Respiratory physiology

Question 1

Which three lung volumes can’t be measured with a spirometer?

Answer 1

Total Lung Capacity (TLC)
Functional Residual Capacity (FRC)
Residual Volume (RV)

Question 2

Explain:

1. Tidal volume
2. Residual volume
3. Inspiratory reserve volume
4. Expiratory reserve volume
5. Vital capacity
6. Functional residual capacity

Answer 2

The volume of gas:

1. inhaled/exhaled in normal resting breath.
2. remaining in lungs after maximal forced expiration.
3. that can be further inhaled after normal TV.
4. that can be further exhaled after normal TV.
5. inhaled after maximal expiration + maximal inspiration.
6. that remains in lungs after normal tidal expiration.

Question 3

Draw a volume/time-diagram of normal respiratory volumes and illustrate it’s Components.

Answer 3

See separate diagram

Question 4

Explain:

1. Closing volume
2. Closing capacity
3. How to calculate these

Answer 4

1. The volume of gas over and above residual volume that remains in the lungs when the small Airways begin to close (ml).
2. The lung capacity at which the small airways begin to close. It’s a combination of RV + closing volume.
3. Closing volume is calculated by measuring the nitrogen concentration in expired gas after a single breath of 100% oxygen. This value is added to RV (calculated by helium dilution) to get closing capacity.

Question 5

Explain the differences in spirometer curve, FVC, FEV1 and FEV1/FVC (%) in:

1. Normal lung
2. Obstructive lung
3. Restrictive lung

Answer 5

1. FVC ca 4500ml, FEV1/FVC ca 75%. So aprox. 75% of FVC is possible to forcibly expire in 1 s (3375ml).
2. FVC is lower (3000ml). Obstructive disease limits the volume of gas that can be forcibly expired in 1 s, so the FEV1/FVC-ratio is lower (33%).
3. Restrictive disease lowers the FVC, but generally does not affect early expiration. FEV1/FVC-ratio is normal or high. An example could be FVC of 3500ml and FEV1 of 3000ml (FEV1/FVC-ratio 85%).

Question 6

Draw Flow-volume loops of:

1. Normal lung
2. Obstructive lung
3. Restrictive lung
4. Variable intrathoracic obstruction
5. Variable extrathoracic obstruction
6. Fixed large airway obstruction

Answer 6

See separate diagrams

Question 7

What is the Alveolar Gas Equation and how is it used?

Answer 7

An equation used to estimate the alveolar oxygen content (PAO2). By comparing that value with arterial oxygen content, one can get an idea of the degree of shunt.

PAO2 = (FiO2 x (Patm - Ph20)) - (PACO2/R)
Ex: PAO2 = (0,21 x (101,3 - 6,3)) - (5,3/0,8) (normal)

Patm is atmospheric pressure, Ph20 is standard vapour pressure of water at 37 C. R is respiratory quotient, usually 0,8 but 1,0 with pure carbohydrate metabolism.

Question 8

What is the Shunt Equation, how is it used and how is it written?

Answer 8

An equation used to give a ratio of shunt blood flow to total blood flow. The normal ratio is 0,3, but will tend to increase under abnormal conditions which will be seen in a reduced PaO2.

Qs/Qt = (Cc’o2 - Cao2) / (Cc’o2 - Cvo2)

Qs = shunted blood flow, Qt (total blood flow), Cc’o2 = end pulmonary capillary O2-content, Cvo2 = shunt blood O2-content and Cao2 = arterial blood O2-content.

Question 9

How do you calculate Pulmonary Vascular Resistance, and what are examples of factors increasing and decreasing PVR?

Answer 9

PVR = ( (Mean Pulmonary Artery Pressure - Left Atrial Pressure) / Cardiac Output ) x 80.

Increased by: PaCO2 up, pH down, PaO2 down, adrenaline, noradrenaline, histamine, high or low lung volume, angiotensin II.

Decreased by: PaCO2 down, pH up, PaO2 up, Isoprenaline, acetylcholine, NO, volatile anaesthetic agents, prostacycline

PVR is at its lowest around the FRC.

Question 10

What are the West Lung Zones?

Answer 10

A 3-4 zone model describing how blood flow varies in the upright lung:

Zone 1 (PA > Pa > Pv, collapse). The apical zone, no blood flow due to Alveolar pressure being higher than arterial. This zone does not exist in the normal lung, but may occur during positive pressure ventilation.

Zone 2 (Pa > PA > Pv, waterfall). Blood flow is determined by the difference between arterial and alveolar pressure, and as both these are cyclic it will be intermittent.

Zone 3 (Pa > Pv > PA, distension). The basal zone. Constant blood flow since both arterial and venous pressure is higher than alveolar. Most of the healthy lung comprises of this zone.

