General Principles of Anaesthetics Flashcards
How do you estimate PaO2 of a patient?
PaO2 should be roughly equivalent to 10kPa less than the percentage inspired concentration of oxygen (FiO2) e.g., room air is 21% oxygen, so should be 11kPa PaO2.
What are the different airway devices for oxygen administration? (x4) Flow and FiO2?
- NASAL CANNULAE: 1-4L/min, 24-36% oxygen.
- HUDSON FACE MASK: 5-10L/min, 30-40% oxygen.
- VENTURI MASK: you can administer at different flow rates depending on coloured valves. Can administer from 2L/min to 15L/min, from 24% O2 to 60% O2.
- NON-REBREATHER MASK (photo): 15L/min, 85-90% oxygen. There is a bag on the mask with valves preventing from rebreathing expired air.
How do Venturi mask valves differ? (x5)
.
What is used for oxygen administration in COPD patients?
Venturi mask, where it is important not to over-oxygenate patients.
What examples are there of non-invasive ventilation?
CPAP and BiPAP.
When is CPAP indicated?
Splint the airways open in type 1 respiratory failure e.g., hypoxia.
When is BiPAP indicated?
Type 2 respiratory failure e.g., COPD exacerbation.
What must you ensure during oxygen administration other than relieving hypoxaemia? (x3)
Ensure adequate clearance of secretions and limit the adverse events of hypothermia and insensible water loss by use of optimal humidification.
What oxygen saturation should you aim for in oxygen administration? (x2)
94-98% in acutely unwell patients; 88-92% in COPD patients because of risk of hypercapnic respiratory failure.
Why do we aim for lower oxygen saturation in COPD patients? (x2)
COPD patients are at increased risk of hypercapnic respiratory failure. Administering oxygen increases this risk for TWO REASONS: OXYGEN-INDUCED VENTILATION PERFUSION: In normal physiological states, anoxia (no oxygen) is a potent stimulus for pulmonary vascular vasoconstriction leading to decreased perfusion of poorly ventilated alveoli. Providing therapeutic oxygen counteracts this mechanism and therefore increases perfusion to diseased lung where CO2 cannot be exchanged. This V/Q mismatch leads to increased PaCO2. AND THE HALDANE EFFECT: increased ability of deoxygenated haemoglobin to carry CO2, and thus in the presence of oxygen the dissociation curve for CO2 from Hb will shift to the right and PaCO2 will increase. Normally, this will cause increased minute ventilation leading to exhalation of CO2, but patients with severe COPD may not be able to increase their minute ventilation.
What are the pathological mechanisms of tissue hypoxia? (x8)
ARTERIAL HYPOXAEMIA: low inspired oxygen, alveolar hypoventilation (apnoea, opiate overdose), ventilation-perfusion mismatch (atelectasis), right to left shunts; FAILURE OF OXYGEN-HAEMOGLOBIN TRANSPORT SYSTEM: inadequate perfusion of tissues, low Hb concentration, abnormal O2 dissociation curve, histotoxic poisoning of intracellular enzymes e.g., cyanide poisoning or septicaemia. ARTERIAL HYPOXAEMIA: low inspired oxygen, alveolar hypoventilation (apnoea, opiate overdose), ventilation-perfusion mismatch (atelectasis), right to left shunts; FAILURE OF OXYGEN-HAEMOGLOBIN TRANSPORT SYSTEM: inadequate perfusion of tissues, low Hb concentration, abnormal O2 dissociation curve, histotoxic poisoning of intracellular enzymes e.g., cyanide poisoning or septicaemia.
Guidelines for oxygen dosing? (x3)
CARDIAC/RESPIRATORY ARREST: 100%, HYPOXAEMIA with PaCO2 less than 5.3kPa: 40-60%, HYPOXAEMIA with PaCO2 more than 5.3kPa: 24% (because of hypercapnic respiratory failure risk).
What are the indications for oxygen therapy? (x5)
- Cardiac/respiratory arrest
- Hypoxaemia (PaO2 less than 7.8kPa or Sats less than 90%)
- Hypotension (SBP less than 100mmHg)
- Low cardiac output and metabolic acidosis
- Respiratory distress (RR >24)
How does the mechanism of a high flow face mask differ from a low flow face mask?
High flow oxygen masks provide the entire ventilatory requirement.
How do high and low flow oxygen masks differ in function and purpose?
High flow delivers low oxygen concentrations but the total ventilatory requirement for a patient. Used in type 2 respiratory failure and COPD as they reduce the risk of CO2 retention. Low flow deliver higher concentrations of oxygen and used mainly in type 1 respiratory failure, but rebreathing may occur which increases risk of CO2 retention and low FiO2.
