Lecture 14 - Hypoxaemia and resp failure Flashcards
Hypoxaemia
Low pO2 in blood
Hypoxia
Low O2 at body tissue level
Hypoxic without hypoxaemia
Severe anaemia - reduced carrying capacity of Hb but same dissolved pO2
Poor circulation - MI
O2 saturation and pO2
94% - 98%
9.3 - 13.3 kPa
Tissue damage
When pO2 less that 8kPa
Less than 90% perfusion
Maintenence of pO2
Normal inspired pO2 Normal alveolar ventilation Ventilation/ perfusion Normal alveolar capillary membrane Normal circulation
Causes if hypoxaemia
- Low inspired O2 - high altitude
- Hypoventilation
- V/Q mismatch
- Diffusion defect - problems with alveolar capillary membrane
- Acute respiratory distress syndrome
- Right to left shunt - cyanosis
Adaptations of people living at high altitudes
Polycythaemia - increased Hb
2,3 - bisphosphoglycerate
Increased capillary density in tissues
Effects of hypoxaemia
Confusion Irritability Agitation Cardiac arrythmias and cardiac ischaemia Central cyanosis - 50% O2 saturation Vasoconstriction of pulmonary arteries
Compensatory mechanisms to increase O2 delivery
- Increased EPO production by kidney - raised Hb (polycythaemia)
- Increased 2,3- BPG
- Chronic hypoxic vasoconstriction of pulmonary blood vessels
Chronic hypoxic vasoconstriction of pulmonary blood vessels
Pulmonary hypertension
Cor pulmonale
RSH hypertrophy and arrythmias
Type 1 respiratory heart failure
Low O2
Normal or low CO2
Type 2 respiratory heart failure
Low O2
High CO2
Hypoventilation
Always type 2 respiratory failure
Hypoxia and hypercapnia
Corrected with O2
Causes of acute hypoventilation
Opiate overdose
Head injury
Very severe acute asthma
Causes of chronic hypoventilation
Severe COPD Acute exacerbations of lower respiratory tract infections Lung fibrosis Severe obesity Scoliosis and kyphoscoliosis
Scoliosis
Sideways curvature of the spine
Kyphoscoliosis
Excessive outward curve of the spine and sideways curvature of spine
Hypercapnia
Causes by hypoventilation Respiratory acidosis - confusion - drowsiness - coma
Peripheral vasodilation - warm hands and bounding pulse
Cerebral vasodilation - headache `
Chronic hypercapnia consequences
Resp acidosis compensated by HCO3-
CNS creates a new CO2 set point
Vasodilation more mild - red face
O2 ventilation in Type 2 dangers
Treatment of O2 improves pO2 however removes the hypoxic resp drive for by peripheral chemoreceptors
Therefore can hypoventilate making pCO2 worse
Also removes hypoxic vasodilation of pulmonary artery branches so blood is diverted away from better ventilated alveoli
Oxygen therapy
Target saturation 88% - 92%
If causes rise in pCO2 - ventilatory support
Type 1 resp failure V/Q mismatch
- Ventilation decreases due to e.g. asthma
- pO2 drops and pCO2 increases initially
- Compensatory hyperventilation and shunting of blood to better diffused alveoli
- pO2 rises a small amount in well perfused alveoli as already saturated
- pCO2 decrease has a greater effect and corrects hypercapnia in normally perfused segments.
- Type 1 resp failure as low O2 and normal or low CO2 `
Causes of V/Q mismatch
Asthma Pneumonia RDS Pulmonary embolism Pulmonary oedema
Pulmonary embolism resp failure
Will cause type 1 resp failure
Poor diffusion across alveolar membrane
Affects O2 more and CO2 more soluble
Type 1 resp failure
Causes of poor diffusion across alveolar membrane
Fibrotic lung disease - thickened alveolar membrane
Pulmonary oedema - fluid in interstitial space increases diffusion pathway
Causes of lung fibrosis
Asbestos
Idiopathic
Extrinsic allergic alveolitis
Acute respiratory distress syndrome ventilation
Ventilation = 0
Still perfused
V/Q < 1 (=0)
Type 1 resp failure develops into type 2
