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Regulation of acid-base balance:
Objectives:
 Defense
WHY?!!
against
the
change
in
pH.
 Because H+ react highly with cellular proteins
resulting in alteration in their function therefore
avoiding acidemia and alkalemia by tightly
regulation H+ which is essential for normal
cellular function.
Why we do Arterial Blood Gas Analysis?
 Oxygenation
 Represented by PaO2
 Ventilation
 Represented by Pa Co2
 Acid Base Status
 Represented by pH, HCO3 and base deficit.
 What information does ABG provide about the patient?
FIRST: ABG provide an assessment of the following:
1)PaO2: Amount of o dissolved in the blood, it give initial
information on efficiency of oxygenation.
2)PaCO2: Adequacy of ventilation is inversely proportional to
Paco2 (when ventilation increase PaCO2 decrease
and vice verse).
3) Acid base status (pH, HCO3, base deficit, anion gap).
4) Hb, Hct, oxygen saturation.
5) Electrolyte e.g. Na+, K+.
SECOND: Calculation of Alveolar Gas Equation and A-a Gradient:
PAO2 = FiO2×(Bp-pH2O)-PaCO2/R.
=
21×(760-47)-40/0.8
= 100 mmHg.
A-a Gradient is alvealo-arterial O2 gradient.
A-a Gradient = PAO2 -PaO2
It is normally = Age/4+4.
It’s Value: concise D.D of hypoxemia.
e.g.:
Decrease FiO2
Hypoventilation
normal A-a Gradient
Ventilation perfusion mismatch
Rt to Lt shunting
Diffusion abnormality
increase A-a Gradient
1) Arterial/alveolar ratio(a/A)
PaO2/PAO2
PAO2 is calculated by the alveolar air equation:
PAO2 = FIO2 (PB – PH2O) – PaCO2/0.8
Normal value for the a/A ratio is 0.8, meaning that 80% of the
alveolar oxygen is reaching the arterial system
2) PaO2/ FIO2 ratio
Normal ratio is 550 (a person breathing FIO2 of 1.0 at sea level
should have a PaO2 of 550 to 600 mmHg)
3) A-a gradient (on 100% oxygen)
PAO2 - PaO2
Where PAO2 is calculated by the alveolar air equation previously
presented
Arterial/alveolar PCO2 Gradient (a-A PCO2)
Arterial PCO2 - Alveolar PCO2
Where Alveolar PCO2 is measured by means of end–tidal PCO2
Normal gradient is an alveolar PCO2 2 mmHg less than arterial,
Acute increase reflects increase in physiologic dead space
Diagnosis of acid Base disorders
Sample source and collection:•Arterial blood sample is common utilized clinically
but venous blood may be useful in determining
acid base status. (Except in CHF and shock).
•Blood sample should be in heparin coated syringe.
•The sample should be analyzed as soon as
possible.
•Air bubble should be eliminated.
•The syringe should be capped and placed in ice.
Problem associated with obtaining ABG:
Arterial puncture may result in acute hyperventilation. To
minimize that: we should use local anesthetic with small
needle.
When would you withdraw ABG sample after beginning or stopping
O2 supplementation?
In absence of significant lung disease we should wait from 5-7
minutes before withdraw ABG sample while patient with
obstructive lung disease we should wait 25 min.
Interpretation of ABG
Normal blood gas values:
Arterial blood
Mixed venous
Venous
PH
7.37-7.47
7.30-7.40
7.30-7.40
PO2
80-100
35-40
30-50
PCO2
36-44
40-50
40-50
O2 saturation
>95%
60%-80%
60%-85%
HCO3
22-26
22-26
22-28
Base difference
(deficit excess)
-2 to 2
Measurement
 What is PH?
PH is –ve log of H+ concentration.
P=protenz (strength-power), H=H+ concentration.
Henderson-Hasselbalch Equation
Relationship between pH & [H+]
pH = pK’a + log ([HCO3] / 0.03 x pCO2)
pH
[H+]
(nanomoles/l)
6.8
6.9
7.0
7.1
7.2
7.3
7.4
7.5
7.6
7.7
7.8
158
125
100
79
63
50
40
31
25
20
15
or more simply: The Henderson
equation:
[H+] = 24 x ( pCO2 / [HCO3] )
 What is base deficit?
