ABG5oct2010 Final

8/2/2019 ABG5oct2010 Final

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Arterial Blood Gases

Interpretation

Amit Kocheta

DNB Trainee

Moderator : Dr Ausim

Anesthesia & Critical Care DepartmentBMHRC


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What is an ABG

Arterial Blood Gas

Drawn from artery- radial, brachial, femoral

It is an invasive procedure.

Caution must be taken with patient on anticoagulants.

Arterial blood gas analysis is an essential part ofdiagnosing and managing the patients oxygenationstatus, ventilation failure and acid base balance.


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When to do ABG

1. Assess the adequacy of ventilation &oxygenation (whether the patient is on aventilator or not)

2. Establish the diagnosis & severity ofrespiratory failure.

3. Guide therapy O2 administration,

mechanical ventilation, weaning4. Assess changes in acid-base homeostasis

5. Guide treatment for acid-base abnormalities


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When to do ABG..

6. Manage patients in ICUs for :-1. Respiratory dysfunction or failure

2. Cardiac failure

3. Renal failure

4. Hepatic failure

5. Polytrauma

6. Multiorgan failure

7. Diabetic ketoacidosis

8. Sepsis

9. Burns

10. Various type of poisoning etc.


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When to do ABG..

7. Monitor patient during :-

Cardiopulmonary surgery

Cardiopulmonary exercise testing

Sleep studies

8. Determine prognosis in critically ill patients


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Explanation of Terms Hb, HCT, FiO2, PaO2, PaCO2, pH, Na+, K+,

Ca++, Sat(%)RQ CO2 produced : O2 consumed, Set value (0.85), Can be fed

HCO3A

(Actual)

Parameter for non respiratory component of acid base balance.

HCO3S

(Standard)

Parameter for non respiratory component of acid base balance but

reported after standarising at PCO2 at 40 mmHg, temp. 37degree C,

SaO2 100%

Base Excess(BE)

HCO3 amount above or below normal content (0) of buffer base, (+)or (-) depends upon entered Hb value & measured pH & PCO2

values

Standard

Base Excess

HCO3 amount above (+) or below (-) normal content (0) of buffer

base. Calculated from a standard Hb value of 6gm % & measured pH

& PCO2 values


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Explanation of TermsBB (buffer

bases)

Sum of all buffer anions in blood, metabolic index (Hb, HCO3,

protein, phosphate)

TCO2 Content HCO3 concentration + Dissolved CO2 in plasma

O2 CT, CaO2,

O2 Content

Hb bound O2 + plasma dissolved O2

A-a DO2 Difference between PO2 Alveolar & PO2 Arterial

P50 Semi saturation pressure = Partial pressure of O2 at which Hb is 50%

saturated

Ca 7.4 Calcium ion conc. Computed for pH 7.4

Li Lithium ion conc.

LAC Lactate conc.

GLU Glucose conc.


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Known Normal & Reference Values for Calculations

Hb (gm%) Measured/Fed/Calcul

ated (HCT/3)HCT (%) Measured/Calculated

(3xHb)

FiO2 Fed

RQ 0.85

PaO2 (mmHg) 80 to 100

PaCO2 (mmHg) 35 to 45

pH 7.35 to 7.45

HCO3 A (mEq/L) 22 to 26

Na+ 135 to 145

K+ 3.5 to 5.1

Ca++ 1.12 to 1.32

Cl- 97 to 100

Base Excess (mEq/L) 0 +/- 2

TCO2 content (mEq/L) ~ 27

BB (mEq/L) 48

O2 Sat (%) >95

O2 CT (ml/dl) 16 to 22

P50 (mmHg) 27

A a DO2 (mmHg) 5 to 25


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TerminologyCompensation

-when the acid base imbalance exists over a period of timeSecondary changes in HCO

