ARTERIAL BLOOD-GAS ANALYSIS

ARTERIAL BLOOD-GAS ANALYSIS
is a blood test which gives 2 sets of information..

1. Acid-Base balance
(pH, CO2, HCO3)‏


2. Blood Oxygenation
(pO2, sO2 )‏
PURPOSE
Evaluation of
lung
kidney functions.

Indicates
Their interaction with each other so as to maintain a normal pH
(Acid-Base balance ).
THE CANDIDATE
Significant respiratory distress sec. to an acute or chronic respiratory,cardiac or hematological problem.
Unconscious person & /on Ventilators
Renal problems.
Critical and Unstable patients.

HOW TO COLLECT SAMPLE?
Perform Modified Allen”s test.

Clean the site. (LA is optional).
Flush the syringe(21 gauge)with Heparin.
Palpate artery with one hand and enter skin at 45 degree angle.
Obtain 2-4mL of arterial blood, preferably without aspiration.
After withdrawal of syringe,apply firm pressure at puncture site.

SPECIAL PRECAUTIONS..
Low friction syringe under arterial pressure.
Avoid pulling (suction) of syringe.. PaO2 and PaCO2 will get reduced.
Avoid excess heparin .. May dilute sample.
Air-bubbles must be tapped to the surface and pushed out… may increase PaO2 and decrease PaCO2.
If lab analysis will be delayed, REFRIGERATE (Place capped syringe in a glass of ice water).. Normal temperature may acidify the sample.

RISKS..
Prolonged bleeding

Bruising

Rarely, arterial thrombotic occlusion.

ABG Interpretation
ABG and Serum electrolytes should be performed simultaneously.

Follow “The SIX STEP method”.
SIX STEP APPROACH
Analyze pH,
Analyze pCO2, HCO3.
Match CO2 and HCO3 with pH
Look for Compensation.
Look for Mixed disorders.
Analyze pO2 and sO2.
Co-relate clinically and establish etiological diagnosis.


pH
Normal value: 7.35 – 7.45
Low pH (<7.35)>7.45) ..ALKALOSIS.
pH is determined by and INVERSELY related to H+ conc.
H+ falls by 20% for each 0.1pH unit increment.
H+ =24 X (PaCO2 / HCO3)‏

BICARBONATES (HCO3)‏
Normal :24 (22 – 26 meqv/l)‏

LOW (<22>26 meqv/l)..MET ALKALOSIS
.
NORMAL HCO3 DOES NOT EXCLUDE ACID BASE DISORDERS.. MIXED DISORDERS CAN GIVE NORMAL HCO3.
PaCO2
NORMAL: 40 (35-45 mm of Hg)‏

High (>45)..resp. acidosis

Low (<35)..resp. style="font-weight: bold; font-style: italic; color: rgb(0, 102, 0);">
ANION GAP(AG)‏ Na –(Cl +HCO3) = 12 +/-2meqv/l
Important unmeasured anions : proteins,phosphate, sulphate and organic acids. Important unmeasured cations: calcium, magnesium and potassium.
ANION GAP- SIGNIFICANCE TO ESTABLISH ETIOLOGICAL DIAGNOSIS of METABOLIC ACIDOSIS. MIXED DISORDERS. PULSE OXIMETRY Measures O2 Saturation of arterial Hb. Normal : 96 – 100% LESS THAN 90% Saturation suggests marked TISSUE HYPOXIA (<60% Pao2). Useful for Hypoxemia screening but Tells Nothing About PaCo2. Hypercapnoea can occur even with 100% O2 saturation. Hypoxia…Hypoxemia HYPOXIA refers to reduced oxygen pressure in the alveolus. HYPOXEMIA refers to low arterial oxygen pressure. NORMAL PaO2 :75 – 100 mm Hg SaO2 :94 – 100 % Oxygen content (O2 CT) :15 – 23% PAO2 and PaO2 PaO2 = 104.2 - (0.27 X AGE)‏ A normal gradient of 10mm of Hg exists between PAO2 AND PaO2 P (A-a) O2 = 10 mm of Hg NO gradient between PACO2 AND PaCO2. BASICS OF ACID-BASE  DISORDERS.. IF the initial disturbance affects HCO3.. Fall - METABOLIC ACIDOSIS Rise - METABOLIC ALKALOSIS IF the initial disturbance affects PaCO2.. Rise - RESPIRATORY ACIDOSIS Fall - RESPIRATORY ALKALOSIS WHEN YOU SEE “METABOLIC”, THINK OF HCO3. WHEN YOU SEE “RESPIRATORY”, THINK OF PaCO2. COMPENSATION The body responds to neutralize the effect of the initial insult on pH homeostasis. HENDERSON-HASSELBACH EQN. pH is maintained by…HCO3/PaCO2. compensation Metabolic acidosis (fall in HCO3) leads to low pH which stimulates the respiratory centre causing hyperventilation. Hyperventilation leads to CO2 washout and decreased PaCO2. PaCO2/HCO3 ratio returns towards normal. This compensation keeps the pH within normal range,as far as possible.
SAME DIRECTION RULE The compensatory changes are in the same direction as the primary change. HCO3 LEADS TO PaCO2 HCO3 LEADS TO PaCO2. PREDICTION OF COMPENSATION METABOLIC ACIDOSIS ( HCO3)‏ PaCO2=(1.5 X HCO3) + 8 PaCO2= HCO3 + 15 METABOLIC ALKALOSIS ( HCO3)‏ PaCO2 = 0.75 X HCO3 COMPENSATION- SIGNIFICANCE Differentiates simple from Mixed disorder. If expected change = actual change, disorder is simple. If actual change is more or less than predicted, disorder is mixed. Compensation follows “same direction rule”. If changes are in opposite direction, think of mixed disorder.

