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Classical Gas:

Blood gases for occasional use

MP Metz

26 June 2013

Country Health/AACB Webinar

Scheme

• Blood gas components & units

• Oxygen consumption and carriage

• Theory: Henderson-Hasselbalch equation

• Interpretation

– Metabolic & respiratory acidosis & alkalosis

– Acid-Base nomogram & plotting gases

– Arterial, venous, capillary

• Collection hazards & Artefacts

Analyte Units Reference Range(arterial)

Reference Range(venous)

pH - 7.35 – 7.45 7.31 – 7.41

pCO2 mmHg 35 - 45 41 – 51

pO2 mmHg 80 - 105 -

Sodium mmol/L 138 - 146 138 – 146

Potassium mmol/L 3.5 – 4.9 3.5 – 4.9

Ionised Calcium mmol/L 1.12 – 1.32 1.12 – 1.32

Chloride mmol/L 98 - 109 98 – 109

Urea mmol/L 2.9 – 9.4 2.9 – 9.4

Creatinine µmol/L 53 - 115 53 - 115

Glucose mmol/L 3.9 – 5.8 3.9 – 5.8

Lactate mmol/L 0.36 – 1.25 0.90 – 1.70

Units for pH

• pH is unitless and is defined as the negative

logarithm of the hydrogen ion concentration.

• Hydrogen ion concentration in nmol/L (10-9 mol/L)[H +] nmol/L pH

20 7.70

30 7.52

40 7.40

50 7.30

60 7.22

70 7.15

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Units for gas pressure

mmHg, kPascal, Torr, atmosphere

1 mmHg = 0.13 kPa = 1 Torr

1 kPa = 7.7 mmHg

1 atmosphere is 760 mmHg

http://www-users.med.cornell.edu/~spon/picu/calc/pressure.htm

We will use mmHg

(figure from Lutgens and Tarbuck, T

he Atm

osphere, 8th edition)

20%

80%

0%

Dry Air

Atmospheric pressure = 760 mmHg

Water vapor pressure varies from about

4 mmHg at 0° to 20 mmHg at 20° to

55 mmHg at 4055 mmHg at 4055 mmHg at 4055 mmHg at 40°°°°

Atmospheric pressure.

• Roughly air is 80% nitrogen and 20% oxygen

• Nitrogen partial pressure (pN2) is 600 mmHg

• Oxygen partial pressure (pO2) is 150 mmHg

• At body temperature, water vapour pressure

(pH20) is about 50 mm HG

Gas mmHg %

N2 570 75

O2 100 13

CO2 40 5

H2O 50 6.6

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Alveolar-arteolar gradient

http://www-users.med.cornell.edu/~spon/picu/calc/aagrad.htm

A normal A-a gradient in a young adult is less than 10 mmHg.The age (years) / 4 + 4 or Age x 0.4 are rough estimates

of a normal gradient.

The difference between alveolar and arterial O2 concentration is a function of the “health” of the lung.

Capillary endo cells, basement membrane and alveolar cells.

Shunting of blood away from alveolar space.

PAO2 = (FiO2 * (760 - 47)) - (PaCO2 / 0.8)(FiO2*700)- (PaCO2*1.25)

A-a gradient = PAO2 - PaO2

PA is alveolar Pa is arterial

Oxygen, the stuff of life

• “Normal” Oxygen consumption at rest is 250 ml/min

• Oxygen in blood is about 2% dissolved in plasma and

about 98% bound to haemoglobin

• At pO2 of 100 mmHg, 3 ml of O2 in 1 liter of plasma

• 1.3 ml of oxygen binds to 1 gram of Hb at 100%

saturation. 120 g of Hb (1 liter of blood) carries 150

ml of O2

• Normal cardiac output is about 5 liters/min

Respiratory Physiology, eds. AC Guyton, JG Widdicombe, Baltimore: University Park Press, pp. 67–104, 1974.

%sat (120

gm/L Hb)

O2 carried

(mls)

100 150

90 135

80 120

75 112.5

Oxygen dissociation curve

http://www.siggaard-andersen.dk/

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Measures of oxygen

• Clear that oxygen delivered to the tissues is

the important measure of oxygen.

• That is a function of haemoglobin

concentration, factors affecting oxygen

dissociation- drugs, pH, Hb itself, and

circulation.

• O2 saturation is often used as a surrogate.

• Our devices don’t measure that so we will use

pO2 as the surrogate.

Analyte Units Reference Range(arterial)

Reference Range(venous)

pH - 7.35 – 7.45 7.31 – 7.41

pCO2 mmHg 35 - 45 41 – 51

pO2 mmHg 80 - 105 -

Oxygenation

Two main features to evaluation of O2 state

A) Oxygen availability (or a surrogate)

B) Lung function –A-a gradient

Carbon dioxide, life’s rubbish

• CO2 is the end product of aerobic metabolism.

