Acid Base
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Acid Base

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Acid Base




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Slide1

Acid Base

Sophie & Mimi

Any questions – S.A.Greenhalgh@warwick.ac.uk

Slide2

Definitions

What is an

Acid?

Any chemical that can

donate

H

+

(proton)

What is a

Base

Any chemical that can

accept

H

+

Slide3

How does the body combat changes in pH (3)?

Buffers systems (e.g. Bicarbonate

HCO

3

)

Henderson-

Hasselbach

equation

H

+

+

HCO

3

-

↔ H

2

CO

3

↔ H

2

O +

CO

2

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

Slide4

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

Slide5

Slide6

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

)?

L

ook

at

[

HCO

3

-

] &

pCO

2

values

Which of these best fits & explains the pH?

If due to pCO

2

then it is a primary

respiratory

disorder

If due to [

HCO

3

-

]

then it is a primary

metabolic

disorder

Look for evidence

of compensation

Has

th

e other value moved out of its normal range (in the same direction) so as to try to minimise the pH change?

Slide7

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 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 compensationWhat type of Respiratory failure is occurring? Type II – Hypoxia (Oxygen<8kPa) + Hypercapnia (high CO2 >6kPa)

Slide8

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 showspH 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 ketoacidosisWhat is the acid-base disorder? Metabolic acidosis with respiratory compensation

Case 2

Slide9

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 showpH 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.

Slide10

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 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 compensationWhy has this happened? Loss of gastric acid from vomiting

Slide11

Summary of acid-base disorders

Disturbance

Primary

change

Compensation

Respiratory

acidosis

pCO

2

[HCO

3

-

]

Respiratory

alkalosis

pCO

2

↓ [HCO

3

-

]

Metabolic

acidosis

↓ [HCO

3

-

]

pCO

2

Metabolic

alkalosis

[HCO

3

-

]

pCO

2