Download - Learning Outcomes ARTERIAL BLOOD GAS … Handouts.pdfSlide 1 'DYLG2 1HLOO06F %6F51103)+($ Associate Lecturer (Non Medical Prescribing) Cardiff University Advanced Nurse Practitioner

David O’Neill MSc BSc RN NMP FHEA

Associate Lecturer (Non Medical Prescribing) Cardiff University

Advanced Nurse Practitioner Respiratory Medicine

ARTERIAL BLOOD GAS

INTERPRETATION

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 2

David O'Neill

Learning Outcomes

• Understand Acid-base balance physiology

• Understand Buffer systems in acid base

balance

• Basic understanding of Oxygen dissociation

• Know normal ABG parameters

• To systematically analyse and interpret ABG

results

• Review Anion gap and its use

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 3

David O'Neill

Acid base terminology

• Acids are substances which have a high

concentration of Hydrogen ions [H+]

– Which two bodily substances have high

concentration of hydrogen ions? (i.e. very acidic)

• Bases (or alkalis) are substances with low

concentration of hydrogen ions and high

concentration of bicarbonate ions [HCO3-]

– Which bodily substance has a high concentration

of bicarbonate ions (i.e. very alkaline)

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

David O'Neill

pH of body fluids

Body Fluid pH

Gastric juices 1.0-3.0

Urine 5.0-6.0

Arterial blood 7.4

Venous blood 7.36

CSF 7.32

Pancreatic fluid 7.8-8.0

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 5

David O'Neill

pH scale

• Hydrogen ion concentration is

expressed as the pH scale

(range 1 to 14)

• Logarithmic scale

• If pH changes by 1 unit (7.0 to 6.0)

Hydrogen ions increase by tenfold

• Greater [H+] lower pH and vice versa

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 6

David O'Neill

Source of acids

• About 100mmols/day is formed as result of

end products of cellular metabolism of

protein, carbohydrates and fats

• It must be neutralised or excreted

• There are three main systems involved in

acid base balance – what are they?

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

David O'Neill

Acid base balance

• Lungs

• Kidneys

• Bones

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 8

David O'Neill

Acid base balance

• Systems interrelated

• Acid exists in two forms

– VOLATILE

• Eliminated as CO2 gas

– NON VOLATILE

• Are eliminated by the renal tubules

and regulated by HCO3-

• Lungs & Kidneys (assisted by buffers) are

main regulators of acid-base balance

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 9

David O'Neill

Carbonic acid

• Carbonic acid (H2CO3) is a weak acid

(Volatile)

• In presence of CARBONIC

ANHYDRASE (an enzyme)

• It easily breaks down into carbon

dioxide and water

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

David O'Neill

Respiratory equation

LUNGS KIDNEYS

• CO2 + H20 H2CO3 HCO3-+ H

+

This is a reversible reaction and can go both

ways

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 11

David O'Neill

Bicarbonate in the blood

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 12

David O'Neill

Oxygen dissociation curve

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

David O'Neill

Buffer systems 1

• Buffers:

– Absorb excessive H+

ions (Acids) or OH-

ions (Bases)

– Exist in ICF and ECF compartments

– FUNCTION AT DIFFERENT RATES

– Exist as buffer pairs of weak acid and

conjugate base

– Can associate and dissociate

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 14

David O'Neill

Buffer systems 2

• The most important PLASMA buffer

systems are:

– CARBONIC ACID-BICARBONATE

and

– HAEMOGLOBIN

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 15

David O'Neill

Buffer systems 3

• The most important INTRACELLULAR

buffer systems are:

– PHOSPHATE

AND

– PROTEIN

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

David O'Neill

Buffer systems

Buffer

pair

Buffer

system

Reaction Rate

HCO3-

/H2CO3

Bicarbonate H++ HCO3

-= H2O + CO2 INSTANT

Hb-/HHb Haemoglobin HHb = H

++ Hb

-INSTANT

HPO42-

/H2PO4-

Phosphate H2PO4-

= H+

+

HPO4-

INSTANT

Pr-/HPr Plasma

proteins

HPr = H+

+ Pr-

INSTANT

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 17

David O'Neill

ORGAN SYSTEMS

ORGANS MECHANISM RATE

Lungs Regulates retention/elimination

of CO2 and hence H2CO3

Minutes-hours

Ionic shifts Exchange intracellular

potassium and sodium for

hydrogen

2-4 hours

Kidneys Bicarbonate reabsorption and

regeneration, ammonia

formation, phosphate buffering

Hours to days

Bone Exchange calcium, phosphate

and release of carbonate

Hours to days

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 18

David O'Neill

Carbonic acid-Bicarbonate buffering

• The most important buffer.

• Operates in lungs AND kidney

• Lungs get rid of CO2 and retain H2O

• Kidneys reabsorb H2CO3-and water.

