1. Physiological comparison of spontaneous and positive-pressure ventilation in laryngotracheal stenosis 
S.A.R. Nouraei, D.A. Giussani, D.J. Howard, G.S. Sandhu, C. Ferguson, A. Patel 
British Journal of Anaesthesia 
Volume 101, Issue 3, Pages (September 2008)
DOI: /bja/aen171
Copyright © 2008 British Journal of Anaesthesia Terms and Conditions

2. Fig 1
(a) Endoscopic appearance of the normal trachea. (b–d) Endoscopic appearances of airway lesion in three of the patients from the present study.
British Journal of Anaesthesia  , DOI: ( /bja/aen171)
Copyright © 2008 British Journal of Anaesthesia Terms and Conditions

3. Fig 2
(a) Awake flow-volume loop obtained from a patient with extrathoracic airway stenosis showing significant diminution of the inspiratory component. (b) Anaesthetized flow-volume loop obtained from the same patient showing near-symmetrical inspiratory and expiratory components. (c) Awake peak expiratory flow (PEF) to peak inspiratory flow (PIF) measurements. During spontaneous ventilation, most extrathoracic intralumenal airway lesions selectively impair inspiratory airflow as a result of negative intratracheal pressure generated during inspiration. This elevates the PEF/PIF ratio.11 (d) Anaesthetized PEF/PIF ratios obtained from the same patients, showing a highly significant reduction in the physiological degree of airway obstruction with positive-pressure ventilation delivered through a laryngeal mask airway (P< ; Student's t-test). This is because with positive-pressure ventilation both inspiration and expiration generate positive intratracheal pressures, which ameliorate the stenosis. As such, the selective impairment to inspiratory airflow which is observed with the patient awake is minimized.
British Journal of Anaesthesia  , DOI: ( /bja/aen171)
Copyright © 2008 British Journal of Anaesthesia Terms and Conditions

4. Fig 3
(a) Correlation between anaesthetized expiratory tidal volume (TV) and awake peak expiratory flow (PEF). (b) Correlation between anaesthetized inspiratory TV and awake peak inspiratory flow (PIF). Both measurements were controlled for forced vital capacity (FVC) to take into account variability in underlying lung performances. The correlation between awake expiratory peak flow and anaesthetized expiratory TV is because expiration generates positive intratracheal pressure under both conditions of spontaneous and positive-pressure ventilation, and therefore affects the stenosis in the same way. On the other hand, inspiration generates negative intratracheal pressure which worsens the stenosis, whereas positive-pressure ventilation generates positive intratracheal pressure which ameliorates the stenosis. The difference in the impact of spontaneous and positive-pressure inspiration on the stenosis explains the lack of correlation between awake PIF and anaesthetized inspiratory TV.
British Journal of Anaesthesia  , DOI: ( /bja/aen171)
Copyright © 2008 British Journal of Anaesthesia Terms and Conditions

5. Fig 4
Schematic representation of spontaneous (a) and ventilator-driven positive-pressure ventilation (b) in the presence of extrathoracic laryngotracheal stenosis. R, resistance; C, capacitance; RM, respiratory musculature; VE, ventilator; R(stenosis), a variable resistor which has a higher resistance (darker colour) with spontaneous ventilation than with positive-pressure ventilation; R(airway), resistance to airflow through the remainder of the tracheobronchial tree; R(lung), resistance of lung tissue which is caused predominantly by resistance to stretching during inspiration. This resistance also stores the potential energy in the pulmonary capacitor [C(lung)] which is then released during, and in part drives expiration. R(thorax) denotes chest wall's resistance to movement, and the potential energy stored C(thorax) during inspiration is also a driver of expiration.
British Journal of Anaesthesia  , DOI: ( /bja/aen171)
Copyright © 2008 British Journal of Anaesthesia Terms and Conditions