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Basic respiratory physiology including oxygen therapy Dr Felix Woodhead Consultant Respiratory Physician.

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Presentation on theme: "Basic respiratory physiology including oxygen therapy Dr Felix Woodhead Consultant Respiratory Physician."— Presentation transcript:

1 Basic respiratory physiology including oxygen therapy Dr Felix Woodhead Consultant Respiratory Physician

2 Aerobic respiration Occurs in mitochondria Chemiosmotic synthesis of ATP Final sink for electrons O 2 consumed, CO 2 generated RQ = [CO 2 ]/[O 2 ] C 6 H 12 O O 2 6 CO H 2 O (RQ=1) C 16 H 32 O O 2 16 CO H 2 O (RQ = 16/23 = 0.7)

3 CO 2 Soluble Stored as HCO 3 - (catalysed by CA) Carboxyhaemoglobin Linear relationship C a CO 2 and pCO 2

4 O2O2 Much less soluble in water Most stored as oxyhaemoglobin Non-linear relationship (sigmoid) Flat top loading still occurs if P A O 2 Maintains good pressure gradient along alveolus Steep portion assists off loading in tissues Shifted to right by temp, acid and pCO 2

5 Acid-base: CO 2 H 2 CO 3 H + + HCO 3 - K` A = ([H + ] x [HCO 3 - ])/[H 2 CO 3 ] K A = [H + ] x [HCO 3 - ] /[CO 2 ] log K A = log [H + ] + log ([HCO 3 - ])/[CO 2 ]) - log [H + ] = - log K A + log ([HCO 3 - ])/[CO 2 ]) pH = pK A + log ([HCO 3 - ])/[CO 2 ])

6 Control of ventilation Acidity of brain stem Blood-brain barrier imperm to ions, perm to CO2 pCO 2 pH CSF few proteins pH changes quickly pCO 2 kept very steady pCO 2 implies ventilatory failure Gradual HCO 3 - by kidney to compensate Low pO 2 only relevant in chronic vent insufficiency

7 Partial pressures Total pressure = sum of partial pressures of constituent gases Atmosphere predom N 2 (80%) and O 2 (20%) p atm = pN 2 + pO 2 F i O 2 = fraction inspired O 2 = atm = 100 kPa pO 2 (at sea level) = 20 kPa

8 A-a gradient Idealised lung arterial pO 2 same as alveolar Alveolar pO 2 = inhaled pO 2 – used up O 2 Used up O 2 related to generated CO 2 by RQ p A O 2 = p I O 2 – (pCO 2 /R) A-a gradient = p A O 2 - p a O 2 Fudge factor for p A CO 2 + p A H Greater A-a gradient implies problem with lungs (V/Q mismatch)

9 V/Q ratio V/Q = 0 –Shunt –PaO2 mixed venous pO2 V/Q = –No flow –Gases approximate P A O 2 but ventilation wasted V/Q mismatching –always pO 2 (A-a gradient ) –pCO 2 may be normal as total vent

10 Partial pressure vs concentration In a perfect gas pp prop conc In a liquid, depends on solubility and chemical binding Pp like voltage determines diffusion Concentration like thermal energy depends on material

11 Oxygen carriage Depends on cardiac output, [Hb] and SaO 2 Once Hb saturated O 2 stored dissolved F i O 2 pO 2 ++ but only C a O 2 a little Possible to pO 2 without C a O 2 pO 2 affects rate of diffusion and control of breathing Cannot increase pO 2 of individual alveolar units by ventilation

12 Respiratory Failure pO2 < 8 kPa Type I –Normal/low pCO 2 –V/Q mismatch/diffusion limitation –Ventilation able to compensate Type II – pCO 2 –pH if acute –Ventilatory failure –Needs controlled O 2 ± ventilation

13 O 2 or ventilation Is there impaired ventilation? If there is impaired ventilation is it the only problem or is it part of multi-organ failure? Is invasive ventilation appropriate?

14 O 2 delivery systems High flow vs low flow systems Does flow rate exceed minute ventilation? Low flow rates, FiO 2 with minute vent Nasal specs comfortable and cheap, not for acutely unwell High flow FiO 2 independent of vent rate (Venturi) Non-rebreathing (reservoir bag) Higher pO 2 requires complete seal

15 Basic respiratory anatomy including radiology Dr Felix Woodhead Consultant Respiratory Physician

16 Lobes Heart on left 3 lung lobes on Right 2 lobes on left Left main bronchus deviates horizontally to miss heart RMB straighter (inhaled foreign body) Lingula (tongue) is analogous to RML, arises LUL Upper and lower lobes pyramidal Lower lobes one apical and four basal segs Oblique (major) fissure bilat (seen on lat), horizontal only on R (PA)

17 Radiological patterns 1 Pleural effusions/mass –Dense white with no air bronchograms –Meniscus –Beware the supine effusion Tumours –Spiculated & single (primary) vs round & multiple (mets) –May present as collapse Collapse –Volume loss (shift of fissure/hilum) –Obliteration of adjacent silhouette

18 Alveolar space filling ( consolidation) Fairly dense, with air bronchograms (patent airways) –Neutrophils ± microrganisms (pneumonia/organising pneumonia) –Eosinophils (eosinophilic pneumonia) –Blood (pulm haemmorhage) –Fluid (severe pulm oedema) –Surfactant (alveolar proteinosis) –Tumour (bronchoalveolar carcinoma – BAC)

19 Interstitial shadowing Less dense than consolidation, dots and lines on CXR (reticulonodular) –Interstitial fluid (pulm odema) –Trapped lymph (lymphangitis carcinomatosis) –Inflammation/fibrosis Interstitial lung disease Diffuse parenchymal lung disease Fibrosing alveolitis pulmonary fibrosis

20 An approach to chest radiographs Normal or abnormal? If abnormal, how long? If consolidation ?pneumononia –treat with antibiotics and repeat film 6-8/52 If interstitial ?pulm oedema –Treat with diuretics and repeat film in a few days Remember –Not all LRTIs are pneumonia (bronchitis/bronchiectasis) –Pulm oedema also seen with fluid resus and renal pts Long-standing shadowing ?diffuse parenchymal lung disease

21 CT radiology terminology Fleischner Society: Glossary of Terms for Thoracic Imaging Radiology ConsolidationDense, white opacity, obliterating vessels Ground glassLess dense, grey. Alveolar filling or fibrosis ReticulationThickened septal lines, usually indicates fibrosis HoneycombingCysts, usually basal and peripheral. Typical in IPF Traction bronchiectasis Airways in abnormal lung pulled apart. Indicates fibrosis Secondary pulm lobule Smallest part of lung surrounded by connective tissue. Central arteriole/bronchiole, periph venule/lymphatics/septal thickening MosaicismPatchy ground glass often of sec pulm lobules. Airways, vessels or interstitium

22 CXRs vs CTs CXR –Cheap –Quick –Good screen –Serial change –Low radiation dose CT –Better spatial resolution –Staging –Patterns of disease –?too sensitive

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