Respiratory Physiology

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Respiratory Physiology

• Division of Critical Care Medicine
• University of Alberta

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Outline

1. Lung Function
2. Oxygen Transportation
3. Carbon Dioxide Transportation
4. Respiratory Failure

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Lung Function - Ventilation

• Tidal volume is about 500 ml per breath.
• 150 ml of the tidal volume remains in the airways
  (anatomical dead space).
• Thus, (500 ml 150 ml) X 12 bpm 4.2 L/min of
  fresh gas enters the respiratory zone.
• This is called alveolar ventilation and
  represents the gas available for exchange.

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Lung Function Dead Space

• This is the air in the lungs that is not
  available for gas exchange.
• Amount varies with tidal volume (increases with
  deep breath from traction on the bronchi by the
  parenchyma) and size and posture of the subject.
• Normal range is 0.2 to 0.35

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Lung Function - Compliance

• During the respiratory cycle, the lungs follow
  the pressure-volume curve below.
• The lung volume is greater at any given pressure
  during deflation than inflation (hysteresis).
• Notice that the lungs still have some air even
  at zero pressure.
• There will still be air left if the pressure
  around lung is raised above zero due to small
  airway closure, trapping residual gas.

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Lung Function - Compliance

• The slope of the pressure-volume curve is the
  compliance (volume change per unit pressure
  change).
• A normal lung is very compliant, normally 200
  mL/cm H2O.
• As distending pressure increases, the lungs
  become stiffer with a smaller compliance and the
  pressure-volume curve flattens.
• Lung compliance can increase or decrease with
  pathological states.

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Lung Function - Resistance

• Airflow is proportional to tube length and
  inversely proportional to the fourth power of the
  radius.
• The bronchi are supported by radial traction of
  the surrounding tissue.
• Therefore, as lung volume decreases, airway
  resistance increases.
• At very low volumes, the small airways may close
  completely.

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Lung Function Regional Differences in
Ventilation

• The lower lung regions ventilate better than the
  upper regions.
• The base of the lungs have a lower volume because
  of the lower pleural pressure whereas the apex is
  normally at a higher volume because of a higher
  pleural pressure.
• This places the base on the steep part of the
  pressure-volume curve. (i.e. the bases are more
  compliant than the apices)
• Therefore, the bases expand more with inspiration
  and ventilate more.

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Lung Function Regional Differences in Blood Flow

• Upright, blood flow in the lungs decreases
  linearly from bottom to top.
• This can be explained by the hydrostatic pressure
  in the blood vessels.
• There are regions at the apex where arterial
  pressure falls below alveolar pressure (Zone 1).
• The capillaries are collapsed and there is no
  flow.
• This region is ventilated but not perfused and
  contributes to alveolar dead space.

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Lung Function Regional Differences in Blood Flow

• In Zone 2, arterial pressure exceeds alveolar but
  venous pressure is still lower.
• This causes blood flow to be dependant on the
  arterial-alveolar pressure.
• In Zone 3, venous pressure exceeds alveolar and
  flow is determined in the usual way by the
  arterial-venous pressure difference.

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Oxygen Transportation

• The partial pressure of oxygen in the alveoli
  determines the driving pressure for oxygenation
  of the blood.
• The alveolar oxygen partial pressure can be
  calculated from the alveolar gas equation.
• PAO2 PiO2 (PACO2/R)
• PiO2 is the partial pressure of inspired,
  humidified oxygen ((PB - 47)X0.21)
• PACO2 is alveolar CO2 partial pressure which is
  approximated to the arterial CO2.
• R is the respiratory quotient which is the ratio
  of CO2 production over O2 consumption in the
  tissues.
• The difference between PAO2 and PaO2 can be used
  to determine the cause of respiratory failure.

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Oxygen Transportation

• Oxygen binds to hemoglobin in an S shaped curve
  called the oxygen dissociation curve.
• The amount of O2 increases rapidly up to a PaO2
  of 50 then becomes flatter.
• The flat portion allows for continued O2 loading
  even if the PAO2 falls while the steep portion
  allows the tissues to extract large amounts of O2
  for only a small drop in capillary PO2.

