PULMONARY FUNCTION. VOLUMES AND CAPACITIES

Transcription

1 PULMONARY FUNCTION. VOLUMES AND CAPACITIES The volume of air a person inhales (inspires) and exhales (expires) can be measured with a spirometer (spiro = breath, meter = to measure). A bell spirometer consists of a double-walled cylinder in which an inverted bell filled with oxygen-enriched air is immersed in water to form a seal (Fig.1). A pulley attaches the bell to a recording pen that writes on a drum rotating at a constant speed. During inspiration, air is removed from the bell and the pen rises, recording an inspired volume. As expired air enters the bell, the pen falls and an expired volume is recorded. The resultant record of volume change vs. time is called a spirogram. Figure 1 Spirometer In this lesson, you will use an airflow transducer and the software will convert airflow to volume, thus approximating the volume reading of a spirometer. This is a much quicker method of obtaining lung capacity data; however, the disadvantage is that the recording procedure must be followed exactly for an accurate conversion from airflow to volume. There are four non-overlapping primary compartments of total lung capacity: Tidal volume Inspiratory reserve volume Expiratory reserve volume Residual volume Figure 2 Respiratory volumes and capacities(miller-keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition by Saunders, an imprint of Elsevier, Inc)

2 Tidal Volume (TV) is the volume of air inspired or expired during a single breath. When a resting person breathes normally, tidal volume is approximately 500 ml. During exercise, tidal volume can be more than 3 liters. Average values for male 600 ml, female 500 ml. Inspiratory Reserve Volume (IRV) is the volume of air that can be maximally inhaled at the end of a tidal inspiration. Average resting values IRV are approximately 3,300 ml in young adult males and 1900 ml in young adult females. Expiratory Reserve Volume (ERV) is the volume of air that can be maximally exhaled at the end of a tidal expiration. Average resting values ERV is approximately 1,200 ml in young adult males and 800 ml in young adult females. Vital capacity is the volume of air that is exhaled by a maximal expiration following a maximal inspiration. Residual Volume (RV) is the volume of gas remaining in the lungs at the end of a maximal expiration. In contrast to IRV, TV, and ERV, residual volume does not change with exercise. Average adult values for RV are 1,200 ml for males and 1,000 ml for females. Residual volume reflects the fact that after the first breath at birth inflates the lungs, they are never completely emptied during any subsequent respiratory cycle. Pulmonary Capacity is the sum of two or more primary lung volumes. There are five pulmonary capacities, which can be calculated as shown below: 1. Inspiratory capacity (IC) IC = TV + IRV Average values male 3.6 L, female 2.4 L 2. Expiratory capacity (EC) EC = TV + ERV 3. Functional Residual capacity (FRC) FRC = ERV + RV Average values male 2.4L, female 1.8L 4. Vital capacity (VC) VC = IRV +TV + ERV 5. Total Lung capacity (TLC) TLC = IRV + TV +ERV + RV Average values male 6L, female 4.2L Pulmonary volumes and capacities are generally measured when assessing health of the respiratory system but can vary with age, weight, height and sex of an individual or can be changed with pulmonary disease. When affected by disease or trauma, the lung volumes and capacities are altered to a certain degree, depending upon the severity of the disorder. For example, inspiratory capacity is normally 60-70% of the vital capacity. Pulmonary tests can show the effects of disease on function, but they cannot be used to give a diagnosis. However these tests do give valuable quantitative data, allowing the progress of a disease to be followed, or the response to a treatment examined. In this lesson, you will measure tidal volume, vital capacity, and expiratory reserve volume. Residual volume cannot be measured using a spirogram or airflow transducer. You will then calculate inspiratory and expiratory capacity for comparison to the average values. Next, you will compare your observed vital capacity with the predicted vital capacity. The following equations can be used to obtain the predicted vital capacities for men or women of your height and age. Vital capacities are dependent on other factors besides age and height. Therefore, 80% of the calculated values are still considered normal. Male V.C. = 0.052H A Female V.C. = H A Table 1 Equation for Predicted Vital Capacity Where VC is vital capacity in liters, H height in centimeters, A age in years.

3 Lung volumes that depend upon the rate at which air flows out of the lungs are termed dynamic lung volumes. There are various dynamic tests: Forced Vital Capacity test, and the Maximum Voluntary Ventilation test. The Forced Vital Capacity (FVC) is the volume of gas that can be exhaled as forcefully and rapidly as possible after a maximal inspiration. Normally FVC = VC, however in certain pulmonary diseases (characterized by increased airway resistance), FVC is reduced. Figure 3 Recording of FEV1 and FVC From the FVC test, we can also determine the Forced Expiratory Volume in 1 sec (FEV1), which is the maximum volume of air that can be exhaled in a 1 sec time period. Normally the percentage of the FVC that can be exhaled during 1 sec is around 80% (i.e. FEV1/FVC=80%). In the obstructed lung, respiration ends prematurely, thus increasing RV and FRC. In the restricted lung, volumes are small because inspiration is limited due to reduced compliance. Common causes of decreased lung compliance are pulmonary fibrosis, pneumonia and pulmonary edema. In an obstructive lung disease, airway obstruction causes an increase in resistance. During normal breathing, the pressure volume relationship is no different from in a normal lung. However, when breathing rapidly, greater pressure is needed to overcome the resistance to flow, and the volume of each breath gets smaller. Common obstructive diseases include asthma, bronchitis, and emphysema. A Figure 4 Disorders of lungs function A, B B The FVC test allows one to clearly distinguish between the two disease types. Notice in the obstructed lung (picture, below left), how FVC is smaller than normal, but also that FEV1 is much smaller than normal. This is because it is very difficult for a person with an obstructive disease (egg. asthma) to exhale quickly due to the increase in airway resistance. Thus, the FEV1/FVC ratio will be much lower than normal, for example 40% as opposed to 80%. In the restricted lung, the FVC is again smaller than normal, but the FEV1 is relatively large in comparison. i.e. the FEV1/FVC ratio can be higher than normal, for example 90% as opposed to 80%. This is because it is easy for a person with a restricted lung (e.g. fibrosis) to breathe out quickly, because of the high elastic recoil of the stiff lungs.

4 Figure 5 The peak expiratory flow rate (PEFR) is the maximal flow rate sustained by a subject for at least 10 mseconds expressed in liter per minute. This test checks lung functioning, and is often used by patients who have asthma. Common asthma symptoms include shortness of breath that worsens with activity, wheezing, and cough. The flow of exhaled air from the lungs may be restricted due to inflammation or congestion from excess mucous. The PEFR test is done with a peak expiratory flow monitor. This is a simple handheld instrument with a mouthpiece on one end and a scale on the other. A small plastic arrow moves when air is blown into the mouthpiece, measuring the airflow speed. Normal test results vary depending on: -anthropometric measurements -age and sex -malnutrition -environmental factors: smoking, pollution The results are most useful when compared to your past rates. Normal values: Use a chart for accurate values, depending on the variables above. In general, normal is L/min in women and L/min for men.

5 Estimated PEFR in Adult Females PEFR = (((Height x3.72) +2.24) - (Age x 0.03)) x 60 where Height in meters or (inches x ) Estimated PEFR in Adult Males PEFR = (((Height x5.48) +1.58) - (Age x 0.041)) x 60 where Height in meters or (inches x )