The base deficit is the amount of acid or base needed to
titrate a serum PH back to normal (7.40) at 37 degree while
the Paco2 is held constant at 40 mmHg, thus eliminating the
respiratory component therefore the base deficit represent only
the metabolic component of acid base disorders.
Its +ve value indicate metabolic alkalosis,
While –ve value indicate metabolic acidosis.
ABG analyzer derives the base deficit from nomogram based on:
1) pH
2) HCO3
3) Hb concentration
STEPS for interpretation of ABG
STEP 1:
Determine if numbers fit:
H+ =
24  PCO
HCO
2
3
H+ = (7.8-PH)×100.
The Rt side of the equation should be within 10% of the Lt
Side. If not so another ABG or chemistry panel for HCO3 should
be done.
STEP 2:
STEP 3:
Determine if:
Acidemia (PH<7.37) OR Alkalemia(PH >7.44) is present.
Identify primary disturbance:
PH
Increase
Decrease
Alkalosis
Acidosis
Look at PCO2
Increased
Metabolic
Alkalosis
Decrease Increased
Respiratory
Alkalosis
Respiratory
acidosis
Decreased
Metabolic
acidosis
STEP 4:
Look at the direction of the change of HCO3/PCO2:
•If it is in the same direction it is either simple or mixed change.
•But if it is in the opposite direction so it is mixed change.
STEP 5:
Calculate rate of change of Hco3 and co2
(Expected compensation)
Disturbance
Response
Expected change
Metabolic acidosis
↓Paco2
1.2×(24-HCO3 measured)
Metabolic alkalosis
↑Paco2
0.7×(HCO3-24)
Acute respiratory acidosis
↑Hco3
0.1×(PaCO2-40)
Chronic respiratory acidosis
↑ Hco3
0.4×(PaCO2-40)
Acute respiratory alkalosis
↓ Hco3
0.2×(40-PaCO2)
Chronic respiratory alkalosis
↓ Hco3
0.4×(40-PaCO2)
N.B.:
There is no over correction or compensation in acid base
balance → if the compensatory response is more or less than
expected → it is mixed acid base disorder
N.B.:
• In respiratory disturbance arterial pH change 0.08 for every 10
mmHg change in PCO2.
• In metabolic disturbance arterial pH change 0.1 for every 6 meq /l
change in HCO3.
Determine the Anion Gap
 The Anion Gap
[(Na+) + (K+)] – [(Cl-) + (HCO3-)]
 The normal anion gap is 12meq ± 4.
Causes ofHyperchloremic Acidosis
 ↑ GIT loss of HCO3 as in: diarrhea, high output fistula




(pancreatic, biliary or small intestinal).
↑ renal HCO3 loss as in: RTA(I, II), CAIs, hypoaldosteronism.
TPN.
Large amount of HCO3 free fluid.
↑ CL containing acids.
 Wide Anion Gap Acidosis





Keto Acidosis
Uremia
Lactic Acidosis
Salicylism
Toxins : Methanol,Paraldehyde,Ethylene glycol
All anions and cations
ANIONS
CATIONS
Proteins 15
Calcium 5
Organic acids 5
Magnesium 1.5
Phosphates 2
Potassium 4.5
Bicarbonate 24
Sodium 140
Sulfates 1
Chloride 104
TOTAL 151
TOTAL 151
Non anion gap metabolic acidosis:
Metabolic acidosis associated with normal AG is
typically characterized by hyperchloremia.
Plasma CL- ↑ to take the place of HCO3 ions lost.
Calculation of AG in urine:
 Urine AG = ( Na+ + K+) – CLIn a patient with a hyperchloraemic metabolic acidosis:
•A negative UAG suggests GIT loss of bicarbonate (eg
diarrhoea)
•A positive UAG suggests impaired renal distal acidification (ie
renal tubular acidosis).
STEP 6:
If there is metabolic acidosis calculate the anion
gap
Anion Gap = Na+ - (Cl- + HCO3-) = 12meq ± 4.
