abg

Arterial Blood GasArterial Blood Gas

Jaime C. Tan,MD

Division of Pulmonary and Critical Care Medicine

Philippine Heart center

Indications of ABGIndications of ABG

Determine acid-base or oxygenation problem

May indicate onset or culmination of cardiopulmonary crises

May serve as a gauge to the appropriateness or effectiveness of therapy

Normal ABG ValuesNormal ABG Values

pH 7.35 – 7.45

PaCO2 35 – 45 mmHg

[BE] 0 2 meq/L

PaO2 80 – 100 mmHg

[HCO3] 24 2 meq/L

SaO2 97 – 98%

Steps for the interpretation of Steps for the interpretation of acid base disturbanceacid base disturbance

Is the px acidemic or alkalemic? Is the disturbance respiratory or

metabolic? If the disturbance is respiratory, is it

acute or chronic? If the disturbance is metabolic, is

the anion gap normal or abnormal?

If the disturbance is metabolic, is the respiratory system compensating?

Severity of Generalized Severity of Generalized Acid-Base DisturbancesAcid-Base Disturbances

pH Degree of Impairment

< 7.20 Severe acidemia

7.20 – 7.29 Moderate acidemia

7.30 – 7.34 Mild acidemia

7.35 – 7.45 Normal pH

7.46 – 7.50 Mild alkalemia

7.51 – 7.55 Moderate alkalemia

> 7.55 Severe alkalemia

Classification of PaOClassification of PaO22 in the Adultin the Adult

Classification PaO2 (mmHg)

Hyperoxemia >100

Normoxemia 80 – 100

Mild hypoxemia 60 – 79

Moderate hypoxemia 45 – 59

Severe hypoxemia < 45

Room Air (Fio2=.21)Room Air (Fio2=.21)

Px < 60 y.o. = Po2 =80-100mhg Px >60 y.o. = subtract 1 from 80 for

every year above 60 years of age

example: 70 y.o.

Ideal PaO2= 80

- 10

- - - - - -

70 mmhg

Supplemental O2Supplemental O2

Fio2= Lpm x 4 + 20 P/F ratio: PaO2/ FiO2 NV: <60 y.o.= 400-500 NV:>60 y.o.= multiply every year above 60 by

5 then subtract the total from 400

ex: 70y.o.

= 10x 5= 50

= 400-50

= 350

Steps in analyzing the Steps in analyzing the oxygenation statusoxygenation status

Is the px hypoxemic or non hypoxemic ?PaO2, Fio2, P/F ratio

Ex: 75 yo, 2 lpmactual P/F= 296

expected P/F= 325Therefore the px is HYPOXEMIC

If the px is hypoxemic, is it corrected, uncorrected or overcorrected?

Compute for pO2 expected for age:

ex: 75 y.o., PaO2: 80

expected Po2 for age:80-15=65

px has CORRECTED HYPOXEMIA

If the px is non hypoxemic , is he receiving adequate oxygenation or more than adequate oxygenation?

Ex: 75y.o., actual Po2:109, Fio2: 0.28, P/F ratio:389 (expected:325)

px is NON HYPOXEMIC

expected Po2 for age:65, actual Po2: 109

Receiving MORE THAN ADEQUATE OXYGENATION

Acid-Base ClassificationAcid-Base Classification

Acid-base disturbance pH PaCO2 HCO3

Respiratory acidosis N0 or

Respiratory alkalosis N0 or

Metabolic acidosis N0 or

Metabolic alkalosis N0 or

CasesCases

CaseCase

V.M., 59 year old male Moderate COPD; NIDDM 2-week cough with yellow sputum Intermittent low-grade fever Cefuroxime, Paracetamol,

Fenoterol+Ipratropium Br Sought consult at ER due to dyspnea

and pleuritic chest pain

x 3 days

CaseCase At ER, he was agitated with the ff v/s:

BP 130/90mmHg HR 110/min

RR 28/min T 380C Chest/Lungs: increased breath

sounds and tactile fremitus at the right lower lung field, crackles on both lower lung fields

