1. Acid Base
Sophie & Mimi
Any questions –

2. Definitions What is an Acid? Any chemical that can donate H+ (proton)
What is a Base
Any chemical that can accept H+
Acids e.g. hydrochloric acid (HCl)
HCl → H+ + Cl-
Bases e.g. sodium hydroxide (NaOH)
NaOH → Na+ + OH-
allowing OH- + H+ → H2O

3. How does the body combat changes in pH (3)?
Buffers systems (e.g. Bicarbonate HCO3 )
Henderson-Hasselbach equation
H+ + HCO3- ↔ H2CO3 ↔ H2O + CO2
Compensation
Lungs
Rapid
can alter levels of CO₂ by increasing or decreasing ventilation
Kidneys
Slow
Can alter levels of bicarbonate by increasing/decreasing reabsorption
Can alter levels of H+ by increases or decreasing secretion
Buffer systems
Rapid chemical reactions that minimise any sudden changes in pH
Unable to change overall body [H+]
Bicarbonate buffer system (extracellular)
Phosphate buffer system (intracellular & urine)
Protein buffer system (mainly intracellular)
Carbonic anhydrase is the enzyme that coverts H2CO3 ↔ H2O + CO2
Compensation
As it is the ratio of [HCO3-] & [CO2] that gives us the pH, this means that an abnormality affecting one parameter can be compensated for to a certain degree by changes in the other
Minimises the changes in pH; tries to restore back towards normal
In compensated disorders, both [HCO3-] & [CO2] values lie outside their normal ranges (& in same direction i.e. both raised or lowered)
Lungs
Can rapidly adjust the excretion of CO2
Kidneys
Can slowly adjust the excretion of H+ into the urine (and alter body bicarbonate (HCO3-) levels)

4. Acidosis or Alkalosis….
What is the normal range of blood pH?
Acidosis - blood is more acidic than normal
(i.e. lower pH)
Alkalosis - blood becoming more basic (alkaline) than normal (i.e. higher pH)
Metabolic – problem with Bicarbonate
Respiratory – problem with CO₂
Acidosis can result from:-
Addition of acid - ↑CO2
Loss of alkali - ↓Bicarbonate
Alkalosis can result from:-
Addition of alkali - ↑Bicarbonate
Loss of acid - ↓CO2

5. pH Acidosis Alkalosis Respiratory Compensation No compensation Mixed
Metabolic
No Compensation
Alkalosis

6. Acid-base interpretation
As with any test result, interpret within the clinical context and look at the patient – are the results consistent?
Look at pH first
Is it normal, low (acidosis), or raised (alkalosis)?
Look at [HCO3-] & pCO2 values
Which of these best fits & explains the pH?
If due to pCO2 then it is a primary respiratory disorder
If due to [HCO3-] then it is a primary metabolic disorder
Look for evidence of compensation
Has the other value moved out of its normal range (in the same direction) so as to try to minimise the pH change?

7. Case 1
A 60 year old man with a history of chronic obstructive pulmonary disease presents to the emergency department with increasing shortness of breath, pyrexia, and a cough productive of yellow-green sputum. He is unable to speak in full sentences. His wife says he has been unwell for two days. On examination, a wheeze can be heard with crackles in the lower lobes; he has a tachycardia and a bounding pulse.
Measurement of arterial blood gas shows
pH ( )
PaCO2 9.3 kPa (4.6—6.4 kPa)
HCO mmol/L (22-30mmol/L)
PaO2 7.9 kPa (11-15 kPa)
What is the acid-base disorder?
Respiratory acidosis with no compensation
What type of Respiratory failure is occurring?
Type II – Hypoxia (Oxygen<8kPa) + Hypercapnia (high CO2 >6kPa)
Any condition leading to inadequate ventilation and consequent retention of carbon dioxide will lead to respiratory acidosis.
Causes include:
Airways disorders—life threatening asthma, acute exacerbation of chronic obstructive pulmonary disease
Drugs—opioids, sedatives, muscle relaxants 
Central nervous system disorders—brainstem stroke, status epilepticus
Neuromuscular disorders—myasthenia gravis.
He is in type II respiratory failure as he is both hypoxaemic and hypercapnic. He should be treated with bronchodilators, oral steroids, antibiotics, and controlled oxygen.
Most patients can be treated safely with oxygen, but a few with chronic obstructive pulmonary disease rely on their hypoxic drive to breathe. Take care when giving them oxygen, and remember to recheck their arterial blood gas levels. If the patient does not improve, he or she may require assisted ventilation either non-invasively with a mask or invasively after sedation and endotracheal intubation.