• Corrected anion gap = observed anion gap + 2.5
(normal albumin - measured albumin).
• If the anion gap ↑ proceed to step 7.
STEP 7:
If the anion gap metabolic acidosis is present we should evaluate
for additional metabolic disorder because the elevation of anion gap
above normal ∆ AG = (AG-12) should be buffered by HCO3.
Adding ∆AG to current HCO3 will yield the corrected Hco3 which should be
normal value 24 meq/l unless there is another disorder present.
Corrected HCO3 = current HCO3 (measured) +∆A.G
(Normal value 24 meq/l)
•If corrected HCO3 >24 → metabolic alkalosis is also present
•If corrected HCO3 <24 → a non gap metabolic acidosis is also present
•If corrected HCO3 = 24 → it is pure gap metabolic acidosis.
Delta ratio = (Increase in anion gap / Decrease in bicarbonate)
Delta Ratio
Assessment Guideline
< 0.4
Hyperchloraemic normal anion gap acidosis
0.4 - 0.8
Consider combined high AG & normal AG acidosis BUT note that the ratio is often <1 in
acidosis associated with renal failure
1 to 2
Usual for uncomplicated high-AG acidosis
Lactic acidosis: average value 1.6
DKA more likely to have a ratio closer to 1 due to urine ketone loss (esp if patient not
dehydrated)
>2
Suggests a pre-existing elevated HCO3 level: consider a concurrent metabolic alkalosis or
a pre-existing compensated respiratory acidosis.
STEP 8:
In case of metabolic alkalosis: measure urinary Cl- concentration
Final step:
Be sure that the interpretation of blood gas is consistent and
correlated with the clinical picture of the patient.
Case 1
 A 75-year-old man presents to the ED after a
witnessed out of hospital VF cardiac arrest.
 Arrived after 10 minutes, CPR had not been
attempted.
 The paramedics had successfully restored
spontaneous circulation after 6 shocks.
 On arrival the man is comatose with a GCS of 3
and his lungs are being ventilated with 50%
oxygen via ET tube.
 He has a ST with rate of 120 min-1 and a blood
pressure of 150/95 mmHg.
ABG Analysis reveals:
 FiO2
 pH
 PaCO2
 PaO2
 HCO3 BE
0.5
7.10
6.0 kPa (45 mmHg)
7.5 kPa(56 mmHg)
14 mmol l-1
- 10 mmol l-1
Case 2
 A 65-year-old man with severe COPD
has just collapsed in the respiratory
high-care unit.
 On initial assessment he is found to be
apnoeic but has an easily palpable
carotid pulse at 90 min-1.
 A nurse is ventilating his lungs with a
BVM and supplementary O2 (with
reservoir)
ABG Analysis reveals:
 FiO2
 pH
 PaCO2
 PaO2
 HCO3 BE
0.85 (estimated)
7.20
20.0 kPa (151 mmHg)
19.5 kPa (147 mmHg)
36 mmol l-1
+ 12 mmol l-1
Case 3
 A 75-year-old lady is admitted to the ED following a
VF cardiac arrest, which was witnessed by the
paramedics.
 A spontaneous circulation was restored after 4 shocks,
but the patient remained comatose and apnoeic.
 The paramedics intubated her trachea, and on
arrival in hospital her lungs are being ventilated with
an automatic ventilator using a tidal volume of 900
ml and a rate of 18 breaths min-1.
ABG Analysis reveals:
 FiO2
 pH
 PaCO2
 PaO2
 HCO3-
 BE
1.0
7.60
2.65 kPa (20 mmHg)
25.4 kPa (192 mmHg)
20 mmol l-1
- 4 mmol l-1
Case 4
 An 18-year-old male insulin dependent diabetic is admitted
to the ED.
 He has been vomiting for 48 hours and because he was
unable to eat, he omitted his insulin.
 He has a ST at a rate of 130 min-1 and his blood pressure is
90/65 mmHg.
 He is breathing spontaneously with deep breaths at a rate
of 35 min-1 and is receiving oxygen 4 l min-1 via a Hudson
mask. His GCS is 12 (E3, M5, V4).
