RESP: Clinical evaluation of respiratory function

Question 1

What does spirometry investigation?

Answer 1

Airway obstruction and lung restriction

Question 2

How can level of airway obstruction be investigated?

Answer 2

If the FEV1/FVC ration is <70%, but FVC is normal -> Indicates Obstructive respiratory disease

If FVC <80% of predicted volume, but FEV1/FVC = Normal -> Indicates Restrictive respiratory disease

If FEV1/FVC <70% and FVC <80% - Indicates mixed Obstructive + Restrictive respiratory disease

Question 3

What is FEV1?

Answer 3

Forced expiratory volume in one second, term used to describe the maximum volume that can be expired during the first second of a maximum forced expiration.

It corresponds to how quickly air can pass through the airways and reflects airway function and health

Question 4

What is FVC?

Answer 4

Forced vital capacity, measures maximum volume an individual can exhale in one breath (after inspiring as much air as possible) and reflects the volume of the lungs that the individual can utilise when breathing

Question 5

The difference between what pressures determines the level of force acting to expand or compress the lung?

Answer 5

Difference b/w alveolar pressure and transpulmonary pressure (intrapleural space pressure)

Question 6

What does a lung volume/intrapleural pressure graph look like during dynamic compliance?

Answer 6

During inspiration (from bottom left), increasing negative intrapleural pressure is generated as lung volume increases, due to elastic properties.

The change in lung volume generated by a specific transpulmonary pressure can be altered by presence of lung pathology due to changes in structure of lung tissue, particularly, density + structure of elastic + structural fibres.

Question 7

How does the area of a pressure-volume loop relate to airway resistance?

Answer 7

Size of internal area of loop is proportional to the level of airway resistance.

Airway resistance increases with increased obstruction and speed of airflow

Question 8

What information do ABGs provide?

Answer 8

• pH
• PaCO2
• [HCO3-]

From this you can determine if a patient has acidosis or alkalosis, and whether its respiratory or metabolic.

Respiratory acidosis would present with a high PaCO2 and a lower pH due to increased retention of CO2

In metabolic acidosis, there would be a deficiency of [HCO3-] as more is being excreted, or not enough is being produced, and there’s increased metabolic activity, so greater amounts of CO2 being unconverted in the blood.

Respirator alkalosis would present with low PaCO2 and a higher pH due to decreased levels of CO2

In metabolic alkalosis, there would be excessive amounts of [HCO3-] as this is what is removing the H+ in the blood, increasing pH

Question 9

What does increased PaCO2in the presence of highpH, or decreasedPaCO2in the presence oflowpHindicate?

Answer 9

Respiratory compensation

Question 10

What does increased [HCO3-]in the presence oflowpH, or decreased [HCO3-]in the presence of high pH indicate?

Answer 10

Metabolic compensation - Whether the degree of compensation is ‘full’ or ‘partial’ depends on whether the level of compensation has restored pH to its normal range (if b/w 7.35 and 7.45 = full)

Question 11

How is mixed acidosis or mixed alkalosis identified?

Answer 11

Mixed acidosis - High PaCO2 + low [HCO3-]

Mixed alkalosis - Low PaCO2 + high [HCO3-]

Question 12

How can we determine the cause of hypoxaemia?

Answer 12

Calculating alveolar oxygen pressure.

It’s not practical in these situations to sample gas directly from alveoli

PAO2 = FiO2 x (PB = PH2O) - PaCO2/RER

• FiO2 - Fraction of oxygen present in inspired gas
• PB = Barometric pressure
• PH2O - Water vapour pressure
• RER - Respiratory exchange ratio

Question 13

What is the respiratory exchange ratio?

Answer 13

Difference b/w O2 and CO2 in inspired and expired air

RER = VCO2 produced/ VO2 consumed

Main determinant of RER is the particular metabolic substrate being used (e.g. fat or carbohydrate, RER for modern diet = 0.8)

Oxygen consumption can be measured indirectly by measuring PACO2, and quantity of CO2 in expired air. This provides a measure of CO2 production by the body.

As usual RER for western diet is 0.8, you can divide VCO2 by RER to indirectly calculate O2 consumed.

Question 14

What is used to investigate hypoxaemia?

Answer 14

Alveolar gas equation and alveolar-arterial oxygen gradient

Alveolar O2 content = O2 inspired - O2 consumed

A-a O2 gradient is difference b/w alveolar + arterial pressure, normally <2kPa.

If hypoventilation contributing to hypoxaemia, PaCO2 >6kPa. It could be lower however, if the hypoxaemia is being caused by V/Q inequality or deficits in oxygenation (but where there is sufficient overall ventilation to remove the excess PaCO2).

If oxygen not diffusing into blood, A-a gradient should be much greater than the normal approx 2kPa. This means the volume of O2 reaching alveoli isn’t diffusing into the blood efficiently

Question 15

What reading indicates obstructive airway diseases?

Answer 15

Reduction in FEV1/FVC ratio (<70%). FVC is typically unchanged as lung function is unaffected

Question 16

What reading indicates restrictive airway diseases?

Answer 16

Restrictive, e.g. pulmonary fibrosis, are indicated by a reduction in FEV1 and FVC (<80% expected value), with a relatively normal FEV1/FVC ration (>70%, i.e. the decrease in FEV1 reflects an overall decrease in lung volume rather than airway obstruction).