wob.pptx

physiology of work of breathing

1. Work of breathing - During normal quiet respiration, work is done only during inspiration (during which respiratory muscles contract), whereas expiration is entirely a passive process. - At rest, work of breathing is about 1-2% of the total body energy expenditure. Even during heavy exercise, work is only about 3-5% of the total energy expenditure of the body.

2. Work of breathing can be divided into: 1- Compliance work (elastic work) 2- Tissue resistance work 3- Airway resistance work 65% of total work 7% of total work 28% of total work - It is the work required to expand the lung against its elastic force. - This can be calculated as follow: Compliance work = volume x pressure needed to inflate this volume - It is the work required to overcome the resistance of non-elastic tissue of the lungs and thoracic cage. - It is the work required to overcome airway resistance during the movement of air into the lung. - The major factor, affecting airway resistance is the diameter of the passage ways. The larger the diameter of the airway, the less work done.

3. Conditions associated with increased work of breathing: 1- Increased compliance work: Decreased elasticity (fibrosis). Lung congestion (heart failure). 2- Increased tissue resistance work: - Diseases affecting thoracic cage e.g. kyphoscoliosis . - Muscle disease. 3- Increased airway resistance work: Obstruction of air way. Chronic bronchitis. Asthma .

4. Inspiratory muscles provide the force necessary to overcome, a. elastic recoil of the lungs and chest wall b. frictional resistance i . caused by deformation of lung tissue and thoracic cage → tissue resistance ii. to air flow in the conducting airways → airway resistance

5. Resistance To Breathing 1. Elastic Resistance ~ 65% 2. Non-elastic resistance ~ 35% i. Airflow ~ 80% ii. Viscous ~ 20%

6. Elastic Resistance to Breathing Elastic Recoil of the Lungs The tendency of elastic lung tissue to recoil from the chest wall results in a sub-atmospheric intrapleural pressure. At FRC, the mean intrapleural pressure ~ 4-5 cmH20 sub-atmospheric The intrapleural pressure is normally estimated by an oesophageal balloon catheter This is more accurate in measuring changes rather than absolute pressure, due to interference from the weight of the heart

7. Compliance Definition : The ability of the lungs to expand is expressed using a measure known as the lung compliance. Lung compliance is the volume change per unit pressure change. It is represented as C = Δ V/ Δ P Static Compliance It is the relationship between volume change of lung and the transpulmonary pressure change, i.e. airway - intrapleural pressure change, measured under known static conditions (zero airflow) Reflects the elastic resistance of the lung and chest wall. Given by C ST = Corrected Vt / (Plateau Pressure- PEEP) The normal value for a 70 kg adult ~ 200 ml/cmH20 The value decreases as lung volume increases, due to the limitations of the non-elastic components of the lung/chest wall system.

8. Work Of Breathing Definition : It is the work required by the respiratory muscles to overcome the mechanical impedance to respiration. It is the sum of work requires to overcome both elastic and airflow resistance. There are two categories that the physical work of breathing can be broken down into. One type is resistance work in which an increase in resistance results in an increase in work. Compliance work is the other type of breathing work done. A decrease in compliance of the lungs requires an increase in work of them. Airway Resistance & Work of Breathing As given in the equation Raw = Δ P/V (Raw = Airway resistance Δ P = PIP-Pplat V= Flow ) The pressure change Δ P, can be treated as the amount of work imposed on the patient. Thus the work of breathing is directly proportional to the airway resistance and an increase in the airway resistance increases the work of breathing. If the work of breathing remains constant then an increase in the airway resistance will decrease the flow. In the clinical setting if the patient is unable to overcome the airway resistanc eby increasing the work of breathing then hypoventilation may result in decrease of the minute ventilation of the patient.

9. Compliance & Work of Breathing Since compliance is inversely related to pressure change, a decrease in compliance will result in increase in the work of breathing. In the clinical setting, atelectasis is one of the most frequent causes of increased work of breathing. If the change in pressure remains constant then the decrease in compliance will cause a decrease in the tidal volume and minute ventilation. Thus in low compliance situations such as ARDS the decrease in minute ventilation is characterized by low tidal volume and high respiratory rates In summary, the work of breathing can be increased by increased airway resistance, reduced lung compliance, or reduced thorax compliance .

10. Work of Breathing during Normal Respiration During normal quite breathing, respiratory muscles work during inspiration to expand the lungs, whereas expiration is a passive process. Normally lungs are highly compliant and airway resistance is low, so only 3% of total energy is used by the body during quite breathing. Clinical Application Work of breathing may be increased: 1. When pulmonary compliance is decreased.– more work is required to expand the lung. eg Pulmonary Fibrosis 2. When airway resistance is increased: more work is required to overcome the resistance. Eg. COAD.

11. Work of Breathing during Normal Respiration During normal quite breathing, respiratory muscles work during inspiration to expand the lungs, whereas expiration is a passive process. Normally lungs are highly compliant and airway resistance is low, so only 3% of total energy is used by the body during quite breathing. Clinical Application Work of breathing may be increased: 1. When pulmonary compliance is decreased.– more work is required to expand the lung. eg Pulmonary Fibrosis 2. When airway resistance is increased: more work is required to overcome the resistance. Eg. COAD.