3

-or PaCO2

Occurring in response to the primary event

To normalize pH

Done by the organ system which is not primarily affected respiratory compensation for metabolic disorders

metabolic compensation for respiratory disorders

Resultant Blood Gas States

Uncompensated Partially Compensated Fully Compensated

Abnormal pH Abnormal pH Normal pH

Other values may remain abnormal


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..Compensations

Respiratory Compensation Renal Compensation

Characteristics Rapid, in 1-3 min, Complete Slow, in hours to days,

Incomplete

Mechanism in Acidosis Wash off excessive CO2 by

increasing ventilation (as in

metabolic acidosis)

Secrete H+ ions out

Reabsorb filtered HCO3 ions

Produce of new HCO3 ions (as

in respiratory acidosis)

Mechanism in Alkalosis Retain CO2 by decreasing

ventilation (as metabolic

alkalosis)

Excess HCO3 filtered into renal

tubules, eliminated in urine (as

in respiratory alkalosis)


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Reading the Report Step-by-Step


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Arterial, Venous or MixedSometimes, there is no way to know if the sample is arterial or venous !

Arterial Venous

Ask the person who

aspirated the sample

Blood pulsates

Syringe plunger may rise

on its own

Blood does not pulsate

Syringe plunger never rises

on its own

PO2 > 40 mmHg < 40 mmHg (often < 30

mmHg)

O2 Saturation values Sao2 > 75% SvO2 < 75%

No. of attempts Single or

Multiple punctures

Single puncture, Rapid

filling

Multiple, Lower PO2 due to

venous admixture

pH Abnormal or normal,

Not diagnostic

Abnormal or normal,

Not diagnostic

PCO2 Abnormal or normal,

Not diagnostic

Abnormal or normal,

Not diagnostic

Venous admixture (CHD) Lower PaO2 values ~ Venous admixture


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Reading the Report Step-by-Step

Step 1Check if the required parameters have been correctly fed ?

Barometric pressure

Patients temperature Hemoglobin

(if machine does not measure, does not calculate)

FiO2

Results in the report are bound to

change, get incorrect and misleading

if the above values are not correctly filled


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Barometric pressure (PB)(Weight of Atmosphere)

Air = O2 21%, N2 78%, Other gases 1%Composition does not change with altitude, PB

decreases with height

PB = sum of Pressures of all constituent gases

P = p1 + p2 + p3 + p4

Each gas exerts its own Partial Pressure

p = % gas x PBp of a gas (O2 & CO2 ) will change according to

Concentration & PB

All machines calculate & adjust readings according to PB


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Alveolar gas equation :

PAO2 = PiO2 1.2(PaCO2)

PiO2 = FiO2 (PB 47)

All machines calculate PAO2 from PiO2

(affected by FiO2 and PB )

A-aDO2 will be affected if PB & FiO2 is not fed

properly

Feed PB if the machine does not measure PB on

its own


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Except in a temporary unsteady state, alveolar PO2(PAO2) is always higher than arterial PO2 (PaO2). As a

result, whenever PAO2 decreases, PaO2 does as well.Thus, from the AG equation:

If FIO2 and PB are constant, then as PaCO2 increases both PAO2

and PaO2 will decrease (hypercapnia causes hypoxemia).

If FIO2 decreases and PB and PaCO2 are constant, both PAO2and PaO2 will decrease (suffocation causes hypoxemia).

If PB decreases (e.g., with altitude), and PaCO2 and FIO2 areconstant, both PAO2 and PaO2 will decrease (mountainclimbing causes hypoxemia).


