Physiology
Respiratory
Regarding compliance, which of the following statements is CORRECT:
Answer:
Compliance changes at different lung volumes. Initially at lower lung volumes the compliance of the lung is poor and greater pressure change is required to cause a change in volume. This occurs if the lungs become collapsed for a period of time. At functional residual capacity (FRC) compliance is optimal since the elastic recoil of the lung tending towards collapse is balanced by the tendency of the chest wall to spring outwards. At higher lung volumes the compliance of the lung again becomes less as the lung becomes stiffer. At all volumes, the base of the lung has a greater compliance than the apex. Patients with emphysema have increased compliance. Compliance is affected by a person's age, sex and height.Lung Compliance
Physiology / Respiratory / Lung Mechanics
Last Updated: 11th April 2019
In order for inspiration to occur, the respiratory muscles must overcome the impedance offered by the lungs and chest wall, mainly in the form of frictional airway resistance and elastic resistance to stretching of the lung and chest wall tissues and the fluid lining the alveoli.
Compliance describes the distensibility or ease of stretch of lung tissue when an external force is applied to it. Like lung volumes, compliance is affected by a person's age, sex and height.
Static Compliance
The static compliance (CL) of the lungs is defined as the change in lung volume per unit change in distending pressure.
Compliance = ΔV/ΔP
The distending pressure is the transmural pressure difference across the lung, which equals alveolar - intrapleural pressure.
Alveolar pressure cannot easily be measured, but when no air is flowing, alveolar pressure must be zero (i.e. equal to atmospheric pressure). The transmural pressure is then equal to the intrapleural pressure.
Intrapleural pressure can be measured with an oesophageal balloon. The subject breathes in steps and measurements are taken while the breath is held and plotted as a static pressure-volume (P-V) curve. The static lung compliance is the slope of the steepest part of this static P-V curve in the region just above the functional residual capacity.
The pressure-volume curve demonstrates hysteresis where the inspiratory curve is slightly different from the expiratory curve even at the same volumes. This is because expiration is deemed a passive process due to the elastic recoil of the lung whereas there is a need to overcome surface tension forces when inflating the lungs.
Dynamic Compliance
Dynamic compliance is measured during continuous breathing and therefore includes a component due to airway resistance. The dynamic pressure-volume loop has a point at each end where the flow is zero: the slope of the line between these points is the dynamic compliance. In health, dynamic compliance is similar to static compliance but is some diseases it may be lower. Between the two zero flow points, the dynamic P-V loop appears fatter than the static P-V loop, as intrapleural pressure must change more to drive airflow. In fact, the area of the dynamic loop is a measure of the work done against airway resistance.
Factors Affecting Compliance
Compliance changes at different lung volumes. Initially at lower lung volumes the compliance of the lung is poor and greater pressure change is required to cause a change in volume. This occurs if the lungs become collapsed for a period of time. At functional residual capacity (FRC) compliance is optimal since the elastic recoil of the lung tending towards collapse is balanced by the tendency of the chest wall to spring outwards. At higher lung volumes the compliance of the lung again becomes less as the lung becomes stiffer. At all volumes, the base of the lung has a greater compliance than the apex.
Other factors increasing compliance:
- Old age
- Emphysema
Other factors decreasing compliance:
- Pulmonary fibrosis
- Pulmonary oedema
- Atelectasis
- Extremes of lung volumes
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- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
