Respiratory Physiology - Dead Space and West Zones Flashcards
What is dead space?
Areas of the respiratory tract that are ventilated but not perfused, and therefore do not undertake gas exchange with the blood
Total dead space is referred to as physiologoical dead space, and is usually around 200-350ml in normal breathing.
Anatomical dead space refers to the volume of the conducting airways (the first 16 airway generations) (Approx 2ml/kg)
Alveolar dead space refers to alveoli that are ventilated but don’t receive enough blood to undertake gas exhange. This can be physiological (such as hypoxic pulmonary vasoconstriction), or pathological (PE)
How is physiological (total) dead space calculated?
Bohr equation images
Physiological (total) dead space can be measured using Bohr’s equation:
This relies on some assumptions - tidal volume is comprised of alveolar and dead space volume alone, and there is no rebreathing of expired CO2
Therefore, all expired CO2 is coming from alveolar minute ventilation
VD/VT = (FACO2-FECO2)/FACO2
Worked example:
VT = 500ml
FACO2 5.5kPa
FECO2 4kPa
(VD/500ml) = (5.5kPa-4kPa)/5.5kPaVD = 1.5kPa/5.5kPa x 500mlVD = 136ml
The Enghoff modification to Bohr’s equation assumes that PACO2 (alveolar) is roughly equal to PaCO2 (arterial)
In reality PACO2 is likely slightly lower than PaCO2 (due to alveolar dead space, shunt, and diffusion impairment), and this means that dead space will be over-estimated.
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How is anatomical dead space calculated?
GRAPH
Anatomical dead space is measured using Fowler’s method:
Step 1 - Patient takes a vital capacity breath of 100% oxygen, removing all nitrogen from anatomical dead space (alveoli still receive nitrogen from the blood)
Step 2 - Patient fully exhales to residual volume into a pneumotachograph, which measures flow over time, calculating volume (flow/time = volume)
Step 3 - the detected concentration of nitrogen measured is plotted against the volume exhaled.
Phase 1 - Pure oxygen is exhaled from dead space, so no nitrogen is detected
Phase 2 - Sigmoid shaped rise in nitrogen concentration (as a result of different alveolar time constants). Initially, alveolar gas mixes with the nitrogen-free dead space, and as time goes on, the proportion of nitrogen-free dead space gas reduces significantly.
Phase 3 - Plateau, representing only alveolar ventilation.
Phase 4 - Inflection and sudden increase in nitrogen - at closing capacity.
This is because basal alveoli are more compliant than apical alveoli, and are emptied first during exhalation. When the closing capacity is reached, the basal alveoli are completely collapsed, while the apical alveoli are not full deflated.
Therefore. prior to the initial 100% oxygen breath, the apical alveoli have not fully deflated, and thus retain some nitrogen while the basal alveoli have 100% oxygen.
The basal alveoli drain first during this exhalation, until closing capacity is reached, and the apical alveoli are the only ones left to empty, and in doing so, release their nitrogen content, significantly increasing the detected nitrogen concentration.
Step 4 - Establish the mid-point of phase 2 on the Y axis, and draw a vertical line down, making areas A and B equal. This cuts through the X axis at the anatomical dead space volume
This method was originally an educated guess of a sensible cut-off point, and was then proven practically afterwards. There is little evidence as to exactly why this is the point where the vertical line is drawn
How is pulmonary vascular resistance calculated?
Pulmonary vascular resistance (PVR) = [Mean pulmonary artery pressure (MPAP) - Left atrial pressure (LAP)]/Cardiac output (CO) x 80
PVR = (MPAP-LAP)/CO x 80
The 80 is a conversion coefficient to adjust for discrepancy between the units used
What factors increase pulmonary vascular resistance (PVR)?
Hypercapnia
Acidosis
Hypoxia
Adrenaline & Noradrenaline
Thromboxane A2
Angiotensin II
Serotinin
Histamine
High or low lung volumes
What factors reduce pulmonary vascular resistance (PVR)?
Hypocanpia
Alkalosis
Hyperoxia
Volatile anaesthetic agents
Isoprenaline
Acetylcholine
Prostacyclin
Nitric Oxide
High intrathoracic pressures
Increased pulmonary venous pressure
How does pulmonary vascular resistance (PVR) change with lung volume?
GRAPH
PVR is at its minimum at FRC
As lung volumes decrease, the compression of pulmonary vessels increases their resistance
As lung volumes increase, the vessels are stretched, increasing their resistance
What are West zones?
IMAGE
When in the upright position, blood distribution is affected by the relationship between arterial, alveolar, and venous pressures. Classically these can be divided into three zones, but a fourth has been added to account for low lung volumes
Zone 1 PA>Pa>Pv
Ventilation far exceeds perfusion, compressing both arteries and veins.
Tendency to form dead space, seen in hypovolaemia and high PEEP
Zone 2 Pa>PA>Pv
Perfusion depends on the difference between arterial and alveolar pressures, varying with cardiac and respiratory cycles. It is higher at the bottom of zone 2 and the top.
Zone 3 Pa>Pv>PA
Both arterial and venous pressures are higher than alveolar pressure, there is consistent blood flow. It represents areas of shunt
Zone 4
Similar to zone 3 but with higher resistance
What is normal V/Q matching?
V represents ventilation, usually 4-5L/minute
Q represents perfusion, usually 5L/minute
A normal V/Q ratio is therefore around 0.9
How does V/Q matching vary with zones of the lung?
Graph
Both perfusion and ventilation gradually decrease from the bottom to the top of the lung, but perfusion decreases more dramatically.
This means that the V/Q ratio is lowest at the bottom, and increases towards the top of the lungs.
The bottom of the lungs demonstrate shunt (West 3), and the top, dead space (West 1)