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Temperature Effect on PaCO2 & PaO2

Machine always analyses blood at 37 C

Hyperthermic Patient = > 37 C (Sample)

Measured PaO2 and PaCO2 will be less than actual

(Pressure Temperaturep decreases when subjected

to lower relative temperature of machine and falselylower pressures get measured)

Hypothermic Patient = < 37 C (Sample)

Measured PaO2 and PaCO2 will be more than actual

(Pressure Temperaturep increases when subjectedto higher relative temperature of machine and falselyhigher pressures get measured)


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For each C above or below 37

degree C

Example = Febrile patient, 39

degree C

Change in PaO2 5 mmHg Measured PaO2 = 80 mmHg

True in vivo PaO2 = 90 mmHg

Change in PaCO2 2 mmHg Measured PaCO2 = 40 mmHg

True PaCO2 = 44 mmHg

Feeding correct Temperature value allows some machines to

correct accordingly, (or Apply formulae manually)


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Hemoglobin

Derived (From Hematocrit)

Measured (Co oximeters)

Not entered ! (Default value Wrong)

Manually entered

True assessment of adequacy of O2 in arterial

blood can only be made if Hb value are entered

SaO2 & PaO2 do not incorporate Hb content in their calculations.

Hb affects Buffer Base values


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Total Oxygen Content (CaO2 ) ml o2/dl

Total O2 attachedto Hb Content

Total Dissolved O2carried by Plasma

Hb content (gm%)

O2 carried by 1 gm Hb(ml)

Saturation Hb

PaO2Solubility Coefficient

15 x 1.34 x 100 (say) = 20.10 + 100 x 0.003 = 0.30= 20.40 ml / dL

Normal = 16 to 22 ml/dl


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FiO2 Entering FiO2 is very important

Most common mistake FiO2 not entered while the sample is fed in the

machine

% FiO2 written on the report later on manually

Hb also not entered at the time of feeding samplebut told later on

If FiO2 not fed properly

Interpretation of PO2 affected adversely A-aDO2 values are wrongly calculated

Interpretation of adequacy of Oxygenationaffected if Hb not fed properly


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Step - 2

Analyse the Adequacy of Oxygenation(i) Look at PaO2 and SaO2 first

Healthy Adult Sea Level, Room Air, A-a O2 = 4 mmHg, PAO2 = 101

PaO2 Important Low PaO2 = Surely something wrong in terms of Oxygenation

Low PaO2 = degree of hypoxemia

Saturation of Hb (SaO2) is dependent upon PaO2

Never rely totally on PaO2 & SaO2 Look at other parameters

also (CaO2)

PaO2 (mmHg) Sao2 (%)

Normal values (on air) >80 >95

Mild Hypoxemia 60-79 90-94

Moderate hypoxemia 40-59 75-89

Severe Hypoxemia


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Acceptable PaO2 Values on Room Air

Age Group Accepable PaO2

(mm Hg)

Adults upto 60 yrs

& Children

> 80

Newborn 40-7070 yrs > 70

80 yrs > 60

90 yrs > 50

60 yrs 80 mm Hg 1mm Hg/yr

Estimate formula of age:

PaO2=100mmHg - (age0.33) 5mmHg


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(ii) Relate PaO2 with FiO2 Classify Hypoxemia

Inspired O2 % PaO2 mmHg

30 >150

40 >200

50 >250

80 >400

100 >500

FiO2 x 5 = PaO2


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Hypoxemia on O2 therapy

Uncorrected: PaO2 < 80 mm Hg

(< expected on RA & FIO2)

Corrected: PaO2 = 80-100 mm Hg

(= expected on RA but < expected for FIO2)

Excessively Corrected: PaO2 > 100 mm Hg(> expected on RA but < expected for FIO2)

PaO2 > expected for FIO2:

1. Error in sample/analyzer

2. Pts O2 consumption reduced3. Pt does not req O2 therapy (if 1 & 2 NA)


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(iii) Find if Oxygenation is adequate or not CaO2

PaO2 & SaO2 may not give true estimate

Low PaO2 but still adequate oxygen content (V/Q

imbalance)

Normal PaO2, Still profound hypoxemia (Anemia,

Altered affinity of Hb for O2) Calculated SaO2 may mislead & show false normal

results (CO, MHb)

If no co-oximeter in the machine, SaO2 is calculated

from PaO2

Total Oxygen Content

CaO2 measured directly or calculated by O2 content equation.

