired flow (V) from the obstructed lung. However, the normal expiratory resistance of clinical ventilation circuits might also generate prolonged, low value exhaled V, that could be confused with bronchial flap-valve obstruction. We reasoned that bronchial flap-valve obstruction would also cause sequential CO2 unloading from each lung and result in a biphasic tracheal capnogram.

METHODS

To test this hypothesis, we ventilated (VT, 650 ml; f, 10 br/min) a dual mechanical test lung, with each side connected to a separate alcohol-burning chamber. An airway adapter-monitor system measured airway V, P, PCO2, and FO2. The circumference of the diaphragm in a respiratory one-way valve was trimmed to generate unidirectional resistance to expiratory V. Measurement sequences were repeated after this flap-valve was interposed in the left "main-stem bronchus."

RESULTS AND DISCUSSION

During moderate or severe left bronchial flap-valve obstruction, left bronchial V was delayed so that the left lung anatomical dead space (devoid of CO2) mixed with normal right exhalate to depress the expiratory upstroke or early plateau of the tracheal capnogram. During severe obstruction, decreased perfusion of the left lung caused lower alveolar PCO2. Then, prolonged low V from the left bronchus also resulted in depression of the end of the tracheal alveolar plateau. In general, the low magnitude of bronchial V from the obstructed lung limited its effect on the tracheal capnogram and the best marker of sequential lung emptying during bronchial flap-valve obstruction may be late exhaled V without reduction in total tidal volume.

Measurement of pulmonary CO2 elimination must exclude inspired CO2 measured at the capnometer sampling site

OBJECTIVE

The pulmonary elimination of the volume of CO2 per breath (VCO2/br, integration of product of airway flow (V) and PCO2 over a single breath) is a sensitive monitor of cardio-pulmonary function and tissue metabolism. Negligible inspired PCO2 results when the capnometry sampling site (SS) is positioned at the entry of the inspiratory limb to the airway circuit. In this study, we test the hypothesis that moving SS lungward will result in significant inspired CO2 (VCO2[I]), that needs to be excluded from VCO2/br.

METHODS

We ventilated a mechanical lung simulator with tidal volume (VT) of 800 mL at 10 breaths/min. CO2 production, generated by burning butane in a separate chamber, was delivered to the lung. Airway V and PCO2 were measured (Capnomac Ultima, Datex), digitized (100 Hz for 60 s), and stored by microcomputer. Then, computer algorithms corrected for phase differences between V and PCO2 and calculated expired and inspired VCO2 (VCO2[E] and VCO2[I]) for each breath, whose difference equalled overall VCO2/br. The lung and Y-adapter (where the inspiratory and expiratory limbs of the circuit joined) were connected by the SS and a connecting tube in varying order.

RESULTS

During ventilation of the lung model (VT = 800 ml) with SS adjacent to the inspiratory limb, VCO2[E] was 16.8 +/- 0.4 ml and VCO2[I] was 1.1 +/- 0.1 ml, resulting in overall VCO2/br (VCO2[E] - VCO2[I]) of 15.7 +/- 0.4 ml. If VCO2[I] was ignored in the determination of VCO2/br, then the %error that VCO2[E] overestimated VCO2/br was 7.2 +/- 0.3%. This %error significantly increased (p < 0.05, Student's t-test) when VT was decreased to 500 mL (%error = 12.4 +/- 0.8%) or when SS was moved to the lungward side of a 60 mL connecting tube (VCO2[I] = 2.8 +/- 0.2, %error = 18.2 +/- 1.6) or a 140 mL tube (VCO2[I] = 5.9 +/- 0.3 mL, %error = 37.5 +/- 3.3).

CONCLUSIONS

When the SS was moved lungward from the inspiratory limb, instrumental dead space (VDINSTR) increased and, at end-expiration, contained exhaled CO2 from the previous breath. During the next inspiration, this CO2 was rebreathed relative to SS (i.e. VCO2[I]), and contributed to VCO2[E]. Thus, VCO2[E] overestimated VCO2/br (%error) by the amount of rebreathing, which was exacerbated by larger VDINSTR (increased VCO2[I]) or smaller VT (increased VCO2[I]-to-VCO2/br ratio).