Asthma Flashcards
What are the characteristic symptoms of an asthma attack?
- Dyspnoea (shortness of breath)
- Wheezing
- Coughing
- Chest tightness
- The unexpected and sudden nature of the attacks can promote anxiety, which exacerbates the symptoms.
How is asthma diagnosed?
- Medical history
- Physical examination
- Objective measures of lung function by spirometry.
- Spirometry is used to measure the FEV1 (forced expiratory volume in 1 second) and PEFR (peak expiratory flow rate) – reductions in these values indicate airway obstruction.
- The reversibility of obstruction is diagnosed by rapid improvement in lung function after inhalation of a short-acting bronchodilator.
How can the severity of an asthma attack be determined?
- Clinical history
- First is the clinical history.
- A recent hospital admission or previous life-threatening attack suggests greater severity
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Intensity of symptoms
- Symptom intensity can be assessed by a clinical examination.
- Measurements include the general appearance of the patient and difficulty in speaking
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Lung function
- Thirdly, lung function can be assessed with spirometry.
- Spirometry is used to measure the:
- FEV1 (forced expiratory volume in one second)
- PEFR (peak expiratory flow rate).
- In addition, the FEV1/PEFR ratio can be calculated.
- Reductions in these values indicate airway obstruction, with the magnitude of reduction correlating with severity.
- The magnitude of the improvement in lung function following bronchodilator administration also indicates severity.
- Can be explained in terms of Poiseuille’s Law
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Blood gases
- Fourthly, arterial blood gas measurements are obtained as indicators of pulmonary ventilation
- These include PaO2, PaCO2, SaO2, and plasma pH
Outline the different grades of severity
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Moderate attack
- A moderate attack is indicated by:
- Worsening symptoms
- FEV > 50% of predicted
- Worsening symptoms
- A moderate attack is indicated by:
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Status asthmaticus
- An acute, severe attack, also known as status asthmaticus (SA), is indicated by:
- Dyspnoea severe enough to impair speech
- Accessory respiratory muscle activity (sternocleidomastoid, scalenes, abdominal muscles)
- Paradoxical pulse > 12 mmHg
- Heart rate > 120 bpm
- Breathing rate > 30 per minute
- FEV1 < 50% of predicted
- SaO2 < 91%
- The patient is hypocapnic (low PaCO2) and alkalotic (high blood pH) because of hyperventilation, which is a result of juxtacapillary receptor activation in response to lung oedema.
- An acute, severe attack, also known as status asthmaticus (SA), is indicated by:
-
Life-threatening attack
- In life-threatening SA, fatigue of the respiratory muscles due to prolonged effort leads to hypoventilation and CO2 retention resulting in:
- A rise in PaCO2 to normal.
- A fall in blood pH to normal.
- Other indicators include
- FEV1 < 30% of predicted
- Silent chest
- FEV1 < 30% of predicted
- In life-threatening SA, fatigue of the respiratory muscles due to prolonged effort leads to hypoventilation and CO2 retention resulting in:
-
Near fatal asthma
- In near fatal asthma, respiratory failure leads to a rise in PaCO2 and fall in plasma pH.
Outline the steps for the pathogenesis of asthma
- Triggers
- Sensitisation
- Inflammatory response on re-exposure
- Pathological consequences
- Late asthmatic response
- Airway Remodelling – long term consequence
Outline the triggers for asthma
-
Extrinsic asthma (atopic)
- Thought to be mediated by a genetic predisposition
- to produce immunoglobulin E (IgE) against common environmental aeroallergens resulting in a type I hypersensitivity reaction
- Type 1 hypersensitivity reaction increases the number of T helper 2 (Th2) cells and immunoglobulin E (IgE) in the airway mucosa.
- to produce immunoglobulin E (IgE) against common environmental aeroallergens resulting in a type I hypersensitivity reaction
- Atopy is defined as a genetic predisposition to produce immunoglobulin E (IgE) against common environmental aeroallergens such as dust mites, moulds, pollens and animal dander. About 80% of patients with asthma are atopic compared with 30% of the general population.
