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98
82

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Physiology

Respiratory

Question 96 of 180

Compliance is decreased by all of the following EXCEPT for:

Answer:

Factors increasing compliance:
  • Old age
  • Emphysema
Factors decreasing compliance:
  • Pulmonary fibrosis
  • Pulmonary oedema
  • Atelectasis
  • Extremes of lung volumes (at higher lung volumes the compliance of the lung becomes less as the lung becomes stiffer)

Lung Compliance

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

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