Importance of airway closure in limiting maximal expiration in normal man

Airway occlusion assessed by single breath N2 test and lung P-V curve in healthy subjects and COPD patients

PURPOSE

To determine whether the analysis of the slow expiratory transpulmonary pressure-volume (P_L-V) curve provides an alternative to the single-breath nitrogen test (SBN) for the assessment of the closing volume (CV).

METHODS

SBN test and slow deflation P_L-V curve were simultaneously recorded in 40 healthy subjects and 43 COPD patients. Onset of phase IV identified CV in SBN test (CV_SBN), whereas in the P_L-V curve CV was identified by: a) deviation from the exponential fit (CV_exp), and b) inflection point of the interpolating sigmoid function (CV_sig).

RESULTS

In the absence of phase IV, COPD patients exhibited a clearly discernible inflection in the P_L-V curve. In the presence of phase IV, CV_SBN and CV_exp coincided (CV_SBN/CV_exp=1.04±0.04 SD), whereas CV_sig was systematically larger (CV_sig/CV_exp=2.1±0.86).

CONCLUSION

The coincidence between CV_SBN and CV_exp, and the presence of the inflection in the absence of phase IV indicate that the deviation of the P_L-V curve from the exponential fit reliably assesses CV.

Expiratory flow limitation

Expiratory flow limitation occurs when flow ceases to increase with increasing expiratory effort. The equal pressure point concept has been largely successful in providing intuitive understanding of the phenomenon, wherein maximal flows are determined by lung recoil and resistance upstream of the site where bronchial transmural pressure is zero (the EPP). Subsequent work on the fluid dynamical foundations led to the wave-speed theory of flow limitation, where flow is limited at a site when the local gas velocity is equal to speed of propagation of pressure waves. Each is a local theory; full predictions require knowledge of both density-dependent Bernoulli pressure drops and viscosity-dependent pressure losses due to dissipation. The former is dominant at mid to high lung volumes, whereas the latter is more important at low lung volumes as the flow-limiting site moves peripherally. The observation of relative effort independence of the maximal flow versus volume curves is important clinically insofar as such maneuvers, when carefully performed, offer a unique window into the mechanics of the lung itself, with little confounding effects. In particular, the important contributions of lung recoil and airways resistance can often be assessed, with implications and applications to diagnosis and management of pulmonary disease.

Immune responses and disease enhancement during respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is one of the commonest and most troublesome viruses of infancy. It causes most cases of bronchiolitis, which is associated with wheezing in later childhood. In primary infection, the peak of disease typically coincides with the development of specific T- and B-cell responses, which seem, in large part, to be responsible for disease. Animal models clearly show that a range of immune responses can enhance disease severity, particularly after vaccination with formalin-inactivated RSV. Prior immune sensitization leads to exuberant chemokine production, an excessive cellular influx, and an overabundance of cytokines during RSV challenge. Under different circumstances, specific mediators and T-cell subsets and antibody-antigen immune complex deposition are incriminated as major factors in disease. Animal models of immune enhancement permit a deep understanding of the role of specific immune responses in RSV disease, assist in vaccine design, and indicate which immunomodulatory therapy might be beneficial to children with bronchiolitis.

Factors determining performance of bronchodilator reversibility tests in middle-aged and elderly

Knowledge of the factors determining the performance of bronchodilator reversibility testing in a general population is lacking. Random samples of all adults aged 47-48 and 71-73 years living in Bergen, Norway, were invited to a cross-sectional study. Altogether 3506 subjects attended (69%). Test quality predictors were examined through multiple logistic regression analyses including gender, age, body mass, educational level, smoking history, respiratory symptoms, and in the elderly cohort cognitive level. Among the participants, 1.6% refused to inhale salbutamol, 2.5% failed the initial spirometry according to the ATS guidelines, and 1.3% failed the post-bronchodilator spirometry. Old age and body mass index > 30 kg/m2 were independent risk factors for an unsuccessful initial spirometry, and never smoking and respiratory symptoms were risk factors for failing the post-bronchodilator spirometry. Cognitive impairment in the elderly was a risk factor for failing both the initial- and post-bronchodilator spirometry. The median number of forced expirations was 7 in subjects obtaining an acceptable reversibility test. One third of these participants needed > or = 8 attempts, with independent predicting factors being old age, Little formal education and never smoking. Although reversibility testing becomes increasingly difficult with age, reliable data are obtained in a vast majority (94%) of subjects in our community study.

Assessment of the development of choked flow during total liquid ventilation

OBJECTIVE

The flow rate of a liquid drainage from the lungs is limited because of the elastic nature of the airways. This study was designed to clarify the relationship between intrapulmonary liquid volume and the development of the flow limitation or choked flow phenomenon as a function of expiratory flow rate during total liquid ventilation with perflubron.

DESIGN

Prospective animal study.

SETTING

University research laboratory.

SUBJECTS

Rabbits with a weight of 3.2 +/- 0.3 kg.

INTERVENTIONS

After the rabbits were killed, the lungs were filled to functional residual capacity with perflubron, followed by administration of an additional volume of 30, 45, or 60 mL of perflubron (initial volume = functional residual capacity + additional volume).

MEASUREMENTS AND RESULTS

In one set of five animals, the intratracheal pressure at the occurrence of choked flow was established at -20 mm Hg. In another set of six animals, we demonstrated that the volume remaining in the lung at the point of development of choked flow (Vch) was stable for the first 40 mins after the animals were killed. Flow rates of 1.25, 2.5, 3.75, 5.0, 7.5, 10.0, and 12.5 mL/sec were then applied at an additional volume of 30, 45, or 60 mL to 34 animals. Vch approximately doubled as the flow rate increased from 1.25 mL/sec to 12.5 mL/sec (p <.001). At the same flow, Vch was higher for an additional volume of 60 mL than 30 mL when the flow was > or =2.5 mL/sec.

