Respiratory Failure Flashcards
What is hypoxaemia?
Low pO2 in arterial blood
<10.6kPa
What is hypoxia?
O2 deficiency at tissue level
What is normal O2 saturation?
94-98%
What is the normal range for PaO2?
10.6-13.3kPa
At what O2 saturation is tissue damage most likely?
<90%
At what pO2 is tissue damage most likely?
<8kPa
What is respiratory failure?
Impairment in gas exchange causing hypoxaemia +/- hypercapnia
Type 1 without
Type 2 with
What is type 1 respiratory failure?
- impairment of gas exchange causing hypoxaemia WITHOUT hypercapnia
- low PaO2 <8kPa or O2 sats <90%
- pCO2 normal or low
What is type 2 respiratory failure?
- impairment of gas exchange causing hypoxaemia WITH hypercapnia
- low PaO2 <8kPa AND high PaCO2 >6.5kPa
- respiratory pump failure
Signs of hypoxaemia
- impaired CNS function > confusion + irritability
- central cyanosis
- Tachypnoea
- tachycardia
- cardiac arrhythmias + cardiac ischaemia
- hypoxic vasoconstriction of pulmonary vessels (can cause pulmonary hypertension)
What are the oxygen stats of a patient with central cyanosis?
<85%
What are compensatory mechanisms to increase oxygen delivery (+ decrease hypoxia)?
- increased EPO secreted by kidney > raised Hb
- increased 2,3 DPG > shifts Hb/O2 dissociation curve to right > O2 released more freely
- increased capillary density
What can chronic hypoxic vasoconstriction of pulmonary vessels result in?
- Pulmonary hypertension
- Right heart failure
- Cor pulmonale: hypertrophy of right side of heart
Causes of hypoxaemia
- low inspired FiO2 e.g. high altitude
- V/Q mismatch
- diffusion defect (problems of alveolar capillary membrane)
- intra-lung shunt - acute respiratory distress syndrome
- hypoventilation
What is the most common cause of hypoxaemia?
V/Q mismatch
List some causes of V/Q mismatch
- asthma
- COPD
- pnemonia
- respiratory distress syndrome in newborn
- pulmonary oedema
- pulmonary embolism
Treatment of V/Q mismatch
- improves with O2 administration
- underlying pathology must be corrected for O2 to completely correct hypoxaemia
What happens to PaO2 and PaCO2 in V/Q mismatch due to inadequate ventilation?
- PaO2 falls
- PaCO2 rises
Blood equilibrates to new alveolar PaO2 + PaCO2
What is the V/Q ratio in a pulmonary embolism?
> 1
Due to poor perfusion
What does poor diffusion across the alveolar membrane cause in terms of respiratory failure?
- initally type 1 respiratory failure pCO2 normal or low + pO2 low
- as disease progresses, restrictive lung disease leads to hypoventilation > hypercapnia: pCO2 high (type 2 respiratory failure)
Causes of diffuse lung fibrosis
- idiopathic
- asbestosis: exposure to asbestos
- extrinsic allergic alveolitis
- pneumoconiosis: inhalation of dust particles
Reasons for intrapulmonary shunts
- collapsed alveolus
- fluid filled alveolus
no gas exchange in that area
Example of intrapulmonary shunt
Acute respiratory distress syndrome
What occurs in acute respiratory distress syndrome?
- loss of surfactant > causes alveolar atelectasis
- exudate in lungs
- lung becomes stiff (less compliant) > lung volume decreases
- loss of hypoxic pulmonary vasoconstriction mechanism
- intrapulmonary shunt due to no ventilation with respect to perfusion
Treatment to acute respiratory distress syndrome
- hard to manage on ventilator
- even 100% O2 may not correct hypoxaemia
- need to add positive pressure ventilation to open the alveoli
What is hypoventilation?
The entire lung is poorly ventilated due to inadequate respiratory rate or volume of alveolar ventilation
What does hypoventilation cause in terms of respiratory failure?
- ventilation reduced
- alveolar pO2 falls > arterial pO2 falls > hypoxaemia
- alveolar pCO2 rises > artieral pCO2 rises > hypercapnia
- Type 2 respiratory failure
Causes of acute hypoventilation
- opiate overdose
- head injury
- very severe acute asthma
Causes of chronic hypoventilation
Severe COPD
Is acute or chronic hypoventilation tolerated better?
Why?
Chronic
Slow onset + progression so there is time for compensation
Effects of acute hypercapnia
- respiratory acidosis
- impaired CN function > drowsy, confusion, flapping tremor, coma
- peripheral vasodilation >warm hands + bounding pulse
- cerebral vasodilation > headache
Why can treatment of hypoaxemia worsen hypercapnia?
-
Correction of hypoxia removes pulmonary arteriole hypoxic vasoconstriction
- this leads to increased perfusion of poorly ventilated alveoli
- blood diverted away from better ventilated alveoli
- V/Q mismatch worsens -
Haldane mechanism
- oxygenated haemoglobin has low affiliated for CO2 > CO2 dissociates from Hb into blood
How does pneumonia cause V/Q mismatch?
Exudate in affected alveoli
Reduced ventilation
What is atelectasis?
Partial collapse or incomplete inflation of lung
Effects of chronic hypercapnia
- respiratory acidosis compensated by retention of HCO3- by kidney
- vasodilation mild but still present
What is scoliosis?
Sideways curvature of the spine
What is kyphosis?
Excessive outward curve of spine
Abnormal rounding of upper back
What is kyphoscolosis?
Both scoliosis + kyphosis
Sideways curvature + excessive outward curve of spine
What conditions can cause hypoventilation?
- gullien barre syndrome
- kypohoscoliosis
- myasthenia gravis
- COPD + asthma due to muscle weakness/fatigue
- motor neurone disease
What is the mechanism explains why a patient with kyphoscoliosis can develop respiratory failure?
- Hypoventilation
- Due to inability to adequately expand the thoracic cavity during inspiration
- Cannot adequately decrease intrapulmonary pressure relative to atm
- inspiration is compromised
