Block 3: Respiratory investigations, Airway obstruction Flashcards
Types of respiratory investigations
- ABG
- Pulmonary function tests: Spirometry, flow volume loops, lung volume, gas transfer
- Imaging: CXR, CT scan
ABG: normal values
- pH: Acidaemia < 7.35 – 7.45 >Alkalaemia
- Pa02:10.7 – 13.3 (on room air)
- PaCO2:4.7 – 6.0
- HCO3:22 – 26
- Base excess -2 - +2
ROME mnemonic
- Respiratory opposite= if the pH is up and Pco2 is down then its respiratory alkalosis. If the pH is down and the Pco2 is up then its respiratory acidosis.
- Metabolic equal= If the pH and HCO3 are up then its metabolic alkalosis. If the pH and HCO3 are down then its Metabolic acidosis
ABG: rule of thumb CO2
- Metabolic problems PH and PCO2 move in the same direction i.e. both go up
- In Respiratory Problems they tend to move in opposite directions
- Problem is mixed defect – both respiratory and metabolic acidosis etc
Respiratory acidosis
- Acute respiratory acidosis= pH goes down, pCo2 goes up, Bicarbonate doesn’t change
- Chronic respiratory acidosis= pH doesn’t change, pCo2 goes up, Bicarbonate goes up
ABG: alkalosis
- Acute respiratory alkalosis= pH goes up, pCo2 goes down, Bicarbonate doesn’t change
- Chronic respiratory alkalosis= pH doesn’t change, pCo2 goes down, Bicarbonate goes down
- Metabolic alkalosis: If there is an excess of HCO3- compared to H+ due to loss of H+ with severe vomiting then this may cause an alkalosis.
Blood gas: giving oxygen
- Rule of thumb to check for hypoxemia is “the Pao2 should be 10-15 kpa less then the FiO2”
- Example= on room air(Fio2 21%) you would expect the Pao2 to be -> 21-10= 11 Kpa (normally 10-14 kpa)
- If someone is on oxygen with a 40 % Venturi mask(FIo2 is 40%) , there PaO2 should be -> 40-15= 25 Kpa( 25-30 Kpa)
The two systems that control pH
- Respiratory system– controlling ventilation and the amount of CO2 in the blood.
- ‘Metabolic’ system– predominantly controlling the production of HCO3 by the kidneys.
Hypoxia and respiratory failure
- Hypoxia is present if PaO2 < 10.7 whilst breathing air (oxygen content 21%, FiO2 0.21).
- Respiratory failurewould be defined as a PaO2 < 8 whilst breathing air.
- Problem with oxygen being transferred from the alveoli into the bloodstream e.g. due to pulmonary fibrosis – may lead to type 1 respiratory failure. If PaO2 is <8 and the PaCO2 is normal or low
- Problem with oxygen being transferred from the air into the lungs e.g. ventilation problems in neuromuscular disease – may lead to type 2 respiratory failure. If PaO2 is <8 and the PaCO2 is high
Step wise approach to ABG interpretation
1.Is there:- An acidaemia (low pH) < 7.35 or an alkalaemia (high pH) > 7.45
- Determine the respiratory component: If the pH is low, is the PaCO2 high (>6.0) – this is a respiratory acidosis. If the pH is high, is the PaCO2 low (<4.7) – this is a respiratory alkalosis
- Determine the metabolic component: If the pH is low, is the HCO3 low (< 22) – this is a metabolic acidosis. If the pH is high, is the HCO3 high (>26) – this is a metabolic alkalosis
- Is there an oxygenation problem? Is the patient hypoxic – PaO2 < 10.7 breathing air. Is the patient in respiratory failure – PaO2 < 8 breathing air
- If present, which type of respiratory failure: PaCO2 is normal/low in T1RF. PaCO2 is high in T2RF
ABG: mixed cause
- Patients may have a mixed cause of an acid-base problem e.g. a mixed respiratory and metabolic acidosis. pH is low, PaCO2 is high and HCO3 is low.
- The body may have compensated for an acid-base problem, and corrected the pH into normal range whilst the PaCO2 and HCO3 are within abnormal ranges
- In compensated T2RF: pH is normal, PaCO2 is mildly high and HCO3 is high
Spirometry measurements
- Forced vital capacity – the total volume of air that a patient has expired at the end of maximal forced expiration (following maximal inspiration)
- Forced expiratory volume in 1 second (FEV1) –the volume of air that a patient can forcefully expire in 1 second (following maximal inspiration)
- The FEV1/FVC ratio – helps us to identify whether the patient has an obstructive or restrictive pathology
Obstructive vs restrictive pattern
- Obstructive: FEV1/FVC <70%. The FEV1 is reduced more than the FVC. For example: COPD, asthma
- Restrictive: FEV1/FVC >70% is maintained. The FEV1 and FVC are reduced by the same amount. For example, interstitial lung disease
Flow volume loops
- A maximum inspiration followed by a maximum expiration. Records flow at any given volume
- The result is plotted on a graph, with a positive expiratory component (above the x axis) and a negative inspiratory component (below the x axis).
- Used to measure peak expiratory flow rate
- Obstructive: have reduced expiratory flow in the small airways. There is a reduction in the peak expiratory flow and a concave (often compared to a church steeple) appearance to the descending portion of the expiratory limb.
- Restrictive: reduced volume in lungs. Normal appearance but reduced expiratory and inspiratory volumes
Large airway abnormalities: extra-thoracic and intra-thoracic
- The extra-thoracic (upper or above the thoracic inlet) airways which go from the nasal cavity to the cervical trachea i.e. vocal cord paralysis, goitre, malignancy
- The intra-thoracic (lower or below the thoracic inlet) airways which go from the thoracic trachea to the bronchi
Large airway abnormalities: flow volume loops and fixed upper airway obstruction
Flow volume loops:
-In variable extra-thoracic obstruction: limitation in the inspiratory flow
- In fixed upper airway obstruction: present in inspiration and expiration, both components of the flow volume loop are reduced. I.e. tracheal stenosis, endotracheal malignancy and foreign body
TLC
- Measured with a whole body plethysmography in an air tight box
- Total lung capacity (TLC)is the total volume of air in the lungs after maximal inspiration. May be increased in patients with obstructive lung disease such as COPD. May be decreased in patients with restrictive lung disease such as kyphoscoliosis
Residual volume
Residual volume (RV)is the amount of air remaining in the lungs after a maximal expiration. May be increased in patients with obstructive lung disease such as COPD where there is air trapping. If excessively raised can have lung volume reduction surgery
Transfer capacity of the lung for carbon monoxide (TLCO)
- A test of functionality of the alveolar-capillary membrane, when reduced there is reduced surface area for gas exchange
- CO is rapidly inspired and held for a short period of time then expired: remaining CO is then measured and compared with inspired amount
- Reduced TLCO: Interstitial lung disease, COPD, obesity, Thoracic abnormality
Transfer co-efficient (KCO)
Is TLCO corrected for alveolar volume. So is gas exchange measured per unit volume of lung. If you only have a half a lung but its functioning perfectly it will be 100%. Is disproportionately high in obesity and neuromuscular disease