CaO2 = Hb(gm%) x 1.34 x SaO2 + 0.003 x PaO2(mmHg).

S 3 A id B di b


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Step 3 : Acid Base disturbances

Analyze pH

pH Analysis7.35 7.45 (7.4) Normal No acid-base disorder

Or, Compensated disorder

< 7.35 Acidemia Uncompensated Acidosis

(or partially compensated)

> 7.45 Alkalemia Uncompensated Alkalosis

(or partially compensated)

Acidemia (pH < 7.35) Alkalemia (pH >7.45)

Mild 7.30 7.34 7.46 7.50

Moderate 7.20 7.29 7.51 7.54

Severe < 7.2 > 7.55

Incompatible to life < 6.8 > 7.8


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Step 4 Analyze the Primary disorder

Respiratory or Metabolic ?


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Change Disorder Change pH Primary Disorder

PaCO2 Respiratory

>45 Respiratory Acidosis

< 35 Respiratory Alkalosis

For every 20 mmHg rises in PaCO2 = pH should fall by 0.10For every 10 mmHg fall in PaCO2 = pH should rise by 0.10

PaCO2 = 65 (20 mm rise from 45)

pH = 7.25 (0.10 fall from 7.35)

PaCO2 = 25 (10 mm fall from 35)

pH = 7.55 (0.10 rise from 7.45)

If pH & PaCO2 move in opposite directionsPrimary defect is Respiratory.

If pH is not moving in opposite direction as PaCO2

Primary defect is Not Respiratory (Metabolic).

Respiratory


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Metabolic

Change Disorder Change pH Primary

disorder

HCO3(base)

Metabolic

> 26 Metabolic

alkalosis< 22 Metabolic

acidosis

If pH moves in same direction as HCO3Primary defect is Metabolic

If pH moves in opposite direction as HCO3

Primary defect is not Metabolic (Respiratory)

l d l


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Step 5 : Analyse and Correlate

Compensation

Body tries to bring pH towards normal, with time Lungs and kidneys are primary buffer response

systems

pH outside normal range Uncompensated orPartially compensated

pH normal range Fully compensated (or no acid

base disturbance)

pH = 7.4 No acid base disturbance or mixed

disorder

Analyze and Correlate pH, PaCO2 and HCO3


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Step-6 : Calculate the Expected

Compensation Match it with actualFor every 10 mmHg change in PaCO2

Disorder Change in PaCO2 Compensation (Kidney)

Respiratory acidosis 10 mmHg Acute rise 1 mEq/L rise in HCO3

10 mmHg Chronic rise 4 mEq/L rise in HCO3

Respiratory alkalosis 10 mmHg Acute fall 2 mEq/L fall in HCO3

10 mmHg Chronic fall 4 mEq/L fall in HCO3

For every 1 mEq/L change in HCO3

Disorder Change in HCO3 Compensation (Lungs)

Metabolic acidosis 1 mEq/L fall 1.25 mmHg fall in PaCO2

Metabolic alkalosis 1 mEq/L rise 0.75 mmHg rise in PaCO2

Match the Calculated Compensation with the Actual (Report)


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Step 7 :

Find out if the Disorder is Mixed ?

(1) Check relative movement of pH PaCO2 and

pH HCO3

If both pairs moving & in correct directions

Mixed disorder

(2) Analyze compensation by Presuming Primary

disorder as Respiratory or Metabolic

If analysis supports no compensation Mixed

disorder


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Venous CO2 and its change from

normal

Index of Plasma HCO3

Total CO2 = Plasma HCO3 + Dissolved CO2 in

Plasma

Normal = 24 30 mEq/L (27 mEq/L)

Change in Venous CO2 from normal

( CO2) = 27 measured CO2


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Bicarbonate GapUnmasks the co-existence of 2 metabolic disorders