PREDICTION…RESPIRATORYACIDOSIS : ACUTE (6-24hrs)‏ Rise in HCO3 = 0.1 X RISE IN PaCO2 FALL IN pH = 0.01 X RISE IN PaCO2 CHRONIC (24hrs)‏ Rise in HCO3 = 0.4 X rise in PaCO2 Fall in pH = 0.003 x fall in PaCO2 PREDICTION.. RESPIRATORY ALKALOSIS.. ACUTE Fall in HCO3 = 0.2 X Fall in PaCO2 Rise in pH = 0.01 x Fall in PaCO2 CHRONIC Fall in HCO3 = 0.4 X Fall in PaCO2 Rise in pH = 0.002 x Fall in PaCO2 Serum Potassium NORMAL : 3.5 – 5.5 meqv/l LOW (<3.5)..>5.5)..
Metabolic ACIDOSIS due to Renal Failure, Type-4 RTA,DKA or Respiratory ACIDOSIS..
CALCULATION 1…
pH 7.23 (7.38 –7.44)‏
pCO2 7.4 (4.7 – 5.9)‏
PO2 8.9 (11 – 13)‏
HCO3 24 (21 – 28)‏
SaO2 90% (94 - 100)

Calculation 2
pH 7.26 (7.38 – 7.45)‏
pCO2 7.1 (4.7 – 5.9 )‏
Po2 10 (11 – 13)‏
HCO3 37 (21 – 28)‏
SaO2 90% (94 – 100)‏

MIXED DISORDERS
IF THE VALUE IS…

MORE THAN EXPECTED COMPENSATION

DOES NOT OBEY “SAME DIRECTION RULE”

NORMAL pH but changes in PCO2 OR HCO3.
MIXED DISORDER..
a non-anionic gap acidosis or a metabolic alkalosis can co-exist with an anion gap acidosis.
corrected HCO3 = measured HCO3 + (anion gap – 12)‏
if corrected HCO3 >24
..metabolic alkalosis co-exists
if corrected HCO3 <24 hco3 =" Measured" hco3="15," hco3="15" 14 =" 29.."> 20meqv/l)‏
Saline responsive Metabolic  Alkalosis
ECF VOLUME DEPLETION
Vomiting
Diuretics
Hypercapnoea Correction

NO ECF VOLUME DEPLETION
NaHCO3 Infusion
Multiple Transfusions
Saline Resistant METABOLIC  ALKALOSIS
HYPERTENSIVE
Hyper-Aldosteronism
Cushing Syndrome

NORMOTENSIVE
Bartter”s Syndrome
Severe Potassium Depletion.

