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Acute respiratory failure. Definitions. acute respiratory failure occurs when: pulmonary system is no longer able to meet the metabolic demands of the body hypoxaemic respiratory failure: PaO 2 60mmHg when breathing room air hypercapnic respiratory failure: PaCO 2 50mmHg kPa.
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Definitions • acute respiratory failure occurs when: • pulmonary system is no longer able to meet the metabolic demands of the body • hypoxaemic respiratory failure: • PaO2 60mmHg when breathing room air • hypercapnic respiratory failure: • PaCO2 50mmHg kPa
CO2 O2
Oxygen in • Depends on • PAO2 • Diffusing capacity • Ventilation • Perfusion • Ventilation-perfusion matching
Oxygen Carbon dioxide Water vapour Nitrogen
Oxygen Carbon dioxide Water vapour Nitrogen
Oxygen Carbon dioxide Water vapour Nitrogen
Oxygen in • Depends on • PAO2 • FIO2 • Alveolar pressure • PACO2 • Ventilation • Ventilation-perfusion matching • Perfusion • Diffusing capacity
V/Q relationships No. of lung units 1 Ventilation:perfusion ratio
Carbon dioxide out • Largely dependent on alveolar ventilation • Anatomical dead space constant but physiological dead space depends on ventilation-perfusion matching
Carbon dioxide out • Respiratory rate • Tidal volume • Ventilation-perfusion matching
Pathophysiology • Low inspired Po2 • Hypoventilation • Ventilation-perfusion mismatch • Shunting • Dead space ventilation • Diffusion abnormality
100% 75% PAO2=110mmHg PACO2=38mmHg
Pathophysiology • Low inspired oxygen concentration • Hypoventilation • Shunting • Dead space ventilation • Diffusion abnormality
PAO2=75 mm Hg PACO2=77 mm Hg PAO2=110 mm Hg PACO2=38 mm Hg
Pathophysiology • Low inspired oxygen concentration • Hypoventilation • Shunting • Dead space ventilation • Diffusion abnormality
75% 75% 100% 75% 87.5%
75% 75% 100% 75% 90%
Shunting • Intra-pulmonary • Pneumonia • Pulmonary oedema • Atelectasis • Collapse • Pulmonary haemorrhage or contusion • Intra-cardiac • Any cause of right to left shunt • eg Fallot’s, Eisenmenger, • Pulmonary hypertension with patent foramen ovale
Pathophysiology • Low inspired oxygen concentration • Hypoventilation • Shunting • Dead space ventilation • Diffusion abnormality
V/Q relationships No. of lung units Diseased Normal 1 Ventilation:perfusion ratio
Pathophysiology • Low inspired oxygen concentration • Hypoventilation • Shunting • Dead space ventilation • Diffusion abnormality
Clinical • Respiratory compensation • Sympathetic stimulation • Tissue hypoxia • Haemoglobin desaturation
Clinical • Respiratory compensation • Tachypnoea • Accessory muscles • Recesssion • Nasal flaring • Sympathetic stimulation • Tissue hypoxia • Haemoglobin desaturation
Clinical • Respiratory compensation • Sympathetic stimulation • HR • BP (early) • sweating • Tissue hypoxia • Haemoglobin desaturation
Clinical • Respiratory compensation • Sympathetic stimulation • Tissue hypoxia • Altered mental state • HR and BP (late) • Haemoglobin desaturation
Clinical • Respiratory compensation • Sympathetic stimulation • Tissue hypoxia • Haemoglobin desaturation • cyanosis
Pulse oximetry 90 Hb saturation (%) 60 PaO2 (mmHg)
Sources of error • Poor peripheral perfusion • Poorly adherent/positioned probe • False nails or nail varnish • Lipaemia • Bright ambient light • Excessive motion • Carboxyhaemoglobin or methaemoglobin
123 8040 87% HR=95
Summary • worry if • RR > 30/min (or < 8/min) • unable to speak 1/2 sentence without pausing • agitated, confused or comatose • cyanosed or SpO2 < 90% • deteriorating despite therapy • remember • normal SpO2 does not mean severe ventilatory problems are not present
Treatment • Treat the cause • Supportive treatment • Oxygen therapy • CPAP • Mechanical ventilation
Oxygen therapy • Progressive hypercarbia due to loss of hypoxic drive is RARE • Hypoxia KILLS • The appropriate response to progressive hypercarbia is assisted ventilation NOT removal of oxygen
Oxygen therapy • Fixed performance devices • Variable performance devices
Variable performance device 30 37% O2 Flow 6 l/min O2 6 0 Time
Variable performance device 24 l/min air 30 37% O2 6 l/min O2 Flow 6 0 Time
Fixed performance device 60% O2 30 l/min 60% O2 15 l/min air 100% O2 15 l/min
Other devices • Bag valve resuscitator
Other devices • Reservoir face mask
CPAP • reduces shunt by recruiting partially collapsed alveoli
Lung compliance and FRC • reduces work of breathing Volume Pressure