what is a spirometer
- a device that measures the volume of air breathed in and out, and records the readings a a spirogram
- spirometry cannot determine all of the lung volumes and capacities but it can determine some measurements
what measurements can spirometry determine
- VT: tidal volume is the volume of air entering or leaving the lung during a single breath. at rest it is typically 500ml
- IRV: inspiratory reserve volume is the extra volume of air that can be maximally inspired above the resting tidal volume. at rest is it around 3000 ml
- IC: inspiratory capacity is the maximal volume of air that can be inhaled starting from the end of a normal expiration at rest. this is usually 3500ml (VT + IRV)
- ERV: expiratory reserve volume is the maximal volume of air that can be expelled starting at the end of a typical tidal volume. at rest is around 1000ml
- RV: residual volume is the volume of air remaining in the lungs after maximal expiration. at rest is usually around 1200ml (measured indirectly)
- FRC: functional residual capacity is the volume of air in the lungs at the end of normal passive expiration. usually around 2200ml (FRC=ERV+RV)
- VC: vital capacity is the maximum volume of air that can be expelled during a single breath following a maximal inspiration. around 4500ml (VC=IRV+VT+ERV)
- TLC: total lung capacity is the maximum volume of air the lungs can hold. usually around 5700ml (TLC=VC+RV)
- FEV1: forced expiratory volume in one second. (FEV1/FVC) or converted to a percentage. usually around 80%
explain obstructive lung disease
- they cannot exhale as much so their FEV1 is lower
- the FRC and RV are greater but the VC is smaller
- these trends would continue to worsen as the breath stacking continued
explain restrictive lung disease
- characterized by low lung volumes
- the absolute amount of air that can be exhaled in 1 second (FEV1) is reduced because the lungs are smaller
- the proportion of the FVC that can be exhaled (FEV1/FVC) is normal since there is no obstruction to airflow
explain using expiratory flow to assess lung function
- during pulmonary function testing, it is more common to use expiratory data than inspiratory data
- in a normal person, flow peaks around 7 L/s, then decreases in a linear fashion
- a person with obstructive lung disease start at higher lung volume, but they cannot achieve the flow rates of a non-diseased person and thy end at higher residual volume
- a person with restrictive lung disease starts off at a lower lung volume, also cannot reach normal peak flow rates, and ends up at a lower residual volume
explain expiratory flow and lung function
the limits on ventilation are during expiration and not inspiration, which is why expiration provides useful data bout pulmonary function
what is minute ventilation
the amount of gas breathed in one minute
what are the steps of how the alveolus changes during inspiration and expiration (step 1)
- in a typical adult person, with a tidal volume of 500ml, not all of the inspired air will reach the alveoli
- about 150 ml remains in the airways and cannot be used for gas exchange
- this means that only 350 ml of air reaches the alveoli
what are the steps of how the alveolus changes during inspiration and expiration (step 2)
when we exhale the tidal volume of 500ml, only 350ml of air is expelled and again 150ml of air is in the airways
what are the steps of how the alveolus changes during inspiration and expiration (step 3)
with each breath you are re-breathing the air of the anatomical dead space
explain alveolar ventilation
tidal volume has to be greater than the anatomical dead space so ventilation is more efficient with “deeper” breaths than a faster ventilation rate
explain the work of breathing
- during normal quiet breathing, the inspiratory muscles overcome the elastic recoil of the lung and airway resistance
- expiration is passive using the lungs recoil
- normally the lung is compliant and airway resistance is low so very little energy, less than 3% of total body energy, is expended during quiet breathing
- the tidal volume and repsiatory rate are optimized to minimize to work of breathing
explain low respiratory rates
- at lower respiratory rates, in order to maintain alveolar ventilation, the tidal volume must increase
- increasing the tidal volume means the inspiratory muscles are working harder, the elastic work of the lung is higher
explain high respiratory rates
when we increase respiratory rates, the tidal volume can decrease, which reduces the elastic work of the lung, but because you are moving more air, the flow-resistance work of the lung increases
what are the 4 conditions that increase the work of breathing
- decreased compliance
- increased resistance
- decreased elastic recoil
- increased demand for ventilation
explain how decreased compliance increases the work of breathing
- when the pulmonary compliance is decreased, the tidal volume decreases and the respiratory rate increases
- EX. pulmonary fibrosis, in which more work is required to expand the lungs due to scarring of lung tissues
explain how increased resistance increases the work of breathing
- the work of breathing is also increased when airway resistance is increased
- this is seen during COPD and asthma when more work is required to overcome the increased flow resistance
- there is a decrease in respiratory frequently, however tidal volume remains roughly the same
explain how decreased elastic recoil increases the work of breathing
- there is a decrease in elastic recoil leads to an increase in the work of breathing
- this is observed in emphysema when passive expiration alone cannot expel air from the lungs so the expiratory muscles are recruited
- there is a decrease in respiratory frequency, however tidal volume remains roughly the same
explain how the increased demand for ventilation increases the work of breathing
- the work of breathing is increased when there is an increased need for ventilation
- this occurs during exercise when there are increases in both tidal volume and respiratory rate
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