• Both systems work well together with

the lungs quickly adjusting acid

concentration and kidneys reabsorb or

regenerate H2CO3-

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

David O'Neill

Why is this important?

• All systems work together to maintain a NORMAL ph (7.35-7.45)

• Outside these parameters enzyme systems start to fail

• Deficit in any of these can affect acid base balance

• Adjustment of abnormal pH is called COMPENSATION but it can leave some abnormal values

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 20

David O'Neill

Renal buffering 1

• Distal tubules:

– Secrete H+

into urine and reabsorbs HCO3-

– Phosphate (HPO42-

) and ammonia (NH3)

– HPO42-

(in tubule) combines with H+

to

create H2PO4-which is lipid insoluble and

hence excreted in urine

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 21

David O'Neill

Renal buffering 2

– H+

combines with NH3 to form ammonium

ions (NH4 +) which are excreted in urine

– Buffering H+

requires use of CO2 and H2O

to form H2CO3-

– New H2CO3-

is added to plasma and H+

are excreted in urine resulting in more

alkaline plasma

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

David O'Neill

Normal ABG

• pH 7.35-7.45

• PaCO2 4.6 – 6 kPa (35-45mmHg*)

• PaO2 10.6- 14.6 kPa (80-110mmHg)

• HCO3 24-26mmols/l

• Base Excess -3 to + 3

*To convert kPa to mmHg multiply by 7.5

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 23

David O'Neill

Base excess

• Derived variable

• Indicates acidity/alkalinity

• Highly NEGATIVE numbers are very ACIDIC(eg -15)

• Highly POSITIVE numbers are very ALKALINE (eg +15)

• It is calculated from how much acid or alkaline is required to return pH to normal at standard temperature and pressure

• It is an alternative to looking at the HCO3 value

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 24

David O'Neill

ABG Interpretation Step 1.

Look at the pH

– Is it Low (equals ACIDIC)

– Is it High (equals ALKALINE)

– Is it Normal

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

David O'Neill

ABG Interpretation Step 2

• Look at the PaCO2

– Is it HIGH (equals ACIDIC)

– Is it LOW (equals ALKALINE)

– Is it NORMAL

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 26

David O'Neill


• Look at PaO2

– Is it HIGH

– Is it LOW

– Is it NORMAL

Does Oxygen have a direct impact on acid

base balance?

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 27

David O'Neill


• Look at the HCO3 OR Base Excess

– Is it LOW (EQUALS ACIDIC)

– Is it HIGH (EQUALS ALKALINE)

– Is it NORMAL

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

David O'Neill

Examples

• pH 7.30

• ACIDIC

• PaCO2 7.3kPa (55mmHg)

• ACIDIC

• PaO2 12kPa (85mmHg)

• NORMAL

• HCO3 24mmols/l

• NORMAL

• Base Excess +1

• NORMAL

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 29

David O'Neill

Interpretation

• Its an ACIDOSIS

• The system causing the acidosis is

RESPIRATORY

(high CO2 causes acidosis)

• The pH is still deranged so its ACUTE

• The HCO3 and BE are still NORMAL so

there is NO COMPENSATION

• So its an ACUTE RESPIRATORY ACIDOSIS

(acute type II respiratory failure)

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 30

David O'Neill

Example 2

• pH 7.37

• PaCO2 7.3kPa (55mmHg)

• PaO2 8.0kPa (60mmHg)

• HCO3 30mmols/l

• Base Excess +7

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

David O'Neill

Interpretation

• pH is NORMAL

• PaCO2 is HIGH (ACID)

• PaO2 is LOW (no effect on pH at this level)

• HCO3 is HIGH (ALKALINE)

• Base excess is HIGH (ALKALINE)

• Original disturbance was respiratory

• COMPENSATION has occurred returning pH to normal range

• Chronic Type II respiratory failure

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 32

David O'Neill

Example 3

• pH 7.25

• PaCO2 10kPa (75mmHg)

• PaO2 7kPa (52.5mmHg)

• HCO3 30mmols/l

• Base Excess +6

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 33

David O'Neill

Interpretation

• pH is deranged (Acidotic)

• Therefore ACUTE

• PaCO2 is HIGH therefore ACID

• PaO2 is LOW (no effect)

• HCO3 and Base Excess are HIGH and

therefore ALKALINE AND CHRONIC

• ACUTE ON CHRONIC TYPE II

RESPIRATORY FAILURE

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

David O'Neill

Example 4

• pH 7.10

• PaCO2 3kPa (22.5mmHg)


• HCO3 14mmols/l

• Base Excess -12

• Blood Glucose 30mmols/l

• Urine Ketones +++ Glucose +++

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 35

David O'Neill

Interpretation

• pH is low therefore ACIDIC

• It is still deranged, therefore ACUTE

• PaCO2 is LOW (ALKALINE)

• PaO2 is high (probably too much O2)

• HCO3 is LOW therefore ACIDIC

• Base Excess is LOW therefore ACIDIC

• So its ACUTE METABOLIC ACIDOSIS with respiratory alkalosis (compensation)

• Given blood glucose and ketones in urine Diabetic Ketoacidosis

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 36

David O'Neill

Example 5

• pH 7.50

• PaCO2 6.6 kPa (50mmHg)


• HCO3 35mmols/l

• Base Excess +10

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

David O'Neill

Interpretation

• pH is HIGH therefore ALKALINE

• It is still deranged therefore ACUTE

• PaCO2 is slightly High (ACIDOTIC) BUT it WON’T get any higher, WHY?