- Thought to be mediated by a genetic predisposition
-
Intrinsic asthma
- Unknown aetiology and is poorly understood.
- Triggering stimuli:
- Exercise
- Stress
- Cold air
- Infections
- Activation of the airway epithelium by pollutants / microbial infection leads to the release of IL-33, IL-25 and thymic stromal lympho-protein
- Cytokines activate dendritic cells that bind the allergens and present them to CD4+ T cells.
- DERP 1 produced by house dust mites cleaves occluding junctions and can be presented by CD4 T cells as well allowing access to adjuvants, stimulating an immune response
- This stimulates a TH2 cell response and increases cytokine secretion
- IL-4 and IL-13, which stimulate B cell IgE production
- IL-5, which stimulates locally recruited eosinophils
- This contributes to the inflammatory soup
- B-cells produce allergen specific IgE that binds to FcE receptors
- This leads to mast cell sensitisation
How is histamine released? What is its effect?
- Histamine released rapidly from pre-packaged granules
- Effect
- Vasodilation
- Mucus secretion
How are leukotrienes/prostaglandins released? What are their effects?
- Formed from arachidonic acid by COX enzyme activation
- Effect
- Bronchoconstriction
- Prostoglandins result in vasodilation
What are cytokines released in response to? What are their effect?
- TNF alpha & IL-4
- Local infalmmation
Discuss the pathological consequences of asthma
- Airway obstruction (decreased lumen diameter) is a consequence of:
- Bronchoconstriction
- Mucus accumulation
- Oedema
- Eosinophil recruitment
Explain bronchoconstriction
- Bronchoconstriction is generated by the action of inflammatory mediators on bronchial smooth muscle.
-
Histamine
- Histamine binds to Gq-linked H1 receptors on smooth myocytes, stimulating contraction via an IP3-mediated increase in [Ca2+]i.
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Other mediators
- Prostaglandins and leukotrienes also stimulate contraction via action on smooth muscle GPCRs.
- Leukotrienes also promote an increase smooth muscle mass that contributes to obstruction.
- Hyperreactivity is also mediated by IL-13
- Shifts the dose response curve of the smooth muscle to the left, so a smaller stimulus is needed for contraction
Explain mucus hypersecretion
- Mediator action on goblet cells leads to mucus hypersecretion and blockage of the small airways with mucus.
- The mucus is initially aqueous, but increases in viscosity as the disease progresses, making it harder for the mucociliary escalator to remove.
- Viscosity increases due to an increased sympathetic drive that increases mucin and a decreased parasympathetic drive that decreases water movement into the lumen
- IL-13, IL-5 and IL-9 from B cells stimulate goblet cell hyperplasia increasing production.
Describe mucolytics
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Mucolytics:
- Reduce the viscosity of the mucus, making clearance from the airway easier
- e.g. N-acetyl-cystine
- It acts by:
- Breaking down disulphide bones in mucoprotein complexes
- Antioxidant effect that reduces oxidative stress and mucus secretion
Explain oedema
- Oedema of the airway wall is a consequence of increased capillary permeability and vasodilation.
- It leads to swelling of the tissue that obstructs the airway.
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Dyspnoea
- Oedema also activates juxtacapillary receptors in the lung parenchyma, which feed in to the respiratory control system and evoke rapid and shallow breathing that gives the sensation of dyspnoea.
- Dyspnoea can result in a decreased PaO2 which is then countered by tachypnoea (exacerbated by anxiety and increased sympathetic nervous control)
- This results in a reduced PaCO2 and respiratory alkalosis (no CO2 retention at this early stage)
When does eosinophil recruitment occur?
- This acute pathology (referred to as the early asthmatic response) is followed by a late asthmatic response.
- It occurs 3-12 hours later
- Caused by the inflammatory processes activated during the acute phase
- Explain the mechanism of eosinophil recruitment.
- What secretes IL-13?
- What do eosinophils release?
- IL-13 is secreted by activated mast cells, TH2 cells and B cells to drive recruitment of eosinophils.