8. Case 2
A six year old boy is taken to the emergency department with vomiting and a decreased level of consciousness. His breathing is slow and deep, and he is lethargic and irritable. He appears to be dehydrated—his eyes are sunken and mucous membranes are dry—and he has a two week history of polydipsia, polyuria, and weight loss.
Measurement of arterial blood gas shows
pH ( )
PaO kPa (11-15 kPa)
PaCO2 3.3 kPa (4.6—6.4 kPa)
HCO mmol/L (22-30mmol/L)
other results are Na+ 126 mmol/L, K+ 5 mmol/L, and Cl- 95 mmol/L.
What do you think is going on?
Probably undiagnosed Diabetes Mellitus Type 1 presenting with diabetic ketoacidosis
What is the acid-base disorder?
Metabolic acidosis with respiratory compensation
The boy has diabetes mellitus with an acute diabetic ketoacidosis. These results show that he has metabolic acidosis (low HCO3 -) with respiratory compensation (low CO2).
This patient should be treated in the paediatric intensive care unit. He should be given intravenous fluids, insulin by infusion, and potassium replacement, and he may need cardiac monitoring.
Metabolic acidosis has many causes including:
Lactic acidosis—shock, infection, tissue ischaemia
Ketoacidosis—diabetes mellitus, alcohol abuse
Urate—renal failure
Poisoning—salicylates, biguanides, ethylene glycol, methanol.
Renal tubular acidosis
Severe diarrhoea (intestinal secretions below the stomach contain a large amount of bicarbonate).

9. Case 3
A 12 year old girl attends the ED after falling and hurting her arm. In triage she is noted to have tachycardia and tachypnoea. She is given some pain killers. While waiting to be seen, she becomes increasingly hysterical, complaining that she is still in pain and now experiencing muscle cramps and paraesthesia.
Measurement of arterial blood gas show
pH ( )
PaO kPa (11-15 kPa)
PaCO2 3.9 kPa (4.6—6.4 kPa)
HCO mmol/L (22-30mmol/L)
What is the acid-base disorder?
Respiratory alkalosis with no compensation
How has this happened?
Pain and anxiety caused her to hyperventilate i.e. blowing off too much CO2
Why no compensation?
Onset is too rapid for metabolic compensation.
Paraesthesia – tingling sensation, “pins and needles”
She should be treated with a stronger analgesic and given reassurance to slow down her breathing. Some people breathe in and out of a paper bag so that CO2 is reinhaled and PaCO2 is brought back to normal.
Note that muscle cramps, tingling, and paraesthesia are caused by low serum calcium, which results from the low H+ ion concentration (increased pH) promoting an increased binding of calcium to proteins and a reduction in ionised serum calcium.
Respiratory alkalosis results from hyperventilation. There are many causes, such as:
Lung disorders—pneumonia, pulmonary embolism, pulmonary oedema
Hypoxia—anaemia, high altitude, right to left cardiac shunt
Central nervous system disorders—meningitis
Psychogenesis—pain and anxiety
Drugs—catecholamines, theophylline, and early stage of salicylates overdose.

10. Case 4
An 80 year old woman presents with a two day history of persistent vomiting. She is lethargic and weak and has myalgia. Her mucous membranes are dry and her capillary refill takes >4 seconds. She is diagnosed as having gastroenteritis and dehydration.
Measurement of arterial blood gas shows
pH (Normal range: )
PaO kPa (Normal range: kPa)
PaCO2 6.0 kPa (Normal range: 4.6—6.4 kPa)
HCO mmol/L. (Normal range: 22-30mmol/L)
What is the acid-base disorder?
Metabolic alkalosis with no compensation
Why has this happened?
Loss of gastric acid from vomiting
The primary disorder is metabolic alkalosis (high HCO3 -). As CO2 is the strongest driver of respiration, it generally will not allow hypoventilation as compensation for metabolic alkalosis. The patient should be treated with normal saline and an appropriate amount of KCl, which should be delivered slowly, to expand the extracellular fluid volume. As the body rehydrates, the kidneys will excrete the excess HCO3 - and correct the alkalosis.
Metabolic alkalosis is most commonly associated with:
Loss of gastric acid from vomiting
Diuretic—hypokalaemia
Burns—due to volume depletion
Antacid overdose
Primary hyperaldosteronism.

11. Summary of acid-base disorders
Disturbance
Primary change
Compensation
Respiratory acidosis
↑ pCO2
↑ [HCO3-]
Respiratory alkalosis
↓ pCO2
↓ [HCO3-]
Metabolic acidosis
Metabolic alkalosis