Lee Goldman MD, in Goldman-Cecil Medicine, 2020 The volume of air in the lung at any given time can be partitioned [Fig. 79-2]. The air that
remains in the lung after a maximal expiratory effort is the residual volume. The amount of air in the lungs at the relaxation point, when muscle effort is minimized and the inward recoil of the lung is balanced by the outward recoil of the chest wall, is the functional residual capacity [FRC]. The difference between FRC and residual volume is the expiratory reserve volume. The volume exhaled in a normal breath is the tidal volume. The volume that can be inhaled above tidal volume is the
inspiratory reserve volume. A series ofcapacities consist of the sum of two or more different volumes. FRC is the sum of expiratory reserve volume plus residual volume. Inspiratory capacity is the sum of tidal volume plus inspiratory reserve volume. Vital capacity is the sum of tidal volume plus inspiratory reserve volume plus expiratory reserve volume. Total lung capacity is the sum of residual volume plus expiratory reserve
volume plus tidal volume plus inspiratory reserve volume. Three of the volumes [tidal volume, inspiratory reserve volume, expiratory reserve volume] can be measured with a spirometer. Measurement of residual volume or any of the capacities that include it, so-called absolute lung volumes, requires more sophisticated methods, such as body plethysmography, the inert gas dilution technique, or the nitrogen washout technique. Body plethysmography, the preferred method for measuring lung volumes, is based on Boyle’s law: at a given temperature, the product of the pressure and volume of a quantity of gas at one time will be equal to the product of the pressure and volume of the gas at another time [P1 × V1 = P2 × V2]. The process of measuring lung volume by plethysmography consists of panting against a closed shutter
to compress and to rarify gas in the chest. The body plethysmograph, a sealed box in which the patient sits, measures the changes in lung volume during panting; pressure measured at the mouth represents the pressure changes within the lung during these volume changes. A similar panting maneuver with the shutter open is used to calculate airway resistance. Although body plethysmography is generally the most accurate method for measurement of lung volumes, particularly in patients with airway
obstruction, it can overestimate lung volumes if panting is too rapid. A plethysmographic total lung capacity greater than 150% of the reference value should be viewed with suspicion.Respiratory Testing and Function
Lung Volumes
Body Plethysmography
Inert Gas Dilution Technique
Lung volumes also can be measured by having the patient rebreathe from a device containing a known volume and concentration of an inert gas [e.g., helium, neon, argon, or methane], which does not react with elements in the blood or tissues, until equilibrium is achieved. The final concentration of helium equals the initial helium concentration times the initial volume of the device divided by the final volume of the lungs plus the device, adjusting for oxygen consumption and carbon dioxide production during the test. The equation can be solved for lung volume. This method underestimates lung volumes when portions of the lung communicate poorly with the central airways, particularly in patients with emphysematous bullae.
Nitrogen Washout Technique
The air that we breathe consists of approximately 21% oxygen, 1% argon, 0.04% carbon dioxide, and a variable amount of water vapor. The remainder is nitrogen. Exhaled air contains a lower concentration of oxygen, usually 14 to 16%, plus 3 to 5% carbon dioxide and water. For the nitrogen washout technique, the test subject inhales 100% oxygen beginning at FRC. As the subject breathes, exhaled gas is collected until the concentration of nitrogen reaches a plateau. Knowing that the initial concentration of exhaled nitrogen is approximately 75% and measuring the final concentration and volume of gases collected, the initial volume of gas in the lungs at FRC can be calculated. This method also underestimates lung volumes in patients with poorly communicating air spaces.
Lung volumes can also be measured from chest radiographs and computed tomography scans. The correlation among the measurement techniques is very good for people with reasonably normal lungs. In the presence of lung disease, however, each of the methods has limitations.
Absolute lung volumes as determined by body plethysmography or one of the gas dilution methods can be used to refine the spirometric evaluation of both obstructive and restrictive disorders. In obstructive disorders, air trapping or hyperinflation can be inferred from an increased residual volume, total lung capacity, or residual volume/total lung capacity [RV/TLC] ratio. If the total lung capacity is greater than 125 to 130% of predicted, hyperinflation is present. A residual volume or RV/TLC ratio greater than the upper limit of normal suggests air trapping. However, in subjects with chest wall limitation or neuromuscular weakness, residual volume may be increased—not because of true airway trapping but because of limitation to expiratory chest wall movement, so the termair trapping should be used with caution.
