Week 1/2 - E - Biochemi test/Acid base balance/Resp failure - Sensitivty/specificity/P.P.V/N.P.V, Bicarbonate/Acidosis/Alkalosis

Question 1

How do you assess the ability of a test to diagnose a disease - lets think So we want to look at * Specificity * Sensitivity * Positive predictive value * Negative predictive value What is sensitivity - which boxes are used to calulate this?

Answer 1

Sensitivity of the test is calculated as The proportion of people who correctly identified as positive for the disease over the total number who have the disease (percentage correctly identified as positive) True positive divided by True positive + False negative (total disease present)

Question 2

* Specificity * Sensitivity * Positive predictive value * Negative predictive value What is specificity - which boxes are used to calulate this?

Answer 2

Specificity of the test is calculated as The proportion of people who correctly identified negative for the disease over the total number who do not have the disease (percentage correctly identified as negative) True negative divided by True negative + False positive (total disease absent)

Question 3

The problem with sensitivity and specificity is that you have to know who has and doesn’t have the disease to be able to work them out. If you’ve got a patient in front of you, you don’t know that – indeed, that is precisely why you’re doing the test. A much more useful parameter would be to know what is the likelihood of disease in people with a positive test. * Positive predictive value * Negative predictive value What is positive predictive value- which boxes are used to calulate this?

Answer 3

Positive predictive value is calculated as the Proportion of those who were correctly identified as positive over the total who tested positive (the likelihood of disease with a positive result) True positive divided by True positive + false positive (total positive)

Question 4

* Specificity * Sensitivity * Positive predictive value * Negative predictive value What is negative predictive value- which boxes are used to calulate this?

Answer 4

Negative predicted value - Proportion who correctly identified as negative for the disease over the total number who tested as negative (the likelihood of health with a negative result ) True negative divided by True negative + false negative (total negative)

Question 5

What happens to the PPV and NPPV if the prevalence of the disease decreases?

Answer 5

If the prevalence of the disease decreases * the PPV decreases - the likelihood of disease with a positive result * the NPPV increases - the likelihood of health with a negative result

Question 6

Why is bicarbonate so important in the buffering of hydrogen ions?

Answer 6

Other buffering systems reach an equilibrium However, because carbonic acid is removed as CO2 through the lungs, the only limiting factor to the buffering of hydrogen ions is bicarbonate

Question 7

When speaking of the respiratory and the metabolic components in acid-base balance, what are these components?

Answer 7

We call CO2 the respiratory component We call HCO3- the metabolic component Respiratory: the primary change is in pCO2 Metabolic: the primary change is in HCO3-

Question 8

Define acidaemia, alkalaemia, acidosis and alkalosis?

Answer 8

Acidaemia - an increase in H+ ions lowering the pH of the blood Alkalaemia - a decrease in H+ ions raising the pH of the blood Acidosis - a process that would cause acidaemia if not compensated Alkalosis - a process that would cause alkalaemia if not compensated

Question 9

What ion changes causes a respiratory/metabolic acidosis/alkalosis?

Answer 9

Respiratory acidosis - increase in H+ ions due increased CO2 Respiratory alkalosis - decrease in H+ ions due to decreased CO2 Metabolic acidosis - increase in H+ ions due to decreased HCO3- Metabolic alkalosis - decrease in H+ ions due to increased HCO3-

Question 10

What is a respiratory compensation for a metabolic acidosis? What is a metabolic compensation for a respiratory acidosis?

Answer 10

Respiratory compensation - lungs blow off CO2 to reduce acidity of the blood Metabolic compensation - H+ ions are excreted in the urine and HCO3- is simultaneously produced

Question 11

Name some causes of respiratory acidosis and respiratory alkalosis? Essentially what causes hypoventilation vs what causes hyperventilation

Answer 11

Respiratory acidosis - Alveolar hypoventilation, * Acute airway obstruction with underventilation, * COPD, * choking, * opiate overdose (respiratory depression via mu receptors) Respiratory alkalosis - Alveolar hyperventilation * Anxiety * PE * Pneumonia * Pulmonary oedema

Question 12

Name some causes of metabolic acidosis and metabolic alkalosis?

Answer 12

Metabolic acidosis - * Impaired excretion of H+ ions eg acute renal failure * Increased production of H+ ions eg diabetic ketoacidosis * Circulatory shock causing increased lactic acid production due to anaerobic respiration Metabolic alkalosis * Loss of H+ ions eg vomiting * Diuretics, K+ loss causing increased HCO3- reabsorption * Alkali ingestion

Question 13

How do diuretics cause hypokalaemia and metabolic alkalosis?

