classical gas: theory: henderson-hasselbalch equation ...€¦ · 7/12/2013 1 classical gas: blood...
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7/12/2013
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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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10
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