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Chest measurements
Physiology and Pharmacology
| Question | Answer |
|---|---|
| Converting between kPa and mmHg | kPa = to convert to mmHg, multiply by 7.5 mmHg = to convert to kPa, divide by 7.5 |
| Normal blood gas values | pH - 7.35 - 7.45 kPa CO2 - 4.5 - 6.0 kPa O2 - 11 - 13 kPa SaO2 - 95-98% Bicarbonate - 22 - 26 mM |
| Venous blood gas parameters | pH - 7.36 CO2 - 5.9 kPa O2 - 5.3 kPa SaO2 - 75% Bicarbonate - 23 mM |
| Arterial blood gas analysis | Needle inserted into radial artery Blood sample taken Placed into a computer which calculates the blood gases |
| Blood gases in acute hypoventilation | pH - 7.26 CO2 - 8 kPa O2 - 9 kPa SaO2 - 90% Bicarbonate - 26 mM |
| Causes of acute hypoventilation | Drugs - opiates, benzodiazepines, muscle relaxants Neurological - severe head injury, massive stroke Respiratory - infective exacerbation of COPD, severe pneumonia |
| Type 2 respiratory failure | Arterial blood gases remain close to alveolar Increased CO2 Decreased O2 Not enough gas moving |
| Type 1 respiratory failure | Due to hyperventilation Arterial CO2 remains close to alveolar Arterial O2 is different to alveolar Failure of gas exchange - particularly oxygen |
| Alveolar gas equation | PAo2 = PIo2 - (PAco2/R) Cannot measure PAco2 PAo2 = PIo2 - (Paco2/0.8) PIo2 = FIo2 x (Pb - Ph2o) = 20 kPa |
| Significance of alveolar - arterial oxygen gradient | Small - only ventilatory issues, no issue in gas exchange Large - issues with gas exchange |
| Blood gases in chronic hypoventilation | pH - 7.37 CO2 - 8.7 kPa O2 - 8.3 kPa SaO2 - 88% Bicarbonate - 36.6 Reduced lung compliance - non invasive ventilation used over night to increase pressure on inspiration |
| Causes of chronic hypoventilation | Weakness of breathing muscles Obesity - alone or combined with COPD |
| Pump model of respiratory system | Healthy - balance between capacity and load Decreased capacity e.g. NM weakness Increased load e.g. COPD, kyphoscoliosis, obesity Decreased drive e.g. respiratory depressents All shift to hypoventilation |
| What can reduce ventilatory drive | Chronic hypercapnia - buffering of decreased pH by bicarbonate decreases drive from central chemoreceptors Oxygen supplementation - removes hypoxic drive to breathe |
| Blood gases in ventilation perfusion mismatch | pH - 7.47 CO2 - 3.6 kPa O2 - 11.5 kPa SaO2 - 94% Bicarbonate - 20 mM Normal O2 with low CO2 - large P(A-a) gradient = issue with gas exchange |
| Causes of ventilation perfusion mismatch | Ventilatory causes - lung fibrosis, heart failure, pneumonia, inhaled foreign body Perfusion causes - large pulmonary embolus |
| V:Q scans | Radioactive isotopes inhaled or injected to view distribution in the lung Inhaled - ventilation Injected - perfusion |
| Blood gases in an extreme altitude climber | pH - 7.53 CO2 - 1.77 kPa O2 - 3.28 kPa - exchange limited by diffusion SaO2 - 54% Bicarbonate - 10.8 mM Even at a decreases Po2 climbers can maintain O2 transport due to an increase in haemoglobin - same amount transported |
| Davenport diagrams | Illustrate shift from normal in pH and bicarbonate Shows how we can compensate for changes in CO2 and bicarbonate to maintain pH e.g. increased CO2 in respiratory acidosis compensated for by increased bicarbonate by kidneys |
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