Tests and biochemistry Flashcards
Define accuracy
How close the result is to the true value
Define precision
The reproducibility of an analytical result
What are the three types of variation?
- pre analytical variation
- analytical variation
- intrinsic biological variation
Name some contributors to pre-analytical variation
- time of day
- posture
- fasting status
- recent exercise
- time in transit to laboratory
- temperature during transit
- anticoagulant or preservative used
- centrifugation time and force
Why is bircarbonate so important?
- other buffer systems reach equilibrium
- carbonic acid (H2CO3) is removed as CO2
- so the only limit is the initial concentration of HCO3-
Define acidosis
Increased [H+] or a process tending to increase [H+]
Define alkalosis
Decreased [H+] or a process tending to increase [H+]
Define acidaemia
Increased [H+]
Define alkalaemia
Decreased [H+]
Define respiratory acidosis
Increased [H+] due to increased pCO2
Define respiratory alkalosis
Decreased [H+] due to decreased pCO2
Define metabolic acidosis
Increased [H+] due to decreased HCO3-
Define metabolic alkalosis
Decreased [H+] due to increased HCO3-
How does the respiratory system compensate for metabolic acidosis?
Primary problem is too much H+, respiratory system compensates by blowing off CO2
How does the metabolic system compensate for respiratory acidosis?
Primary problem is too much CO2, compensate by excreting more H+
What does compensation aim to do?
Restore the ratio opposite to normal
Name some causes of acidosis
- choking
- bronchopneumonia
- COAD
- increased H+ production or ingestion
- impaired H+ excretion
- loss of HCO3-
Name some causes of alkalosis
- hysterical overbreathing
- mechanical over-ventilation
- raised intracranial pressure
- loss of H+ in vomit
- alkali ingestion
- potassium deficiency
Name some effort dependant tests
- Forced expiratory volume / flow rates
- spirometry
Name some effort independent tests
- relaxed vital capacity - spirometry
- helium / N2 washout static lung volumes
- whole body plethysmography
- impulse oscillometry
- exhaled breath nitric oxide
Describe the FVC in asthma
FVC is usually reached except in patients with airway remodelling
What is a normal FEV1/FVC ratio
More than 75% is normal
Describe the FVC in COPD
Decreased FVC
Describe FEV1, FVC and the FEV1/FVC ratio in restrictive disease
FEV1 = decreased FVC= decreased FEV1/FVC= >75%
If the FEV is reduced in proportion to FVC it is restrictive disease
Describe exercise testing
- decreased FEV1 or decreased PEF post exercise; asthma
- decreased SaO2 during exercise in interstitial lung disease; to monitor treatment response
- full cardiopulmonary exercise test (CPET) to differentiate cardiac vs respiratory dyspnoea. Heart rate vs oxygen uptake vs ventilatory rate
Describe the measurement of airway resistance
- measured by either whole body plethysmography or more commonly/easily with impulse oscillometer
- impulse oscillometer (IOS) measures airway resistance during quiet tidal breathing at different resonant frequencies to give total resistance
- useful in patients (eg.kids) where it is easier to breathe at tidal volume rather than doing forced expiratory manoeuvres
Describe pH testing
- it is a logarithmic scale of hydrogen ion concentration
- being a log scale, a small change in the pH signifies a big change in hydrogen ion concentration
- pH is regulated over a small range because of the effect of an acidosis or alkalosis on protein structure and hence protein function
Describe PaO2
- it is the partial pressure of oxygen in arterial blood. It is generally abbreviated to PO2 in blood gas results
- normal range for breathing air is 9.3-13.3 kPa
- oxygen is vital for human survival because of its importance in energy production
- failure to oxygenate a patient appropriately will result in significant harm or death
- the PaO2 must be interpreted with the inspired oxygen concentration
Describe PaCO2
- normal range is 4.7-6 kPa
- carbon dioxide is a by-product of metabolism
- accumulation results in an acidosis
- a high PaCO2 indicates a respiratory acidosis
- a high PaCO2 indicates a metabolic alkalosis
- the body will try to compensate for a metabolic alkalosis by raising PaCO2 (hypoventilating)
- the bod will try to compensate for a metabolic acidosis by lowering PaCO2 (by hyperventilating)
Describe base excess (BE)
- normal range is -2 to +2 mmol/l
- base excess is a measure of how much base has to be removed or added to the system to correct the pH in standard conditions
- it is therefore a measure of metabolic acidosis or alkalosis
- a negative BE indicates acidosis
- a positive BE indicates alkalosis
- the kidney will try to compensate for respiratory acidosis by creating a metabolic alkalosis (by retaining bicarbonate)
- the kidney will try to compensate for a respiratory alkalosis by creating a metabolic acidosis (by excreting carbonate)
A decreased PO2 suggests what?
Hypoxia