Question 11

Describe V/Q-mismatch and draw a graph (with Flow and VQ-ratio on x-axis 1 and 2, and Region of Lung on Y). What is the normal V/Q-ratio?

Answer 11

V/Q-ratio describes the imbalance between ventilation (V) and perfusion (Q) in different areas of the lung. Given an alveolar ventilation of 4,5 liters/min and a pulmonary arterial blood flow of 5 liters/min, V/Q in a normal person would be 0,9. The graph (separate) shows that higher lung regions tend towards being ventilated but not perfused (dead space, V/Q = infinite) and vice versa (shunt, V/Q = 0).

Question 12

Describe Dead space and these subcategories:

• Anatomical
• Alveolar
• Physiological

Answer 12

Dead space is the volume of the Airways in which no gas exchange occurs (ml).

Anatomical dead space is the volume of the Airways that does not contain respiratory epithelium (nasal cavity to generation 16 terminal bronchioles. Usually 2 ml/kg in adults (can be calculated by Fowlers method).

Alveolar dead space is the volume of those alveoli that are ventilated but not perfused, and so cannot take part in gas exchange.

Physiological dead space is the sum of the anatomical and the alveolar dead space. It can be calculated using the Bohr equation.

Question 13

Describe Fowler’s method, and what it’s used for.

Answer 13

Fowlers method (aka Nitrogen Washout-test) is a way to measure the anatomical dead space. This is done by taking a single vital capacity breath of pure O2, and then exhaling through a N2-analyser. The first part of the exhalation will be pure 02, and this volume equals the anatomical dead space (since no gas exchange happens in that area).

Question 14

Describe the Bohr Equation, and what it’s used for.

Answer 14

The Bohr Equation is used to give a ratio of physiological dead space volume to tidal volume. This is normally around 30%, but increases under abnormal conditions.

The equation can be derived into:
Vd/Vt = (PaCO2 - PECO2) / PaCO2

Where PaCO2 is CO2 in arterial blood (equals alveolar CO2) and PECO2 is CO2 in expired air.

Question 15

Describe and draw the oxygen cascade.

Answer 15

The oxygen cascade is a way of describing the flux of O2 to the tissues and to understand the changes in PO2 according to the location in the body.

1. Air (PO2 = FiO2 x Patm)
2. Trachea, humidification (PO2 = FiO2 (Patm - PH20))
3. Alveolus, ventilation. PAO2 = tracheal air - PACO2/R
4. Capillary, diffusion. Negligible barrier for O2.
5. Artery, shunt & V/Q-mismatch, usually < 2 kPa.
6. Mitochondria. Low PO2 of around 1,5 kPa is usual.
7. Veins. Normal PVO2 = 6,3 kPa.

Question 16

What’s the pasteur point?

Answer 16

It’s the oxygen concentration below which oxidative phosphorylation cannot occur in the mitochondria. It’s considered to be around 1 mmHg / 0,13 kPa.

Question 17

What is Hüffner’s constant, and how is it used?

Answer 17

Hüffners constant describes the volume of O2 (ml) that can combine with each 1 g Hb. It is 1,34 in vivo and 1,39 in vitro. It is used when calculating the arterial O2-content.

Question 18

How do you calculate the arterial O2-content (CaO2)?

Answer 18

CaO2 = (1,34 x Hb x Sat) + (0,0225 x PaO2) kPa

Question 19

How do you calculate the delivery of O2 to organs?

Answer 19

DO2 (delivery of O2) = Cardiac Output x CaO2 x 10

where CaO2 is arterial O2-content. The multiplier (10) is because CaO2 is measured in ml/min whereas CO is measured in l/min. Normal DO2 is 1000 ml/min.

Question 20

Draw a supply-demand curve showing the relationship between oxygen delivery (DO2) and oxygen consumption (VO2). Whats a normal oxygen consumption, and what is the “Critical DO2”?

Answer 20

Normal oxygen consumption is usually around 250-500 ml/min.

Critical DO2 is the degree of oxygen delivery below which supply is inadequate to meet oxygen demand. Usually between 4-8 ml/kg/min.

Question 21

Describe the Oxygen Extration Ratio (O2ER), what it usually is and how it may play a role in the oxygenation process.

Does the O2ER vary between organs? If so, name one particular organ that stands out.

Answer 21

O2ER is the fraction of delivered oxygen that is taken up by the tissues. Usually it’s around 0,2-0,3, indicating that only 20-30% of oxygen is utilized. This spare capacity enables the body to cope with a fall in oxygen without initially compromising aerobic respiration.

The heart has a normal O2ER of about 0,6, making it particularly sensitive to ischemia.

Question 22

What is Hypoxia? Describe it’s four main subgroups (cause dependent).

