Respiratory Physiology FRCA – Dead Space and Shunt

Flashcard maker : Patrick Turner
What is anatomical dead space?
Volume of gas in conducting airways without any possibility of participating in gas exchange
~1500mls per tidal breath
What is alveolar dead space?
Volume of alveoli that are ventilated but not perfused
What is physiological dead space?
Anatomical dead space + Alveolar dead space
How is anatomical dead space measured?
Fowler’s method:
Patient takes a breath of 100% O2 and then exhales through a nitrogen analyser
Phase 1: Anatomical dead space gas = 100% O2
Phase 2: Mixed anatomical and alveolar dead space gas = rising N2 concentration
Phase 3: Pure alveolar gas = plateau of N2 concentration
Upstroke of phase 2 curve is divided equally in half to give areas A and B.
Anatomical dead space = 0 up to and including B
How is physiological dead space estimated?
Bohr’s Equation:
Used to determine the volume of lung that does not excrete CO2
The concentration of CO2 in air is minimal
All expired CO2 must come from alveolar ventilation

Vd/Vt = (PaCO2 – PeCO2) / PaCO2

What factors increase anatomical dead space?
Increasing body size
Increasing age
Bronchodilatation
Lung disease e.g. emphysema
Jaw thrust
What factors decrease anatomical dead space?
Supine posture
Bronchoconstriction e.g. asthma
Lung disease e.g. obstructing tumours
ET tubes
What factors increase alveolar dead space?
Increasing age
Anaesthetic gases (unknown mechanism)
Reduced pulmonary artery pressure (e.g. hypotension)
Reduced perfusion to apices of lung
IPPV due to exaggerated hydrostatic failure of perfusion and reduced pulmonary blood flow
Lung distension e.g ARDS
Oxygen resulting in hyperopic vasodilatation
What factors reduce alveolar dead space?
Hypoxic vasoconstriction
What is anatomical shunt?
Deoxygenated blood enters left side of the heart for anatomical reasons:
Thebesian veins
Bronchial circulation
Right to left shunt
What is physiological shunt?
Sum of anatomical shunt and functional shunt
What is functional shunt?
Alveolar perfusion without adequate ventilation resulting in V/Q mismatch
E.g.
Odema
Infection
ARDS
Atelectasis
What is normal V/Q matching?
Alveolar ventilation = 4.5L/min
Pulmonary arterial blood flow = 5L/min
Normal V/Q ratio = 0.9
What is the shunt equation?
Qs/Qt = (CcO2 – CaO2)/(CcO2 – CvO2)
What is a normal shunt fraction?
4%
What is the effect of shunt on PaCO2?
Shunt does not normally case increased PaCO2 because higher concentrations of carbon dioxide are sensed by central chemoreceptors increasing ventilatory rate.
PaCO2 will therefore only increase when the shunt fraction increases beyond 50%.
Why is giving a patient 100% oxygen to breathe a very accurate measure of shunt?
The alveolar-arterial pO2 difference increases with alveolar pO2 in a non-linear manner governed by the oxygen dissociation curve. At high alveolar pO2, a plateau of alveolar/arterial pO2 difference is reached, but the alveolar pO2 at which the plateau is reached is higher with larger shunts.

Due to the nature of the ODC, on the flat upper part, increasing FiO2 will only slightly increase sO2 as Hb is fully saturated In the presence of high pAO2 a huge gradient will be present between the predicted arterial pO2 and the actual. This would otherwise be much smaller.

Where is perfusion greatest?
Bases of lungs
Where is ventilation greatest?
Bases of lungs
Where is ventilation better than perfusion i.e. high V/Q?
Top of the lungs
Decreased pACO2 and increased pAO2
Where is perfusion better than ventilation i.e. low V/Q?
Bottom of the lungs
Increased pACO2 and decreased pAO2
Where in the lungs is V/Q 1?
2/3 of the way up (approx rib number 3)
Describe West Zone 1
Not normally seen in healthy lungs:
PA > pa > pv
Only seen at apex of lungs in haemorrhage or PPV
Normally pulmonary arterial pressure is sufficient to pump blood to the top of the lungs, however, in this situation, pa pressure is less than PA pressure compressing blood vessels and preventing flow
The lung is ventilated but not perfused
Alveolar dead space created
Describe West Zone 2
Optimal V/Q matching:
pa > PA > pv
Arterial pressure exceeds alveolar pressure in systole, however alveolar pressure is still greater than venous pressure creating a “waterfall effect”
Describe West Zone 3
Represents the majority of the lungs in health:
pa > pv > PA
No external resistance to blood flow
Flow is determined by the pa-pv pressure difference
Capillaries open independent of respiratory cycle increasing perfusion at bases and resulting in a low V/Q
Describe West Zone 4
Seen in upright posture/atelectasis:
pa > pi > pv > PA
Weight of lung tissues causes increased interstitial pressure which compresses extra-alveolar capillaries thus reducing blood flow. The pa-pi pressure difference determines whether there is blood flow. pi affects pa and pv equally so the pa>pv pressure difference remains the same.
What will happen to blood flow in supine patient?
Blood flow to posterior regions of lungs will become greater than to the anterior portions of the lungs. The difference between the apex and base of the lung will diminish in the supine patient.
What will happen to V/Q in spontaneously breathing patient in lateral position?
Dependent lung:
Increased ventilation compared to top lung (more compressed = greater compliance = better ventilation)
Increased perfusion compared to top lung (effect of gravity)
V/Q will be lower in comparison to top lung
What will happen to V/Q in anaesthetised patient in lateral position?
Due to mediastinal weight increase in relaxed anaesthetised patient, the lungs move down the compliance curve.
Dependent lung:
Poorly compliant
Poor ventilation
Increased perfusion initially until hypoxic vasoconstriction occurs
V/Q low (lower than upper lung)
How can you measure blood flow in the lungs?
Radioactive xenon can be dissolved in normal saline and injected into a peripheral vein. It has a very low solubility so when it reaches the pulmonary capillaries it will quickly move from the blood into the alveolar gas.
What is the effect of an area of V/Q inequality on overall gas exchange?
High V/Q units e.g. apices there is high PAO2 and low PACO2 but they are less well perfused and therefore contribute less to overall gas exchange. These units are on the upper part of the ODC where small changes have little effect on sO2.

In low V/Q units e.g. bases there is a low PAO2 and higher PACO2 and are better perfused so contribute more to overall gas exchange. These units are on the steep upslope of the ODC where small changes have a big effect on the degree of sO2. The net result is often a reduction in pO2.

The same does not apply for CO2 as this is a more linear curve so high V/Q units compensate for low V/Q units.

What is the Bohr Equation?
Gives a ratio of physiological dead space volume to tidal volume.
VD/VT = (PaCO2 – PECO2)/PaCO2

VD: Physiological dead space
VT: Tidal volume
PECO2: Expired CO2 concentration
PaCO2 = Surrogate for PACO2

What percentage of tidal volume does dead space normally represent?
30%

Get instant access to
all materials

Become a Member