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Respiratory function

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Respiration

  1. Cellular: process of converting glucose into energy (can be aerobic or anaerobic)
  2. Physiological: process of gas exchange

 

Control of respiration

  1. Cerebral cortex - voluntary control
  2. Brainstem - pons and medulla: autonomic control
    • Medullary respiratory centre (Reticular formation)
      1. Dorsal group: inspiration
      2. Ventral group: expiration
    • Apneustic area - prolongs inspiratory phase
    • Pneumotaxic area - Inhibits inspiratory area - "fine tunes" respiratory
  3. Chemoreceptors
    • Central: ventral surface of medulla - sensitive to PaCO2 (which diffuses across BBB as H+)
    • Peripheral: carotid/aortic bodies - sensitive to PaO2, pH, PaCO2
  4. Mechanoceptors
    • Pulmonary stretch receptors (Hering-Breuer inflation reflex - distension leads to slowing of inspiration/increase expiratory time)
    • J-receptor (located airways close to capillaries) - stimulate respiration following increase in pulmonary blood flow

 

Oxygen dissocation curve

  1. Sigmoidal curve: Progressive co-operative binding of oxygen
  2. Bohr effect = Right shift of curve (reduce oxygen affinity)
    • Acidosis
    • Increased temperature
    • DPG
  3. Fetal ODC - right shifted (has higher affinity) to extract maternal blood

Pulse Oximetry

  1. Measures haemoglobin saturation and pulse rate
  2. Works on principle of spectrophotometry - differing amount of light absorbed by saturated and unsaturated Hb molecules
  3. Sources of error (1) poor peripheral perfusion (2) unreliable below 70% sats (3) Ambient light (4) Nail varnish/pigments - jaundice (5) irregular cardiac rhythms

Gas diffusion

  1. Fluid lining alveoli
  2. Alveolar epithelium
  3. Interstitial space
  4. Basement membrane of capillary endothelium
  5. Capillary endothelium
  6. Plasma
  7. Red cell membrane 

 

 

Oxygen delivery

Equivalent to total oxygen capacity of blood x cardiac output

DO2 = [(Hb x sats x 1.34)1 + (0.03 x PaO2)2] x Cardic output = 200ml/L arterial blood

 

Oxygen content is determined by

  1. Bound to Hb 99%
    • 1.34ml/g oxygen carried by haemoglobin
  2. dissolved in solution 1%
    • Henry's Law = Gas content = product of solubility and partial pressure of gas
    • Oxygen dissolved = 0.03 x PaO2

Incremental drops in pO2 from the atmosphere to blood

Alveolar-Arterial gradient:

  • Increased in "lung" pathology- VQ mismatch
  • Normal in mechanical failure

Alveolar gas equation

PaO2 = PiO2 - PaCO2/R

PiO2 = Inspired PO2
R = Respiratory exchange ration (0.8)

 

Oxygen therapy

  1. Variable performance
    • Nasal cannulae
    • Face mask (Hudson)
  2. Fixed performance
    • Venturi mask
    • Reservoir bag
    • Oxygen tent
    • CPAP
    • Invasive ventilation

Complications of Oxygen therapy

  1. Loss of hypoxic drive
  2. Absorption atelectasis (due to loss of splinting)
  3. Oxygen radicals
    • Direct pulmonary injury - irritates mucosa, loss of surfactant, progressive fibrosis
    • Retinopathy - retrolenticular fibrosis
  4. Risk of fire / explosions

 

Haemoglobin structure

  • Haem component + 2alpha + 2beta chains
  • Fe2+ in protoporphoryn ring (Cf Methaemoglobin which is Fe3+ - due to oxidation/loss of reducing enzymes)
  • Can bind total of 4 oxygen molecules (8 atoms)
  • Also binds: CO2, protons (H+), DPG
  • Production in (1) Bone marrow (2) Liver + spleen (3) yolk sac in first few weeks of gestation

 

Carbon dioxide transport

  1. As bicarbonate: CO2 + H2O -- H2CO3 -- H+ + HCO3-
    • Reaction catalysed by carbonic anhydrase
  2. As carbanimo compounds
    • formed when CO2 binds with plasma proteins (ie Haemoglobin)
  3. Dissolved in solution (5%)
    • CO2 has x24 more solubility than oxygen

Bicarbonate generated increases intracellular osmotic pressure - resulting in increased venous haematocrit
CO2 can never be expressed as "percentage" saturations as it's solubility is not saturated!

