acid base sophie & mimi any questions – [email protected]
TRANSCRIPT
Definitions• What is an Acid?
Any chemical that can donate H+ (proton)
• What is a BaseAny chemical that can accept H+
How does the body combat changes in pH (3)?
1. Buffers systems (e.g. Bicarbonate HCO3 )
• Henderson-Hasselbach equationH+ + HCO3
- ↔ H2CO3 ↔ H2O + CO2
• Compensation
2. Lungs• Rapid
o can alter levels of CO₂ by increasing or decreasing ventilation
3. Kidneys• Slow
o Can alter levels of bicarbonate by increasing/decreasing reabsorptiono Can alter levels of H+ by increases or decreasing secretion
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 BicarbonateRespiratory – problem with CO₂
pH
Low
Ph
<7.3
5
Acidosis
High CO₂
RespiratoryHigh Bicarb
Compensation
Normal Bicarb No compensationLow Bicarb
Mixed
Low Bicarbonate Metabolic
High CO₂Mixed
Normal CO₂ No CompensationLow CO₂
Compensation
High pH >7.45
Alkalosis Low CO₂ Respiratory
Low BicarbCompensation
Normal CO₂ No compensationHigh CO₂
MixedHigh Bicarbonate
Metabolic
Low CO₂Mixed
Normal CO₂ No CompensationHigh CO₂
Compensation
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?
Case 1A 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 7.2 (7.35-7.45)PaCO2 9.3 kPa (4.6—6.4 kPa)HCO3 - 27 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)
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 7.2 (7.35-7.45)PaO2 13.3 kPa (11-15 kPa)PaCO2 3.3 kPa (4.6—6.4 kPa)HCO3 - 10 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
Case 2
Case 3A 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 7.5 (7.35-7.45)
PaO2 15.3 kPa (11-15 kPa)
PaCO2 3.9 kPa (4.6—6.4 kPa)
HCO3 - 24 mmol/L (22-30mmol/L)
What is the acid-base disorder?
Respiratory alkalosis with no compensationHow has this happened?
Pain and anxiety caused her to hyperventilate i.e. blowing off too much CO2Why no compensation?
Onset is too rapid for metabolic compensation.
Case 4An 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 7.5 (Normal range: 7.35-7.45)PaO2 11.3 kPa (Normal range: 11-15 kPa)PaCO2 6.0 kPa (Normal range: 4.6—6.4 kPa)HCO3 - 37 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
Summary of acid-base disorders
Disturbance Primary change CompensationRespiratory acidosis ↑ pCO2 ↑ [HCO3
-]
Respiratory alkalosis ↓ pCO2 ↓ [HCO3-]
Metabolic acidosis ↓ [HCO3-] ↓ pCO2
Metabolic alkalosis ↑ [HCO3-] ↑ pCO2