CONCLUSIONS

From these data, we conclude that choked flow occurs at intratracheal pressure of less than -20 mm Hg, that Vch is stable for the first 40 mins after the animals are killed, and that Vch is a function of flow rate and initial volume.

Predictors of spirometric test failure: a comparison of the 1983 and 1993 acceptability criteria from the European Community for Coal and Steel

OBJECTIVES

To identify, in this general population study, predictors of spirometric test failure on the 1983 and 1993 acceptability criteria from the European Community for Coal and Steel (ECCS).

METHODS

All men aged 30-46 years living in western Norway (n = 45,380) were invited to join a cross sectional community survey. Respiratory symptoms, smoking habits, and marital status were found from self administered questionnaires, and measurements of forced expiratory volume in one second (FEV1) and forced vital capacity (FVC) were performed with dry wedge bellow spirometers.

RESULTS

Altogether 29,611 subjects (65%) participated in this survey. Spirometric recordings were obtained in 26,803 of these (91%) of whom 1.6% failed the 1983 criterion (< or = 300 ml between the two highest FVC values) and 9.5% failed the 1993 criteria (< or = 5% or 100 ml between the two highest FEV1 and the two highest FVC values). Spirometric failures on both criteria were more prevalent in never smokers, single men, and subjects with respiratory symptoms than in ever smokers, married, and asymptomatic subjects. Failure of the 1993 criteria increased with age and declining height. Morning cough and phlegm, breathlessness uphill, attacks of breathlessness, and wheezing were related to failure of the 1993 criteria after adjustment for demographic variables and smoking, whereas only breathlessness uphill was related to failure of the 1983 criterion.

CONCLUSIONS

In men aged 30-46 years, spirometric test failures on both the 1983 and 1993 ECCS acceptability criteria occurred more often in never smokers than in smokers and ex-smokers after adjustment for other covariables. Spirometric test failure with the 1993 criteria also varied with height and most respiratory symptoms. The higher failure rates found in non-smokers, in shorter, and in single men could be due to late compression of the airways, smaller lung volumes, and poor general health, respectively.

Airway closure and trapped gas during low volume breathing

We used a plethysmographic technique to estimate the trapped gas volume (TGV) in seven normal subjects after breathing air or O2 normally at functional residual capacity (FRC-air, FRC-O2) and near residual volume (RV) with small breaths (LVB-air, LVB-O2). If airway closure occurs, lung units subtended by closed airways would tend to collapse as TGV is absorbed; rapidly if the gas is O2 and slowly if it is air. Both inspiratory and expiratory quasi-static and static pressure-volume curves after LVB-O2 for 180 sec demonstrated a shift to the right and increased hysteresis, reduced vital capacity (VC) and significantly increased transpulmonary pressures at 25% and 50% control VC (PL25, PL50) as compared with those of FRC-O2 controls (P less than 0.01-P less than 0.001). Three VC breaths reversed these changes which suggested that they were related to atelectasis. The absolute plethysmographic RV level decreased and was also restored after three VC breaths. We used this reproducible decrease in RV level to represent the absorbed TGV (delta TGV). The delta TGV after LVB-O2 (180 sec) was significantly greater than that after LVB-air (P less than 0.001) or FRC-O2 (P less than 0.001). There was a significant correlation between the increase of delta TGV (DTGV) and the increase of PL25 (delta PL25) (P less than 0.01). The delta TGV after FRC-O2 was 128.6 +/- 50.3 ml (1.61 +/- 0.63% of TLC) and the maximal delta TGV after LVB-O2 (180 sec) averaged 504.3 +/- 40.5 m (7.5 +/- 0.37% TLC). The significant increase in delta TGV near RV during O2 as compared to air breathing is best explained by reversible atelectasis and further supports the concept of dependent airway closure at low lung volumes.

Repeatability of the moments of the truncated forced expiratory spirogram

It is proposed that if the spirogram is truncated for moment analysis, this should be done with respect to volume and not time. Errors are incurred when the moments of one spirogram are compared with those of another. These errors are maximal with no truncation and are reduced by truncation. A method is described for deriving sequential truncated moments of the forced expiratory spirogram. The repeatability and discriminatory power of the truncated moments were assessed over five consecutive days in 21 symptom-free subjects and were compared with conventional spirometric tests. The first and second moments about the origin of the spirogram (alpha 1 and alpha 2), the moment ratio (square root alpha 2/alpha 1) and the forced expiratory time to truncation (FET) are progressively less repeatable within individuals the later the truncation point. The discriminatory power of alpha 1 and alpha 2 and FET declines with later truncation but the discriminatory power of the moment ratio is maximal with truncation at 85% forced vital capacity (FVC) an diminishes sharply if truncation is beyond 95% FVC. At 75% FVC truncation alpha 1 is as good as FEV1% in discriminating between our subjects, whereas alpha 1 at 100% FVC is only half as good as FEV1%. The moment ratio at 90% FVC truncation is highly reproducible (mean within person coefficient of variation 2.1%), has important discriminatory power and is little influenced by events early in the spirogram (correlation with FEV1% r = -0.61, p less than 0.001). The moment ratio at 90% of FVC has attributes which may be useful in detecting early airway obstruction and its further study is warranted in order to establish its normal range and predictive value.