ABG Analysis reveals:
 FiO2
 pH
 PaCO2
 PaO2
 HCO3-
 BE
0.4
6.79
1.48 kPa (11.3 mmHg)
17.0 kPa (129.2 mmHg)
4.7 mmol l-1
- 29.2 mmol l-1
Case 5
His vital signs are:
 Heart rate
120 min-1 – sinus
tachycardia – warm peripheries
 Blood pressure 70/40 mmHg
 Respiratory rate
35 breaths min-1
 SpO2 on oxygen
92%
 Urine output
50 ml in the last 6 hours
 GCS
13 (E3, M6, V4)
ABG Analysis reveals:
 FiO2
 pH
 PaCO2
 PaO2
 HCO3-
 BE
0.4 (approx)
7.12
4.75 kPa (36 mmHg)
8.2 kPa
(62 mmHg)
12 mmol l-1
- 15 mmol l-1
Case 6
Which patient is more hypoxemic, and why?
Patient A: pH 7.48, PaCO2 34 mm Hg, PaO2 85 mm Hg, SaO2 95%,
Hemoglobin 7 gm%
 Patient B: pH 7.32, PaCO2 74 mm Hg, PaO2 55 mm Hg, SaO2 85%,
Hemoglobin 15 gm%
 Patient A: Arterial oxygen content = .95 x 7 x 1.34 = 8.9 ml O2/dl
 Patient B: Arterial oxygen content = .85 x 15 x 1.34 = 17.1 ml O2/dl
Patient A, with the higher PaO2 but the lower
hemoglobin content, is more hypoxemic.
Case 7
The PO2 in a cup of water open to the
atmosphere is always higher than the arterial
PO2 in a healthy person (breathing room air)
who is holding the cup.
True or False
Case 8
A patient is admitted to the ICU with the following lab values:
BLOOD GASES
pH: 7.40
PCO2: 38
HCO3: 24
PO2: 72
ELECTROLYTES, BUN & CREATININE
Na: 149
K: 3.8
Cl: 100
CO2: 24
BUN: 110
Creatinine: 8.7
What is(are) the acid-base disorder(s)?
Step 1: Anion gap
AG = Na+ - (Cl- + CO2)= 149 - (100 + 24) = 25
This high an AG indicates an anion gap metabolic acidosis
Step 2: Delta anion gap
calculated AG 25 mEq/L
normal AG = 12 mEq/L
25 - 12 = 13 mEq/L; this is the excess or delta anion gap
Step 3: Delta serum CO2
= normal CO2 - measured CO2
=27 (average normal venous CO2) - 24 = 3 mEq/L
Step 4: Bicarbonate Gap = delta AG - delta CO2
= 13 - 3 = 10 mEq/L
The patient was both uremic (causing metabolic acidosis) and had been
vomiting (metabolic alkalosis).
Case 9
In the clinical setting, which of the following statements concerning blood
gas physiology is(are) true?
a) End-tidal PCO2 should always be higher than arterial PCO2.
b) In a steady state situation, alveolar PO2 should always be higher than arterial PO2.
c) The %oxyhemoglobin + %carboxyhemoglobin + %methemoglobin should never
exceed 100%.
d) The ratio of dead space to tidal volume should never exceed 1.0.
e) The average airway pressure does not exceed barometric pressure in a
spontaneously-breathing patient.
a) is false; End-tidal PCO2 should always be equal or lower than PaCO2.
b) is true.
c) is true.
d) is true.
e) is true; the average airway pressure is always equal to barometric
pressure in a spontaneously-breathing patient.
Case 10
55 yrs old pt. who drink fifth of wesky per day has 2 wks
history of diarrhea , Anion gap is 17, HCO3 = 10, PH =
7.30, PO2 =0.90 mmHg, PCO2 = 30 mmHg.
What is(are) the acid-base disorder(s)?
Case 11
25 yrs pt. come to ER with fever, abd. pain , vomiting,
with the history of migrane PH = 7.33, PCO2 = 10mmHg,
PO2 = 80 mmHg, HCO3 = 5, Sodium = 140 mmol, K = 3
mmol, CL = 108 mmol.
What is(are) the acid-base disorder(s)?
Any Question?
Thank You