- Eosinophils release:
-
Major basic protein
- Induce epithelial apoptosis and epithelial damage
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Cationic protein
- Stimulates histamine release from mast cells
- Thus acts as a positive feedback loop – mast cells secrete cytokines that activate eosinophils, which secrete protein that degranulates mast cells
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Major basic protein
Describe evidence for airway remodelling as a long-term pathological consequence of asthma
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Cadaver analysis: Houston et al, 1953
- Selected patients who had died in asthma attacks with no other underlying cause of death
- Analysed their lungs with sections that were stained with H&E
- Showed that there was:
- Thickening of the airway wall
- Increased collagen and mucus
- Goblet cell metaplasia and hyperplasia
- Increased collagen and mucus
- Sub-mucosal fibrosis
- Cellular spirals of leukocytes
- Thickening of the airway wall
- This all pointed to the inflammatory nature of asthma, but the mediators for it were not yet known.
Describe epithelial desquamation - how it occurs and what it can contribute to
- Desquamation = skin peeling
- Shedding of outermost membrane of tissue
- Another important change is desquamation of the bronchial epithelium by the actions of:
- Major basic protein (MBP)
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Eosinophil cationic protein (ECP)
- From eosinophils
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Bronchial hyper-responsiveness
- Removal of overlying epithelial cells exposes subepithelial irritant receptors, C fibres and immune cells to irritants in the lumen, contributing to the bronchial hyper-responsiveness that is characteristic of asthma
- Loss of cilia
- It also increases the risk of infection that can exacerbate the asthma due to the loss of cilia to move mucus
Explain an experiment which shows that major basic protein contributes to bronchial hyper-responsiveness
- Experiment by Flavahan et al.
- Method
- Rings of guinea pig trachea were suspended between two steel wires connected to strain gauges to record tension.
-
Indomethacin (cyclo-oxygenase inhibitor) was administered
- Prevent the generation of prostaglandins and leukotrienes during the experiment that could interfere with tension
- Incubation of the trachea in solution containing MBP was associated with an increase in the tension generated by smooth muscle contraction in response to acetylcholine and histamine application.
- Mechanical removal of the epithelium produced a similar increase in tension, suggesting that the effects of MBP are mediated by epithelial damage

What is the consequence of remodelling? What does this highlight?
- Chronic remodelling presents a poor prognosis for asthma patients
- Studies show declines in FEV1 throughout life
- even in patients with asthma considered ‘well-managed’ by corticosteroid therapy [Copenhagen Heart Study]
- Studies show declines in FEV1 throughout life
- This highlights that current treatment of asthma (only symptom-based) is not sufficient and the chronic remodelling/inflammation should be targeted early for true disease-modifying effects.
- Asthma that is not well controlled can progress to COPD through the remodelling effects
List the long-term physiological consequences of asthma:
-
Increased airway resistance leads to decreased VQ
- Hypoxia
- Hypercapnia
- Respiratory acidosis
-
Decreased expiratory volume leads to lung hyperinflation
- Resulting in:
- Compression of the pulmonary vasculature
- Pulmonary hypertension
-
Cor pulmonale
- Abnormal enlargement of right side of heart
-
Cor pulmonale
- Resulting in:
-
Status ashtmaticus
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Explain how asthma can result in hypoxia and respiratory acidosis
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Increased airway resistance leads to an increase in force required to move air out of the lungs
- Manifests as wheezing and is a sign of the increased work of breathing
- This decreases the ventilation of some alveoli, decreasing VQ
- leading to VQ mismatch
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Ventilation-perfusion mismatch (V/Q) leads to reduced gas exchange, resulting in progressive:
- hypoxia,
- hypercapnia,
- respiratory acidosis.
- Lactic acidaemia can occur in some patients, this leads to decreased CNS activity and fatigue and fatal ventilatory depression
Explain how asthma can result in pulmonary hypertension
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Decreased expiratory flow leads to lung hyperinflation resulting:
- Compression of the pulmonary vasculature
-
Pulmonary hypertension
- Cor pulmonale
- Abnormal enlargement of right side of heart
- Cor pulmonale
- Increased risk of right heart failure
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Increased risk of pneumothorax
- due to a higher alveolar pressure