• The acid that is the product of life.

• A huge role of the lung is to excrete CO2.

Moiety mmol/dayCO2 13000

Lactate 1500

Ammonium 40

Phosphate Sulfate 30

pO2 (or better yet- oxygen saturation, spO2) can be

measured well with an oxygen saturation device

pCO2 can be measured with end-tidal CO2 device

pH can only be measured via blood gas device.

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Acid-Base

and blood gases

Henderson-Hasselbalch Equation

pH = 6.1 + log [HCO3-]

pCO2 x 0.03

mmol/L

mm Hg

CO2 + H2O HCO3- + H+

Carbonic anhydrase

They were contemporaries in the first half of the 20th century.Henderson was a Harvard prof who developed this equation.Hasselbalch was a Dane who first measured pH in blood & “logged” the equation.

pH = 6.1 + log [HCO3-]

pCO2 x 0.03

mmol/L

mm Hg

pH go up→ HCO3- goes up

or pCO2 goes down.pH go down→ HCO3

- goes down or pCO2 goes up.

Analyte Units Reference Range(arterial)

Reference Range(venous)

pH - 7.35 – 7.45 7.31 – 7.41

pCO2 mmHg 35 - 45 41 – 51

pO2 mmHg 80 - 105 -

HCO3- = 0.03 * pCO2 * 10 (pH - 6.1)

www-users.med.cornell.edu/~spon/picu/calc/basecalc.htm

HCO3- AKA bicarb

When reported by gas machines as part of a blood gas is a calculated number.

When reported as part of EUC or electrolyte panel is measured.

Not enough difference to discuss here.

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pC

O2

→→

pH →→ 7.40

40

Metabolic

Acidosis

Metabolic

Alkalosis

Respiratory

Acidosis

Respiratory

Alkalosis

pH - 7.35 – 7.45

pCO2 mmHg 35 - 45

pCO2 ↑Bicarb ↔↑

pCO2 ↑Bicarb ↑

pCO2 ↓Bicarb ↓

pCO2 ↓Bicarb ↔↓

0

20

40

60

80

100

120

140

6.90 7.00 7.10 7.20 7.30 7.40 7.50 7.60 7.70

pC

O2

pH

1500 ABGs from ED

pH 7.35 – 7.45

pCO2 35 - 45

Interpretation

• Start with the pH.

– Acidaemia pH<7.35

– Alkalaemia pH>7.45

• Compensatory schemes are very potent

but don’t return pH to normal

pH<7.35

Acidaemia

Check pCO2

• >45 mmHg

– Respiratory acidosis

• < 35 mmHg

– Metabolic acidosis with respiratory compensation

– Low Bicarb

pH ↓HCO3- ↓ pCO2 ↑

pH = 6.1 + log [HCO 3-] pCO2 x 0.03

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pH>7.45

AlkalaemiaCheck pCO2

• >45 mmHg

– Metabolic alkalosis with respiratory compensation

– Raised bicarb

• < 35 mmHg

– Respiratory alkalosis

pH ↑HCO3- ↑ pCO2 ↓

pH = 6.1 + log [HCO 3-] pCO2 x 0.03

Courtesy of Noel Walmsley

Courtesy of Noel Walmsley

Rules of thumb

• Acute change in ventilation

– For 10 mmHg change in pCO2 one way.

– A 0.1 pH change in the other.

– ie. pCO2 of 50 mmHg causes pH of 7.3.

– Working from that one can try to estimate how

much compensation has occurred.

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Rules of thumb

• Maxima of compensation

– Metabolic Acidosis- pCO2 down to 10 mmHg

– Metabolic Alkalosis-pCO2 up to 60 mmHg

– Respiratory Acidosis- Bicarb up to 40 mmol/L

– Respiratory Alkalosis- Bicarb down to 10 mmol/L

J. Clin. C

hem. C

lin. Biochein.

Vol. 25, 1987, pp. 795-798O

. Müller-P

latheD

erived from S

iggaard-Andersen w

ork

http://www.siggaard-andersen.dk/

http://web.squ.edu.om

/med-Lib/

Sultan Q

aboos University M

edical Library of Om

an.

0

20

40

60

80

100

120

140

6.90 7.00 7.10 7.20 7.30 7.40 7.50 7.60 7.70

pC

O2

pH

Arterial Gases

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Evaluating acid base state

• In addition to having a primary disorder with

compensation.

• One can always have two (or more) primary

disorders.

• Sometimes it is extremely difficult to sort.

• Lists for causes of acidaemia & alkalosis.

Suitable samples

• Arterial- gold standard

– Challenging & Painful

– Risk of artery occlusion & ischaemic injury

Suitable samples

• Venous

– pH (-0.03) & pCO2 (+5 mmHg) reflect arterial values closely

– No good for pO2.