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Oxygen Transportation

• The curve is shifted to the right (O2 affinity
  for hemoglobin is reduced) by an increase in
  PCO2, temperature, acidity, and 2,3 DPG.
• This allows for more unloading of O2 at a given
  PO2 in the tissue.

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Oxygen Transportation

• Oxygen is carried in the blood by hemoglobin and
  dissolved in solution.
• For each mmHg of PO2, there is 0.03 mL O2/L of
  blood (obviously inadequate to meet tissue needs)
• One gram of hemoglobin can combine with 1.39 mL
  O2 and factoring the percentage saturated, the
  full equation is
• (Hgb X 1.39 X SaO2/100) 0.03PaO2
• Normal value is about 200 mL O2/L blood.
• Multiply by the cardiac output and you get the
  oxygen delivery to the tissue (about 1000 mL
  O2/min).

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Carbon Dioxide Transportation

• The PaCO2 is proportional to the amount of CO2
  produced in the tissue and partial pressure of
  humidified gas and inversely proportional to the
  respiratory rate, tidal volume and dead space.

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Carbon Dioxide Transportation

• CO2 is carried in three forms in the blood
  dissolved, as bicarbonate, and in combination
  with proteins.
• Dissolved CO2 is 20X more soluble than O2 and
  plays a significant role in its carriage.
• Bicarbonate is formed by the reaction of CO2 with
  H20 in the presence of carbonic anhydrase.
• The bulk of the CO2 is in the form of
  bicarbonate.
• Carbamino compounds are formed by the combination
  of CO2 with terminal amine groups in blood
  proteins.
• Binding CO2 to hemoglobin facilitates O2
  unloading (Haldane effect).

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Shunt

• Shunt occurs when blood passes to the pulmonary
  venous system without going through gas
  exchanging areas.
• A shunt can occur through congenital defects in
  the heart or blood vessels or through areas of
  atelectasis or consolidation in the lungs.

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• As can be seen above, if there is a 50 shunt,
  the oxygen content returning to the heart will be
  lower.
• If we give 100 oxygen, the PAO2 will increase to
  670 but this will only cause a small increase in
  the total oxygen content.

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Shunt

• Hemoglobin is fully saturated above a PaO2 of 150
  so all additional oxygen content is due to
  dissolved oxygen.
• If the mixed venous oxygen content is directly
  measured (with a Swan-Ganz catheter) then the
  shunt fraction can be calculated from the shunt
  equation.

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V/Q Mismatch

• The average V/Q is about 0.8 ((4 L/min)/(5
  L/min)).
• In the normal lung, blood flow is the greatest at
  the bottom of the lung.
• Ventilation also greater at the bottom.
• The differences in perfusion from bottom to top
  are greater than the differences in ventilation
  so the ratio of ventilation to perfusion is low
  in the bottom of the lung and high at the top.

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V/Q Mismatch

• Shunt (discussed above) represents an extreme
  form of V/Q mismatch with no ventilation and
  normal blood flow such that the ratio is 0.
• If an alveolus is unperfused, the blood flow is
  zero and the V/Q ratio increases until it
  approaches infinity. This is called dead space.
• The gas in the alveolus also approach inspired
  gas (PO2 150 and PCO2 0).

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V/Q Mismatch

• CO2 excretion is higher in areas of higher V/Q
  mismatch but because the blood flow is small,
  this only has a modest effect on PaCO2.
• From the standpoint of total ventilation, this is
  inefficient gas exchange since the ventilation
  going to the high V/Q units carries away less CO2.

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V/Q Mismatch

• If PCO2 production is constant, then the PaCO2
  will rise in the face of high V/Q ratio due to
  inefficient CO2 elimination.
• The response to this is to increase tidal volume
  or respiratory rate if the respiratory system is
  able.
• If hypoxia is present, it is usually easily
  corrected with a small amount of supplemental
  oxygen to raise the PAO2 in the rest of the
  alveoli.

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Summary

1. There are regional differences in ventilation and
   perfusion, both decreasing from the base to the
   apex.
2. Shunt occurs in areas of ventilation but no
   perfusion. Causes hypoxia that is not responsive
   to supplemental oxygen.
3. Dead space ventilation occurs in areas of
   ventilation but no perfusion.
4. Dead space ventilation causes easily corrected
   hypoxia and elevated CO2 due to inefficient
   ventilation.