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Formula to work alveolar ventilation
Alveolar ventilation (measured in litres/min) is defined as (tidal volume - dead space) x respiratory frequency.
Describe the change of FEV1/FEV
Patients with restrictive disease have a lower FVC because of the restriction to airflow and subsequent decrease in lung volumes. Not all patients, however, have a lower FEV1 - because the airways are unaffected, they can usually expire air normally and quickly.
In a healthy adult, changes in arteriolar diameter ensure that blood flow in the lungs is proportional to ventilation. How does the same mechanism lead to pulmonary hypertension in someone with Chronic Obstructive Pulmonary Disease (COPD)?
The principle underlying this phenomenon is the response of pulmonary vasculature to hypoxia. Unlike systemic vessels, which dilate under hypoxic conditions to increase oxygen delivery to tissues, pulmonary vessels constrict. This response is simply the body’s way of matching ventilation to perfusion – the vasoconstriction ensures that blood flow to non-perfused alveoli reduces so that blood is diverted away from alveoli that cannot perform gas exchange.
In COPD, emphysema and chronic bronchitis lead to air trapping in the alveolar spaces, which produces a hypoxic environment. Pulmonary vessels vasoconstrict in response, which leads to increased blood pressure in the pulmonary circulation. Pulmonary hypertension can lead to changes in cardiac morphology because the right ventricle needs to generate greater force of contraction to overcome the increased pressure in the pulmonary circulation. The typical responses include right ventricular hypertrophy and congestive heart failure.
a) Jane is 8 months pregnant. Indicate how the following lung volumes would change: Increase / Decrease / No change (2 marks; 0.5 marks each)
Answers:
ERV: Decrease
RV: Decrease
TV: Increase
Respiratory rate: No change
Explanation:
Progressive uterine distension is the major cause of changes in lung volume parameters during pregnancy, including an elevation of the diaphragm. The growing uterus increases the abdominal pressure, which pushes the diaphragm upwards. While the rib cage expands horizontally to compensate for the smaller vertical dimensions, the overall compliance of the chest wall reduces as the chest cannot expand as freely as before.
The reduced chest wall compliance reduces the ERV and RV which leads to a subsequent decrease in TLC and FRC. Metabolic demand increases during pregnancy due to the growing foetus, which elicits an increase in minute ventilation. This increase is achieved by an increase in tidal volume while respiratory rate is unaffected.
Which of the following statements about a subject without lung disease but suffering from anaemia (Hb concentration roughly half “normal”) is INCORRECT?
If given oxygen to breathe, the oxygen content of arterial blood will more than double.
Which of the following provides the best “fit” for the following measurements made in an adult female; Haemoglobin concentration 12 g/dl; arterial PCO2 4.6kPa (35 mmHg); arterial PO2 6.8kPa (60mmHg)?
V/Q mismatching
How is blood flow affected in a person with chronic mountain sickness?
Chronic mountain sickness occurs in acclimatised individuals because of overcompensation by adaptive mechanisms. In these individuals, an increase in number of red blood cells (polycythaemia) to increase the oxygen-carrying capacity of the blood increases the haematocrit significantly. As a result of the increased blood viscosity (0.5 mark), the rate of blood flow decreases (0.5 mark).
Which of the following statements about the rate of alveolar ventilation is correct?
In a standing human, the bases of the lungs are better ventilated (per cc of lung tissue) than the apices
in healthy individual is the physiological or anatomical dead space more?
In any healthy individual the physiological dead space is little more than the anatomical dead space.
In which of the following are the vocal cords located?
The Larynx (voice box or Adam’s apple), is a valve that allows air into the lower airways but excludes liquids and solids. The walls of the larynx, trachea and bronchi are held open by plates or crescents of cartilage (a non-mineralised connective tissue, that is supporting but flexible).
What effect would Ipraprotium, a muscarinic antagonist, have on airway structures?
Topic summary
Upper airway structures are innervated by the vagus nerve, a major component of the parasympathetic nervous system. Acetylcholine released by this nerve acts on muscarinic receptors on airway smooth muscle cells, endothelial cells and submucosal glands to stimulate bronchoconstriction, vasodilation and mucous secretion. Inhibition of these receptors results in the opposite response – bronchodilation, vasoconstriction and reduced mucous secretion. Note that plasma exudation is a consequence of increased vascular permeability due to activation of a sensory nerve.