• Capillary (Ear lobe or finger)

– pH and pCO2 very similar

– pO2 is always less, and may have a fair correlation

– Collection technique makes a difference for cap.It can be learned.

• Arterialised (warm and perfused)

0

20

40

60

80

100

120

6.90 7.00 7.10 7.20 7.30 7.40 7.50 7.60

pH

pCO

2

2000 VBG from ED

pH 7.31 – 7.41

pCO2 41 – 51

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0

20

40

60

80

100

120

6.90 7.00 7.10 7.20 7.30 7.40 7.50 7.60

pH

pCO

2

0

20

40

60

80

100

120

140

6.90 7.00 7.10 7.20 7.30 7.40 7.50 7.60 7.70

pC

O2

pHArterial

Venous

Pre- Analytical artefact

• Things have changed.

• No longer ice slurry for a glass syringe.

• Supplied syringes do well.

• Heparin concentration effects some

non-gas analytes & pH.

• Analyse within 30 minutes.

Pre- Analytical artefact

0.03 mm Hg/10 min2 mm Hg/10 minpO2

0.1 mm Hg/10 min1 mm Hg/10 minpCO2

0.002/10 min0.01/10 minpH

4o C37o CParameter

With increased white cell counts, all bets are off.

WBC >100,000, pO2 falls 20 mmHg in 2 min

even with icing.

Pre- Analytical artefact

• Plastic syringes are gas permeable.

• Gases will equilibrate with the atmosphere.

• pCO2 decreases.

• pO2 moves toward 150 mmHg.

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Pre- Analytical artefact

• Settling artefact.

• Often gases are needed with sick patients.

• Too much to do and too few people to do it.

• Collect a gas, set it down, and analyse later.

• Within the syringe, red cells will settle, one

ends up with sampling artefact.

• Always mix (roll).

Examples

78 year woman SOB on room air.pO2 111 mmHg(NR: 65-83 )

pCO2 14 mmHg(NR: 35-45 )

pH 7.31 (NR: 7.35-7.45 )

Bicarb 8 mmol/L(NR: 21-31)

pH (7.34-7.45) 7.51

pCO2 (34-45) mmHg 50

HCO3 (18.0-26.0) mmol/L 40

5 week old boy, vomiting

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83 yo woman hypo … met alk

pH 7.55 (NR: 7.35-7.45 )

pCO2 55 mmHg (NR: 35-45 )

HCO3 47 mmol/L (NR: 22-32)

13 year boy feeling poorly.

Bicarb (19.0-30.0) mmol/L < 2.0

pH (7.34-7.45) 7.07

pCO2 (34-45) mmHg 6

78 y/o man chronic lung disease

p02 121mmHg (90-110)

pCO2 87mmHg (35-45)

pH 7.35 (7.35-7.45)

Bicarb 47mmol/L (22-31)

77 y woman on 1.5 liter/minute

pO2 46 mmHg (NR: 65-83 )

pCO2 113 mmHg (NR: 35-45 )

pH 7.16 (NR: 7.35-7.45 )

Bicarb 39 mmol/L (NR: 22-32 )

Oxygen Sat 67.1 % (NR: 95.0-98.0 )

Is that venous

or arterial?

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27 year woman in ED with asthma

pH 7.45 (7.35 - 7.45)

pCO2 33 mmHg (34 - 45)

pO2 220 mmHg (80 - 100)

Bicarb 22.7 mmol/L (22 - 28)

Inspired Oxygen 40%

A-a gradient (pAO2-paO2)pAO2=FiO2(700)-CO2*1.25

=0.4(700)-33*1.25pAO2= 280-40 = 240A-a grad= 240-220 =20

59 year man unwell pneumonia

pO2 57mmHg (NR: 74-94 )

pCO2 85mmHg (NR: 35-45 )

pH 7.32 (NR: 7.35-7.45 )

Bicarb 42mmol/L (NR: 22-32 )

Oxygen Sat 88.3% (NR: 95.0-98.0 )

Is this arterial or venous?

6 year girl wheezingpH (7.34-7.45) 7.04

pCO2 (34-45) mmHg 100

pO2 (83-108) mmHg 225

HCO3 (20.0-28.0) mmol/L 26

59 year man breathing hardpO2 87mmHg (NR: 74-94 )

pCO2 22mmHg (NR: 35-45 )

pH 7.60 (NR: 7.35-7.45 )

Bicarb 22mmol/L (NR: 22-32 )

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Classical Gas:

Blood gases for occasional use

MP Metz

26 June 2013

Country Health/AACB Webinar

Brand newborn child

pH (7.34-7.45) 6.93

pCO2 (34-45) mmHg 40

pO2 (80-90) mmHg 71

HCO3 (18.0-26.0) mmol/L 8

Little one with movement disorder

pH (7.34-7.45) 7.55

pCO2 (34-45) mmHg 17

pO2 (80-90) mmHg 428

HCO3 (18.0-26.0) mmol/L 15.1