Causes of Hypoxaemia and Hypercapnia Flashcards
How does insufficient ventilation cause hypoxaemia and tissue hypoxia?
→ Obstruction of airways
→ Failure to breath adequately
How does insufficient gas exchange cause hypoxaemia and tissue hypoxia?
→ Exchange surface dysfunction
→ V/Q mismatch
How does insufficient oxygen carrying capacity lead to hypoxia?
→ eg. there isn’t enough Hb
How does insufficient oxygen in the atmosphere cause hypoxia?
Not enough being exchanged
→ ability to extract oxygen from the blood decreases
What are the types of respiratory failure?
→ Type I respiratory failure involves low oxygen, and normal or low carbon dioxide levels.
→ Type II respiratory failure involves low oxygen, with high carbon dioxide.
Describe the PAO2, PaO2, PaCO2, and pH in type 1 respiratory failure
PAO2= normal PaO2= reduced PaCO2= normal pH= normal → increased A-a gradient
Describe the PAO2, PaO2, PaCO2, and pH in type 2 respiratory failure
PAO2= reduced PaO2= reduced PaCO2= increased(hypercapnia) pH= reduced
What are the causes of type 1 and type 2 respiratory failure?
TYPE 1:
↓ oxygenation
(altitude, VQ mismatch, pneumonia)
TYPE 2:
↓ ventilation
(asthma, COPD, NMD, drug overdose, apnoea)
→ inability to overcome increased resistance to ventilation
What is respiratory failure?
PaO2 < 8kPa when breathing air at sea level
What are the causes of hypoventilation?
→ Aphyxia, choking etc.
→ Obstructive sleep apnoea
→ Bonchial obstruction (asthma, chronic bronchitis, cystic fibrosis- excess mucus secretion)r
Problems with initiation or mechanics of breathing…
→ Drug overdose- neural impulses are impaired (typically opioids
→ Neuromuscular disease (muscular dystrophy, motor neuron disease)
→ Issues with lung mechanics (COPD, pulmonary fibrosis, NRDS, pneumothorax)
→ Stroke- blood supply is interacted
What are the causes of increased distance for diffusion?
↑ basement membrane thickness (fibrosis)
↑ thickness of fluid layer/oedema (pulmonary oedema, pneumonia)
→ Alveolar injury (ARDS)
What can cause reduced surface area for diffusion?
emphysema
What is the result of reduced perfusion?
increase in 𝑉̇/𝑄̇ ratio -‘dead-space effect’
→ Heart failure (cardiac arrest)
→ Blocked vessels (pulmonary embolism)
→ Loss/damage to capillaries (emphysema)
What are the results of reduced ventilation?
→ a decrease in 𝑉̇/𝑄̇ ratio -shunt effect
→ Pneumonia,
→ atelectasis
→respiratory distress syndrome
What is atelectasis?
the airways and air sacs in the lung collapse or do not expand properly.
What are the causes of insufficient oxygen carrying capacity?
→Iron deficiency (↓ production)
→Haemorrhage (↑ loss)
→Carbon monoxide poisoning (CO prevents O2-Hb binding):
What are the clinical signs/symptoms of respiratory distress?
→Shortness of breath, tachypnoea, dyspnoea
→Tachycardia- CV system tries to compensate for lack of O2 by increasing CO
→Laboured breathing, audible lung sounds- e.g. asthma, or pulmonary fibrosis
→Tiredness, drowsiness, loss of consciousness
→Fatigue (generally or upon exercise)- reduced oxygenation
→Cyanosis- due to ratio between oxy- and deoxy- blood
Effects of acute hypoxaemia…
→Clinical signs = dyspnoea (shortness of breath), cyanosis, fatigue, coma, seizure
→Acute = Hypoxaemia → tissue hypoxia (e.g. cerebral hypoxia) → organ failure → death
→Chronic = polycythaemia, hypoxic pulmonary vasoconstriction → pulmonary heart failure
→Reliance on anaerobic respiration which produces lactic acid
Effects of insufficient carbon dioxide removal:
→Clinical signs = dyspnoea, confusion, seizure, unconsciousness
→Hypercapnia → acidosis → organ failure + cardiac arrhythmia → death
What two main factors affect total arterial content?
→oxygen dissolved in plasma
→oxygen bound to the Hb
What influences oxygen dissolved in plasma?
composition of inspired air- eg altitude, O2 therapy
→alveolar ventilation
→oxygen diffusion between alveoli and blood
→adequate perfusion of alveoli
What influences alveolar ventilation?
rate and depth of breathing- eg drug overdose, stroke, neuromuscular problems
→lung compliance- eg fibrosis
→airway resistance
What affects oxygen diffusion between alveoli and blood?
surface area
→diffusion distance
What affects diffusion distance?
→membrane thickness- eg fibrosis
→amount of interstitial fluid - eg oedema
What affects oxygen bound to Hb?
→%saturation of Hb
→total number of binding sites- affected by CO poisoning
What affects %saturation of Hb?
PCO2
→pH
→2,3DPG
→temperature
