Understand the measurement of respiratory function
PreparationRead the handout for background on the respiratory system and measurement methods.
General functionThe primary function of the lungs is rapid gas exchange to deliver oxygen to the body and release carbon dioxide. This function is accomplished by the coordinated action of the lungs, the central nervous system, the diaphragm and chest wall musculature, and the circulatory system.
The entire blood volume of the body passes through the lungs each minute in the resting state, that is 5 liters per minute. The total surface area of the lung is about 80 meters square, equivalent to the size of a tennis court.
About 10% of the lung is occupied by solid tissue; the remainder is air and blood. The supporting structures are delicate to allow gas exchange, yet strong enough to maintain the structure. The lung can be divided into the conducting airways (dead air space) and the gas exchange portions.
MeasurementRespiratory function can be measured in a number of ways. Some devices directly measure volume of gases entering or leaving the lungs (spirometer) and record a plot of volume versus time (spirogram). Other devices, such as you will use, are pneumotachometers which measure flow rate from the lungs. Flow versus time is simply the derivative of volume versus time. A respiratory effort transducer measures respiratory effort by measuring the change in thoracic or abdominal circumference using a belt around the chest with an attached sensor which outputs voltage proportional to the change. These devices are useful for evaluation of respiratory function and patient monitoring.
PneumotachometerA pneumotachometer measures flow directly using a fixed calibrated resistance and a pressure transducer (MPX50) where the output voltage is directly proportional to the applied pressure. The flow across this resistance (a mesh or screen) is proportional to the pressure drop across the resistance. Numerical integration of the flow signal can be used to calculate the volume of air passing through the pneumotachometer.
Volumes and capacitiesLung capacity is divided into four volumes:
PathologiesAbnormalities of lung function can be divided into three types: 1) Restrictive ventilatory defects; 2) Obstructive ventilatory defects; 3) Defects in gas transfer. The causes of restriction could be stiff lungs (e.g. pneumonia, edema), stiff chest wall (e.g. rib fractures) or respiratory muscle weakness (e.g. polio, spinal injury). The causes of obstruction could be increased airway resistance (e.g. bronchitis), decreased lung elastic recoil (e.g. emphysema) or increased tendency for airway closure (e.g. asthma).
Pulmonary function tests are useful in determining the degree of impairment of lung function and are used in conjunction with the patients history and physical examination in order to formulate a clinical diagnosis. With obstructive defects, FEV1/VC is below normal. Vital capacity may also be below normal. With restrictive defects, vital capacity is below normal and FEV1/VC is higher than normal (as high as 100%). With mixed defects, vital capacity is below normal and FEV1/VC is below 70%.