Answer 22

Hypoxia is a condition of insufficient supply of oxygen to the tissues to maintain cellular function. It might be regional or generalized. Depending on cause:

1. Hypoxaemic hypoxia. An abnormal reduction in the partial pressure of oxygen in arterial blood. (hypoventilation, V/Q-mismatch, difusion defect etc.).
2. Anaemic hypoxia. Reduced/failed oxygen-carrying capacity of blood, despite normal partial pressure of O2. (Severe anaemia, carbon monoxide poisoning)
3. Ischaemic hypoxia. Failure of perfusion. (septic shock, embolus etc.)
4. Histotoxic hypoxia. Failure of oxidative phosphorylation. An inability of the tissues to utilize the oxygen that is being supplied. (Cyanide poisoning -> uncouples the respiratory chain).

Question 23

Draw the Oxyhaemoglobin (y:sat%, x:PaO2) dissociation curve and give correct values for the three key points: arterial, venous and P50. Explain the reason of the sigmoid curve.

Answer 23

Arterial Point: 100% saturation, 13,3 kPa.
Venous Point: 75% saturation, 5,3 kPa.
P50: 50% saturation, 3,5 kPa.

Hb exhibits cooperative binding. At first binding of O2 is slow and difficult, but as more O2 binds the affinity of the Hb-molecule (4 binding sites) increases. This give rise to a sigmoid curve shape.

Question 24

Describe factors that change the oxyhaemoglobin curve (and therefore the position of P50) rightwards and leftwards. Also, explain the Bohr effect.

Answer 24

Left shift (increased affinity): Decreased PaCO2, alkalosis, decreased temperature, decreased DPG, fetal Hb, carbon monoxide, methaemoglobin.

Right shift: Increased PaCO2, acidosis, increased temperature, increased DPG, pregnancy, altitude, haemoglobin S.

The Bohr effect is the situation whereby the affinity of Hb for oxygen is reduced by a reduction in pH and vice versa. Peripherally the reduction i pH facilitates the offloading of O2, and in the lungs the increase in pH (CO2 i offloaded) facilitates the binding of O2.

Question 25

Describe CO2 carriage in blood. What is the solubility of CO2 in blood compared to O2? Which reaction facilitates CO2-carrying?

Answer 25

CO2 is 20 times more soluble in blood than O2. It is carried in three forms: Bicarbonate (arterial 90%, venous 60%), Carbamino compounds (arterial 5%, venous 30%) and Dissolved (arterial 5%, venous 10%).

The following reaction occurs in erythrocytes in tissues (and reverse in lung capillaries) and explains how CO2 is carried as HCO3- :

CO2 + H20 H2CO3 H+ + HCO3-

Question 26

Describe the Haldane and the Hamburger effect in carriage of CO2.

Answer 26

HALDANE EFFECT is the phenomenon by which deoxy-Hb is able to carry more CO2 than oxy-Hb. It’s because deoxy-Hb more readily forms carbamino-complexes with CO2. Also, deoxy-Hb is a better buffer of H+ than oxy-Hb, which increases the amount of HCO3- produced.

Once formed, HCO3- diffuses out of the erythrocyte. To maintain electrical neutrality Cl- moves in. This is known as the HAMBURGER EFFECT.

Question 27

Draw a graph describing “Work of breath”, with Lung volume above FRC (y) and Pressure in kPa (x). What is the starting pressure Point (aka pleural pressure)?

Answer 27

Pleural pressure is around -0,5 kPa

Question 28

Draw a graph describing Control of ventilation, minute ventilation l/m (y) and Oxygen partial pressure kPa (X). Whats the effect of differing PaCO2 (5 vs 10 kPa).

Answer 28

Under normal conditions the minute volume remains relatively constant around 6 l/m until the PaO2 falls below 8 kPa. The rise in MV under this Point is steep, illustrating the hypoxic drive.

In a setting with coexisting hypercarbia the line is plotted above and to the right of the first curve.

Question 29

Draw a graph depicting the effect that ventilation (MV, l/m) has on PACO2.

Answer 29

See graph (p 235)

Question 30

Define compliance. What is the difference between static and dynamic compliance? Which is usually the highest?

Answer 30

Compliance is the volume change per unit change in pressure (ml/cmH20 or l/kPa). Total compliance is lung compliance + chest compliance.

Static compliance is the compliance of the lung measured when all gas flow has ceased. It is usually higher than dynamic compliance.

Dynamic compliance is the compliance of the lung measured during the respiratory cycle when gas flow is still ongoing (ml/cmH20 or l/kPa).

Question 31

Define resistance.

Answer 31

Resistance is the pressure change per unit change in flow. Total resistance is chest wall resistance + lung resistance.