BG = AG - CO2

BG = (Measured AG 12) (27 Measured CO2)

Positive (+) or Elevated BG = > + 6 mEq/L

Metabolic Alkalosis

Bicarbonate retention as compensation for

Respiratory Acidosis

Negative (-) or Low BG =


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Conditions Invalidating or

Modifying ABG Results

DELAYED ANALYSIS

Consumption of O2 & Production of CO2continues after blood drawn into syringe

Iced Sample maintains values for 1-2 hoursUniced sample quickly becomes invalid

PaCO2 3-10 mmHg/hour

PaO2 at a rate related to initial value &dependant on Hb Sat


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EXCESSIVE HEPARIN

Dilutional effect on results HCO3-& PaCO2

Syringe be emptied of heparin after flushingRisk of alteration of results with:

1. size of syringe/needle

2.

vol of sample25% lower values if 1ml sample taken in 10 ml syringe(0.25 ml heparin in needle)

Syringes must be > 50% full with blood sample

HYPERVENTILATION OR BREATH HOLDING May lead toerroneous lab results


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7. ? Use Nomogram to convert values at 37C topts temp

8. Some analysers calculate values at both 37C andpts temp automatically if entered

9. Pts temp should be mentioned while sendingsample & lab should mention whether values

being given in report at 37 C/pts actual temp

WBC COUNT

0.1 ml of O2 consumed/dL of blood in 10 min inpts with N TLC

Marked increase in pts with very high TLC/pltcounts hence imm chilling/analysis essential


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TYPE OF SYRINGE

1. pH & PCO2 values unaffected

2. PO2 values drop more rapidly in plastic syringes(ONLY if PO2 > 400 mm Hg)

3. Other adv of glass syringes:

Min friction of barrel with syringe wall

Usually no need to pull back barrel lesschance of air bubbles entering syringe

Small air bubbles adhere to sides of plasticsyringes difficult to expel

Though glass syringes preferred, differencesusually not of clinical significance plasticsyringes can be and continue to be used


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SERIAL ABGs

CLINICAL PROFILE SUPPORTING LAB DATA/

INVESTIGATIONAL TOOLS

CLINICIANS JUDGEMENT

CORRECT INTERPRETATION

SIMPLE DISORDER

(DEG OF COMPENSATION)

MIXED DISORDER

(ORDER OF PRIMARY &

SUBSEQUENT DISORDERS)

SUMMARY

OXYGENATION /VENTILATORY STATUS


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pH

N

AcidemiaResp and/orMet Acidosis

Alkalemia Resp and/orMet Alkalosis

No acidemia

/alkalemia

Resp Acidosisand

Met Alkalosis

Met Acidosisand

Resp Alkalosis

No A-B Dis


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pH

pCO2 , HCO3

pCO2 , HCO3 N

Resp + Met Alkalosis

Uncomp Resp Alkalosis

pCO2 N, HCO3 Uncomp Met Alkalosis

pCO2 , HCO3 Comp(F/P) Met Alkalosis

pCO2 , HCO3 Comp(F/P) Resp Alkalosis


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pH

pCO2 , HCO3

pCO2 , HCO3 N

Resp + Met Acidosis

Uncomp Resp Acidosis

pCO2 N, HCO3 Uncomp Met Acidosis

pCO2 , HCO3 Comp(F/P) Resp Acidosis

pCO2 , HCO3 Comp(F/P) Met Acidosis

Comp(F) Resp Acidosis


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pH

NorN

pCO2 , HCO3 Comp(F) Met Alkalosis

pCO2 N, HCO3 N N Acid Base Homeostasis

pCO2 , HCO3 Met acidosis

+Resp alkalosis

Comp(F) Met Acidosis

Comp(F) Resp Alkalosis

Comp(F) Resp Acidosis

Resp Acidosis+

Met Alkalosis


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Thank You

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ABG5oct2010 Final