Haldane effect : Reduced affinity for CO2 in light of increased PaO2 (downshift of CO2 dissociation curve)

 

Ventilation

Flow of gas per unit time

  1. Minute ventilation = total volume of air entering respiratory tree every minute = Tidal volume x Respiratory rate
  2. Alveolar ventilation = amount of gas entering alveoli each minute = (Tidal volume - dead space) x Respiratory rate
    • More accurate measure of ventilation (only gas that interfaces with respiration)
    • Rapid shallow breaths are inadequate (due to dead space)

Dead space = volume of gas not involved in respiration

  1. anatomical - upper airways not involved in respiration; mouth, nose etc
  2. Alveolar - alveoli ventilated but not perfused (shunts)

Shunt

  • Perfused but not ventilated
  • Normal: bronchial circulation, cardiac thebsian veins (drain directly into left side of heart)
  • Pathological: Left-to-right cardiac defects (cyanotic septal defects - tetralogy)

 

Pulmonary blood flow

  1. Normal CO - 5-6l/min
  2. Normal Pulmonary artery pressure = 25/8 (pulmonary vascular resistance is approximately one tenth of systemic vascular resistance
  3. Pulmonary Vascular Resistance
    • falls with rising pulmonary pressure (due to distension of thin walled pulmonary vessels or to recruitment of collapsed vessels)
    • Increasing radial traction reduces resistance to flow (poiseulles)
    • As lung expands, radial traction forces on blood vessels increases, increasing calibre
    • Controlled by (1) pulmonary artery and venous pressure (2) Lung volume (3) Pulmonary vascular smooth muscle tone (4) Hypoxia
  4. Blood distribution
    • Standing: lowest parts of lungs have greatest flow (hydrostatic pressure of dependent portions)
    • Exercise: Increased upper lobe blood flow

 

 

Respiratory concepts

  1. Muscles of respiration
    • Diaphragm (c345)
    • External intercostals
    • Accessory muscles - SCM, scalenes, strap muscles
  2. Lung
    1. Determined by poiseuille's Law
    2. Greatest resistance in upper airways, trachea
    1. Compliance differs in inspiration and expiration - "Hysteresis"
    2. Laplace Law: P = 2T/r; smaller the radius, the more the tension
    3. Increased compliance with bigger alveolar volumes (hence CPAP)
    4. Improved with surfactant (lipid-protein) from Type II pneumocytes reducing surface tension
    5. Decreased compliance with restrictive lung disease, fibrosis
    • Airflow
    • Compliance: rate of change of volume / rate of change in pressure = 200ml/cmH20
    • Elastance: measure of elastic recoil of lung (1/compliance)

 

Respiratory assessment

  1. Non-invasive
    • Sputum
    • Pulse oximetry
    • Capnography
    • Lung function
      1. PEFR
      2. Spirometry
        • Tidal Volume = 7ml/kg = 500mls
        • IRV = 3L
        • ERV = 1.3L
        • RV = Volume remainin in lung following maximal respiration (measured by helium dilution, nitrogen washout, plethysmography)
        • Vital Capacity = 10-15ml/kg

        Capacity = Sum of two or more volumes

        FRC: Amount of gas remaining in lung at end of quiet expiration

        		Obstructive airways disease: loss of flow
        Restrictive airways disease: loss of volume

      3. Gas transfer
    • Imaging
      1. CXR
      2. CT
      3. MRI
      1. V/Q scanning
    • Echo: assess pulmonary artery pressure and right heart function
  2. Invasive
    • ABG
    • Bronchoscopy
    • Mediastinoscopy - performed via incision at root of neck, permits biopsies of regional lymph nodes
    • Lung biopsy - open / radiologically-guided

 

 

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