CASE 1

Pt WITH POORLY CONTROLLED TYPE 1 DM…
pH 7.1 (7.38 – 7.45)‏
HCO3 8 (21 – 28)‏
PaCO2 20 mm of Hg (35 – 45)‏
Na 140 (135 – 150)‏
Cl 106 (95 – 110)‏
Urinary Ketones +++
? ? ?

CLINICAL CONDITIONS…

CNS
COMA … RESP ACIDOSIS/ALKALOSIS
SEIZURES … METABOLIC ACIDOSIS
CVS
CCF … RESP ALKALOSIS
SHOCK …MET ACIDOSIS / RESP ALKALOSIS
RS
TACHYPNOEA … RESP ALKALOSIS
BRADYPNOEA … RESP ACIDOSIS
CLINICAL CONDITIONS…
G I
VOMITING … MET ALKALOSIS
DIARRHOEA … MET ACIDOSIS
ABD PAIN … RESP ALKALOSIS
RENAL
OLIGURIA … MET ACIDOSIS
POLYURIA … MET ACIDOSIS/ ALKALOSIS
ENDOCRINAL
MYXOEDEMA … RESP ACIDOSIS
HYPERTENSION … MET ALKALOSIS

COMMON MIXED DISORDERS…

METABOLIC AND RESP ACIDOSIS
1.CARDIAC ARREST (HYPOVENTILATION + LACTIC ACIDOSIS)‏
2.SHOCK WITH RESPIRATORY FAILURE
3.DKA WITH RESP DISEASE

MIXED DISORDERS…

METABOLIC ACIDOSIS AND RESP ALKALOSIS
1.GRAM NEG SEPSIS
2.LIVER FAILURE

MIXED DISORDERS…

METBOLICALKALOSIS AND RESP ACIDOSIS
1.COPD WITH DIURETICS

MIXED DISORDERS…

METABOLIC ALKALOSIS WITH RESP ALKALOSIS
1. LIVER FAILURE WITH VOMITING
2.Pt ON VENTILATOR WITH CONTINUOUS NASOGASTRIC ASPIRATION
MIXED DISORDERS…

METABOLICACIDOSIS AND METABOLIC ALKALOSIS
1.DKA WITH VOMITING
2. VOMITING WITH SEV VOL DEPLETION CAUSING LACTIC ACIDOSIS
LACTIC ACIDOSIS

TYPE A:IMPAIRED TISSUE OXYGENATION
1.SHOCK (CARDIO/SEPTIC)‏
2.RESP FAILURE
3.CO OR CYANIDE POISONING
4.SEVERE ANAEMIA
MIXED DISORDERS…
RESPIRATORY ACIDOSIS WITH RESPIRATORY ALKALOSIS…
DOES NOT EXIST!
LACTIC ACIDOSIS

TYPE B:NO HYPOXIA. MITOCHONDRIAL RESP IS IMPAIRED
1.DM
2.HEPATIC FAILURE
3.SEV INFECTION
4.TOXINS-ETHANOL, METHANOL
5.DRUGS-BIGUANIDES
Case 1
Expected Compensation, PaCO2 = HCO3 X 1.5 + 8 =8 X 1.5 + 8 = 20

Expected PaCO2 Matches Actual PaCO2 … Simple Acid-base Disorder.

ANION GAP = Na – (Cl + HCO3)
=140 – (106 + 8)‏
= 26 >12… high anion gap

HIGH ANION GAP METABOLIC ACIDOSIS
DUE TO DKA !!!

CASE 2
ABG of a Pt in shock on ventilatory support For 4 Hrs..
pH 7.48 (7.35 – 7.45)‏
HCO3 14 (22 – 26)‏
PaCO2 22 mm of Hg (35 – 45)‏

? ? ?
Case 2
Respiratory alkalosis

Expected acute compensation
= 0.2 x fall in PaCO2
= 0.2 x (40 – 22)‏
= 0.2 X 18 = 3.6 meqv/l
So expected HCO3 24 – 3.6 = 20.4

Actual HCO3 < 20.4 … Additional Metabolic Acidosis (Shock-induced Lactic Acidosis )‏



CALCULATION 3
pH 7.41 (7.38 – 7.45)‏
Pco2 3.8 (4.7 – 5.9)‏
HCO3 20 (21 – 28)‏
SaO2 96% (94 – 100)‏