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Respiratory Sytem Part 4

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Simon Fraser University
Biomedical Physio & Kines
BPK 306
Mike Walsh

Respiratory Section #4 IV. PULMONARY VOLUMES, CAPACITIES, AND RATES A. Volumes and Capacities 1. Functional Residual Capacity i. Cannot be measured using just air in respirometer ii. Most common measurement: closed circuit method using helium (very low solubility in blood) 1. Known conc’n of helium (C1) is put in respirometer of known volume (V1) 2. After a normal expiration, air left in lungs is the FRC 3. Subject switched from breathing room air to breathing gas inside respirometer 4. After a few breaths, helium is well mixed with both lung air and respirometer air 5. Now measure Helium conc’n (C2) 6. FRC = (C1/C2 -1)V1 7. After finding FRC we can find RV (residual volume). ERV (Expiratory reserve volume) a. RV = FRC – ERV 2. Dead Space i. Anatomical dead space volume (conducting zone) most commonly determined by Fowler’s method. ii. Need rapid responding nitrogen analyzer and 100% O2 to breathe. iii. Method works b/c nitrogen conc’n changes very little during expiration and inspiration iv. Fowler’s Method: 1. Subject takes a breath of pure O2, then expires deeply. The first air expired is the last air in which is pure O2 w/ no nitrogen 2. Nitrogen conc’n begins to rise thereafter, reflecting beginning of expiration of alveolar gas. Doesn’t rise instantaneously b/c of some mixing of gas betw. dead space volume and alveolar volume at the interface as well as some anatomical anomalies. 3. Can determine dead space from nitrogen expiration trace. Can determine from volume of air corresponding to the hatched area. 4. Alveolar volume  V = A – VE D B. Rates 1. Maximal Breathing Capacity (MBC) i. Measurement actually a rate aka maximal voluntary ventilation (MVV) ii. Subject connected to respirometer. Ventilates as much air as possible in 12 sec (L/min) 1. Method not very useful b/c optimal combination of breathing freque
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