Answer 13

Both loop and thiazide diuretics prevent the reabsopriton of sodium proximal to the distal convoluted tubule Due to the low sodium concentration in the blood, at the collecting duct the K+ and H+ ion transporters cause loss of these ions in order to try and retain sodium (HCO3- is also reabsorbed ) This leads to a hypokalaemia and a metabolic alkalsosis

Question 14

Try and rememeber the transporters both loop and thiazide diuretics act on

Answer 14

Loop diuretics act on the Na+/K+/2Cl- cotransporter in the distal thick ascending loop of henle - decreases sodium reabsorption and leading to water enter the tubule osmotically Thiazide diuretics act on the Na/Cl symporter in the distal convoluted tubule preventing sodium reabsorption

Question 15

Classify the acid-base disorder and explain the results

Answer 15

There is haematemesis - blood being lost causing the patient to enter - likely - hypovalaemic shock Therefore in this reduced oxygen state, anaerobic respiration will be taking place leading to increased lactate/lactic acid decreasing the pH f the blood. PCO2 is reduced in an attempt to compensate * METABOLIC ACIDOSIS WITH PARTIAL RESPIRATORY COMPENSATION

Question 16

Classify the acid-base disorder and explain the results

Answer 16

Hydrogen ions raised so acidosis, CO2 rasied so indicating resp acidosis HCO3 also raised indicating partial compensation COPD makes respiratory acidosis more likley due to hypoventilation leading to hypercapnia and decreased O2 * RESPIRATORY ACIDOSIS WITH PARTIAL METABOLIC COMPENSATION

Question 17

Type 1 and Type 2 respiratory failure What’s the difference?

Answer 17

Type 1 respiratory failure - defined as hypoxia (PaO2 6kPa)

Question 18

There are no diseases that will only causes type 1 or type 2 failure. The physiology of hypoventilation leads initially to hypoxaemia, then to hypercabia. Some diseases more characteristically lead to chronic type 2 respiratory failure, but not always. If in doubt, assume type 2 failure and do a blood gas. What are the causes of type 1 respiratory failure? What is the primary cause?

Answer 18

Causes of type 1 respiratory failure - primarily ventilation/perfusion mismatching Also, hypoventilation, abnormal diffusion, right tp left shunts

Question 19

What is a ventilation perfusion mismatch? What is the V/Q ratio of a healthy persons lungs approximately equal to?

Answer 19

Ventilation perfusion mismatch or “V/Q defects” are defects in total lung ventilation perfusion ratio. It is a condition in which one or more areas of the lung receive oxygen but no blood flow, or they receive blood flow but no oxygen due to some diseases and disorders. The V/Q ratio of a healthy lung is approximately equal to 0.8, as normal lungs are not perfectly matched.,[1] which means the rate of alveolar ventilation to the rate of pulmonary blood flow is roughly equal.

Question 20

V/Q - ventilation but no blood flow (perfusion) or perfusion but no ventilation Give examples of diseases that can cause this?

Answer 20

Pneumonia, pulmonary oedema, PE, asthma, emphysema, pulmonary fibrosis

Question 21

Why would a PE cause a V/Q mismatch? How would this lead to type 1 resp filure?

Answer 21

PE for exam - adequate ventilation to the alveoli however there is a perfusion defect with defect in blood flow. Gas exchange thus becomes highly inefficient leading to hypoxemia as measured by arterial oxygenation In asthma or COPD - the ventilation is the problem as the blood flow is adequate - meaning there is a hypoxaemia again

Question 22

What is ventilation without perfusion known as? What is perfusion without ventilation known as? Which area of the lung has which?

Answer 22

Ventilation without perfusion - dead space - lung apex Perfusion without ventilation - shunting - lung base

Question 23

How could a V/Q mismatch be diagnosed? What would be the problem in Pneumoia, PE, Asthma, COPD?

Answer 23

1. Ventilation Scan is abnormal but perfusion scan is normal indicating abnormal airway suggesting COPD or ASTHMA. 2. Ventilation Scan is normal but perfusion is abnormal indicating any obstruction to the blood flow (perfusion), may be because of the PULMONARY EMBOLISM obstructing the flow. * 3. Both scans are abnormal. It may be found in PNEUMONIA or COPD.

Question 24

How is the type 1 respiratory failure treated?

Answer 24

Treat the underlying cause Also give oxygen via facemask - usually 60% Assisted ventilation if PaO2 drops below 8kPa despite 60% O2

Question 25

What are the causes of Type 2 respiratory failure?

Answer 25

Caused by alveolar hypoventilation with or without V/Q mismatch eg Asthma, COPD, OSA Reduced respiratory drive - sedative drugs, opiates Many other reasons

Question 26

What are the signs and symptoms of hypoxia and hypercapnia?

Answer 26

Hypoxia - dyspnoea, restlesness, confusion, central cyanosis Hypercapnia - headache, peripheral vasodilation, tachycardia, tremor/flap, confusion

Question 27

What is the treatment of Type 2 respiratory failure?

Answer 27

Treat underlying cause Controlled oxygen therapy eg 24%-28% O2 - be wary of rising CO2

Question 28

If CO2 rises when giving the patient oxygen and the patient is still hypoxic, what is the treatment?

Answer 28

Patient may need non invasive positive pressure ventilation (NIPPV) If PCO2 continues to rise, consider intubation and invasive ventilation

Question 29

Important to also understand causes of metabolic acidosis

This is done by calculating the anion gap
How is the anion gap calculated?
What is the normal anion gap range?

Answer 29

Anion gap = (sodium + potassium) - (bicarbonate + chloride)
= +ve minus -ve ions

Normal range is 8-12mmol/l

Question 30

Calculating the anion gap allows you to try and separate the possible causes of metabolic acidosis

What are the causes of a
* raised anion gap metabolic acidosis (MUDPILES)
* normal anion gap or hyperchloraemic metabolic acidosis (DURHAM)

Answer 30