• HCO3 is HIGH (ALKALINE) as is the

• Base Excess (ALKALINE)

• Therefore its an ACUTE METABOLIC ALKALOSIS with respiratory acidosis as compensation

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 38

David O'Neill

Example 6

• pH 7.10

• PaCO2 8kPa (60mmHg)


• HCO3 15mmols/l

• Base Excess -10

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 39

David O'Neill

Interpretation 1

• pH is LOW therefore ACID

• pH is still deranged, therefore ACUTE

• PaCO2 is HIGH therefore ACID

• PaO2 is ? Normal

• HCO3 is LOW therefore ACID

• Base Excess is LOW, therefore ACID

• So what’s going on?

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

David O'Neill

Interpretation 2

• It’s a MIXED ACIDOSIS,

• Predominantly METABOLIC IN ORIGIN

• Action would be to CORRECT THE

UNDERLYING METABOLIC COMPONENT

(DKA, renal Failure, Cardiac failure leading to

lactic acidosis)

• Then review the Pa CO2 which will probably

then have little effect on the pH

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 41

David O'Neill

Anion gap

• Is an estimate of unmeasured Anions

• It is the difference between:

• Cations (Na and K) and

• Anions (Cl and HCO3-

)

• NORMAL RANGE 10-18mols/l

• Eg Na + K – Cl + HCO3-

• (135 + 4.0) – (100 + 24) = 139 – 124 = 15

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 42

David O'Neill

Metabolic Acidosis with increased anion

gap

• Increased production of fixed or organic acids causes:

– HCO3-

to fall

– Unmeasured anions associated with the acids accumulate

• Caused by Increased Lactic acid (shock infection, hypoxia)

• Urea (renal failure)

• Ketones (diabetes, alcohol

• Drugs/toxins (salicylates, biguanides, ethylene glycol, methanol)

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

David O'Neill

Metabolic Acidosis with normal anion

gap

• Due to loss of bicarbonate or ingestion

of hydrogen ions (Cl-

) retained

– Caused by renal tubular acidosis

– Profuse diarrhoea

– Drugs (acetazolamide)

– Addisons disease

– Pancreatic fistula

– Ammonium chloride ingestion

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 44

David O'Neill

Questions

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 45

David O'Neill

References

• Longmore, M, Wilkinson, I, Torok, E (2001) Oxford Hnadbook of

Clinical Medicine Oxford Oxford University Press.

• McCance KL, Heuther, SE. (2006). Pathophysiology-The biologic

basis for disease in adults and children. St Louis. Elsevier-Mosby

• Parson PE Heffner JE. (2002) Pulmonary/Respiratory Therapy

Secrets (2nd Ed.) Philadelphia. Hanley & Belfus

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

David O'Neill

Example 1

pH 7.494

PaCO2 (kPa) 3.2 (24mmHg)

PaO2 (kPa) 21.6 (162.5)

HCO3 (mmols/l) 21

BE -3.2

• ALKALOSIS

• RESPIRATORY CAUSE

• ACUTE (pH still deranged)

• Minimal compensation

• hyperventilation, anxiety

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 47

David O'Neill

Example 2

pH 7.28

PaCO2 10.6 (79.8)

PaO2 6.9 (51.8)

HCO3 32

BE 7

• ACIDOSIS

• RESPIRATORY CAUSE

• ACUTE

• COMPENSATION

• Acute respiratory acidosis with partial compensation

• Acute on chronic Type II respiratory failure

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

Slide 48

David O'Neill

Examples

pH PaCO2

kPa

PaO2

kPa

HCO3

(mmols/l)

BE

7.494 3.2

(24mmHg)

21.6

(162.5)

21 -3.2

7.28 10.6

79.8

6.9

51.8

32 7

7.274 11.1

(83.3)

6.48

(48.6)

35 10.4

7.17 3.9

(29.5)

53

(400.5)

8 -16.6

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

David O'Neill

Examples

pH PaCO2

kPa

PaO2

kPa

HCO3

(mmols/l)

BE

7.278 5.2

(39.2)

8.1

(61.2)

16 -7.9

6.867 105.1 60.3 8 -16.6

7.335 12.2

(91.8)

10.5

(78.8)

16.7 -9

7.365 10.1

(76.9)

14.6

(110)

40 15.4

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________

___________________________________