Intrathoracic airways

Experimental data, from several investigators who measured intrathoracic particle deposition during mouth breathing, were summarized by Stahlhofen et al. (184). Figure 12 shows these data as a function of the aerodynamic particle diameter. Because large particles are effectively collected by the extrathoracic airways, intrathoracic deposition does not increase or decrease monotonically with particle diameter, but exhibits maximal values for particles with diameters between 4 and 8 pm. It appears to be lowest for particles smaller than 1 pm. 

An intrathoracic obstruction is more likely to allow gas flow during inspiration as the negative intrathoracic pressure generated helps to pull the airways open. As such, the inspiratory limb of the curve may be near normal. In contrast, the positive pressure generated during forced expiration serves only to exacerbate the obstruction, and as such the expiratory limb appears similar to that seen in obstructive disease. Both TLC and RV are generally unaffected. 

Naughton MT, Rahman M A, Hara K, Floras JS, Bradley TD (1995) Effect of continuous positive airway pressure on intrathoracic and left ventricular transmural pressures in patients with congestive heart failure. Circulation 91 1725-1731... 

The advantage of measuring airway resistanee is that it is a simple, rapid, and extremely sensitive test of airway caliber, but the major disadvantage is that it cannot distinguish between resistance changes due to obstruetion of the extrathoracic airway (pharynx, larynx, glottis, and trachea) or the intrathoracic airway (bronchi, bronchioles, and alveoli). 

The best way to determine the caliber of the intrathoracic airways is to utilize pulmonary tests measured at maximal flow rates such as forced expiratory volume in 1 second (FEVj) and maximal expiratory flow-volume curves. The forced expiratory volume maneuver requires a subject to inspire maximally and then exhale as hard as possible into a spirometer. The typieal... 

As with other obstructive pulmonary diseases (e.g., asthma), spirometric tests will indicate a decrease in FEVj and an increase in the total forced expiratory time (the total time required to exhale the entire vital capacity of the lung). In emphysema, all of these pulmonary function parameters are altered due to a loss of the elastic recoil properties of the lung and a collapse of the intrathoracic airways during forced expiration, both of these phenomena being caused by the destruction of the interstitial connective tissue. 

Airway resistance may increase because of toxic inhalational injury, resulting in increased work of respiration. Air trapping secondary to increased airway resistance increases intrathoracic pressure. Increased work of respiration and decreased venous return result in exercise limitation. Ventilation-perfusion abnormalities of disordered airway function limit oxygen delivery and carbon dioxide clearance, which also compromises exercise tolerance. 

Tests of pulmonary function provide objective, quantifiable measurements that are used for various purposes (232). The fact that forced expiratory maneuvers are relatively independent of the resistance of the upper airways and hence more sensitive to changes in intrathoracic caliber has made these measurements a popular means of assessing airway disease and response to aerosol therapy. 

Patients with functioning ectopic gastric mucosa will present with pain, bleeding and perforation. However, patients with intrathoracic foregut duplication cysts, mainly younger patients, will present with respiratory distress caused by compression of the airway. 

As a byproduct of work on cough CPR (Section 18.3.1.3 and Section 18.3.1.10), Rosborough found that using ventilation to increase intrathoracic pressure with simultaneous compression of the abdominal compartment maintained carotid artery flows similar to flows from standard CPR in dog experiments [36]. This model, based on a pure thoracic pump concept, has not been demonstrated to improve outcome in humans, and has been associated with pulmonary complications attributed to high airway pressures. 

A double-blind, randomized clinical trial over 7 days compared oral pure 1,8-cineole (3 x 200 mg/day) to Ambroxol (3 x 30 mg/day) in 29 patients with chronic obstructive pulmonary disease (COPD). Vital capacity, airway resistance, and speci c airway conductance improved signi cantly for both drugs, while the intrathoracic gas volume was reduced by 1,8-cineole but not Ambroxol. All parameters of lung function, peak ow, and symptoms of dyspnea were improved by 1,8-cineole therapy but were not statistically signi cant in comparison to Ambroxol due to the small number of patients. In addition to other properties, it was noted that the oxide seemed to have bronchodilatory effects (Wittman et ah, 1998). 

Reinhardt JM, D Souza ND, Hoffinan EA (1997) Accurate measurement of intrathoracic airways. IEEE Trans Med Imaging 16 820-827... 

Prolonging inspiratory time may recruit more slowly recruitable alveoli (24) and improve V/Q matching (24,25). The development of intrinsic PEEP from the short expiratory times can have similar effects to that of applied PEEP. Since long inspiratory times increase the total intrathoracic pressures, cardiac output may be affected. Finally, inspiratory-to-expiratory time ratio (I/E) > 1 1— known as inverse ratio ventilation—is uncomfortable, in the absence of airway pressure release ventilation (26,27). 

Another aspeet to eonsider beyond deep inspiration is the inspiratory apnea, assured through the glottie funetion. The deep inspiration stretches the airways, and increases the contraction force of the expiratory muscles as well as the retraction force of the lung parenehyma the inspiratory apnea (with glottic closure) facilitates the airway distribution to the most peripheral areas of the lung and increases intrathoracic pressure. 

Fig. 3.3.11. Pressure-dependent airways collapse in a patient with smoking-related emphysema. The flow cuts off abruptly after peak flow. The volume contained in the expiratory limb is far less than during inspiration, where the negative intrathoracic pressure keeps the airways open. (Flow in liters/second, expiration upwards, inspiration downwards, volume in liters)...

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