A restrictive disorder can be inferred from a spirometry pattern showing a reduced FVC with a normal or increased FEV1/FVC ratio. To confirm the presence of true restriction, lung volumes are required to demonstrate a TLC less than the lower limit of normal. If TLC is normal, the pattern is called a nonspecific pattern [see online supplement].
Lung Volumes and Airway Resistance
Joseph Feher, in Quantitative Human Physiology [Second Edition], 2017
Abstract
Spirometers can measure three of four lung volumes, inspiratory reserve volume, tidal volume, expiratory reserve volume, but cannot measure residual volume. Four lung capacities are also defined: inspiratory capacity, vital capacity, functional residual capacity, and the total lung capacity. Pulmonary ventilation is the product of tidal volume and respiratory frequency. The maximum voluntary ventilation is the maximum air that can be moved per minute. Spirometry also provides a measure of airway resistance by use of the forced expiratory volume test. The clinical spirogram presents the forced vital capacity differently. In laminar flow, pressure necessary to drive flow increases linearly with the flow. In turbulent flow, pressure increases with the square of the flow. The Reynolds number is used to estimate whether flow is laminar or turbulent. Airway resistance also increases inversely with lung volume because stretch of the lungs opens airways. Dynamic compression limits flow at high expiratory effort.
Read full chapter
URL: //www.sciencedirect.com/science/article/pii/B9780128008836000616
P
Cynthia C. Chernecky PhD, RN, CNS, AOCN, FAAN, in Laboratory Tests and Diagnostic Procedures , 2013
Usage.
Diagnosis and monitor the progress of pulmonary dysfunction [asthma, bronchitis, bronchiolitis obliterans, emphysema, and myasthenia gravis]; quantify the severity of known lung disease; evaluate the effectiveness of medications [bronchodilators]; determination of whether a functional abnormality is obstructive or restrictive; identification of clients at high risk for postoperative pulmonary complications; evaluation of the risk of pulmonary resection; used in conjunction with a cardiopulmonary exercise stress test for evaluation of functional ability; serial measurements used to evaluate response to treatment in cardiopulmonary vascular disease.
| Total Lung Capacity [TLC] = [VT + ERV + RV + IRV] | Overdistention of the lungs associated with obstructive disease | Restrictive disease |
| [Total volume of lungs when maximally inflated is divided into four volumes] | ||
| Tidal Volume [VT] | May indicate bronchiolar obstruction with hyperinflation or emphysema | May indicate fatigue, restrictive parenchymal lung disease, atelectasis, cancer, edema, pulmonary congestion, pneumothorax or thoracic tumor; decreased VT necessitates further testing |
| [Volume of air inhaled and exhaled in normal quiet breathing] | ||
| Inspiratory Reserve Volume [IRV] | n/a | Decreased IRV as an isolated value does not indicate disease |
| [Maximum volume that can be inhaled after a normal quiet inhalation] | ||
| Expiratory Reserve Volume [ERV] | n/a | May occur with obesity, pregnancy, or thoracoplasty |
| [Maximum volume that can be exhaled after a normal quiet exhalation] | ||
| Residual Volume [RV] | Increased RV above 35% of the TLC indicates obstructive disease; RV is also increased with aging | n/a |
| [Volume remaining in lungs after maximal exhalation] | ||
| Forced Expiratory Volume [FEV] | Restrictive disease | Decreased FEV1 after administration of beta-blockers may indicate presence of bronchospasm and contraindicate continued use of specific pharmacologic therapy involved |
| [Volume expired during specified time intervals [0.5 and 1 second]] | ||
| Forced Expiratory Volume 1 [FEV1] | Restrictive disease | Decreased FEV1 as percentage of vital capacity [FEV1/FVC] indicates obstructive disease: 65%-80% of predicted = mild disease 50%-65% of predicted = moderate disease Chủ Đề |