CaseCase

Chest x-ray: lobar pneumonia at the

right lower lobe, hazy densities at the

left base and blunting of the right

costophrenic angle

CaseCase ABG taken at room air

pH 7.50

PaCO2 31 mmHg

PaO2 60 mmHg

HCO3 20 meq/L

BE 1.6 meq/L

SaO2 90%

What is the acid-base problem?A. Respiratory alkalosis

B. Respiratory acidosis

C. Metabolic alkalosis

D. Metabolic acidosisA. Respiratory alkalosis

Respiratory AlkalosisRespiratory Alkalosis

Hallmark

CompensationCellular bufferingRenal response: retention of

endogenous acids, excretion of HCO3

pH, PaCO2

Respiratory AlkalosisRespiratory Alkalosis

Formula for compensation

HCO3 by 2 - 4 meq/L

Respiratory AlkalosisRespiratory AlkalosisPrimary central disorders Hyperventilation

syndrome, anxiety Cerebrovascular disease Meningitis, encephalitisPulmonary disease Interstitial fibrosis Pneumonia Pulmonary embolism Pulmonary edema (some

patients)

Hypoxia

Septicemia, hypotension

Hepatic failure

Drugs Salicylates Nicotine Xanthines Progestational hormones

High altitude

Mechanical ventilators

Respiratory AlkalosisRespiratory Alkalosis Treatment

Treat the primary cause Control the patient’s ventilation

Paralyze the patient CMV

Carbonic anhydrase inhibitor (acetazolamide)

To decrease HCO3 of limited value

* Most frequently, requires no specific therapy

CaseCase Given O2 at 4L/min via nasal cannula

SaO2 98%

Given Ceftazidime, Amikacin

WBC 30 x 106/uL

Segmenters 98%; (+) toxic granules

Potassium 4 mmol/L

Sodium 135 mmol/L

Creatinine 0.12 mmol/L

CaseCase Eight hours later drowsy with

labored breathing and cyanosis

ABG at 10L/min O2

pH 7.23

PaCO2 86 mmHg

PaO2 69 mmHg

HCO3 25 meq/L

BE - 4.8 meq/L

SaO2 91%

CaseCase Repeat Chest X-Ray: progression of

hazy densities on the left lower lung field, with no significant interval change in the previously noted right lower lobe pneumonia.

Patient admitted to ICU




D. Metabolic acidosisB. Respiratory acidosis

Respiratory Respiratory AcidosisAcidosis

Hallmark

CompensationCellular buffering: HCO3

Renal adaptation: H+ secretion, Cl- reabsorption, net acid excretion

pH, PaCO2

Respiratory AcidosisRespiratory Acidosis

Formula for compensation

HCO3 by 3 - 4 meq/L

Respiratory AcidosisRespiratory Acidosis COPD O2 excess in COPD Drugs

Barbiturates Anesthetics Narcotics Sedatives

Extreme ventilation-perfusion mismatch

Exhaustion Inadequate MV Neurologic disorders

Neuromuscular disease Poliomyelitis ALL G-B syndrome Electrolyte deficiencies

(K+, PO4-)

Myasthenia gravis Excessive CO2

production TPN Sepsis Severe burns NaHCO3 administration

Respiratory AcidosisRespiratory Acidosis

Treatment:Correct precipitating causeRestore alveolar ventilationCorrect CO2 retention Intubation and assisted ventilationO2 administration

CaseCase

Third ICU day blood C/S: Klebsiella pneumoniae

Sensitive: piperacillin/tazobactam,

meropenem, cefepime

Resistant: ceftazidime, amikacin

Decreasing urine output at 10ml/hr

Impression: acute renal failure

Referred to nephrologist

CaseCase

sodium 145 mmol/L

potassium 5 mmol/L

chloride 106 mmol/L

creatinine 0.48 mmol/L

CBG 300 mg/dL

Laboratories

CaseCase ABG at 40% FiO2, BUR 20/min

400ml VT, PEEP 5cmH2OpH 7.20

PaCO2 25 mmHg

PaO2 114 mmHg

HCO3 11 meq/L

BE - 15.9 meq/L

SaO2 98.4%




D. Metabolic acidosisD. Metabolic acidosis

Metabolic Metabolic AcidosisAcidosis

Metabolic AcidosisMetabolic Acidosis

Hallmark: pH HCO3

base deficit accumulation of fixed acids


Abnormalities: Overproduction of acids Loss of buffer stores Underexcretion of acids


Compensation pCO2 (hyperventilation) Pathway:

pCO2

HCO3

ratio H+ conc

Acidification of ECF ECF pH

Stimulation of brainstem RR pCO2

Normalization of pH

HCO3


Compensation Ionic shift

K+ moves extracellularly for H+

HCO3 generation, H+ excretion


Effects Stimulate epinephrine release Leukocytosis Hyperkalemia Hypercalcemia / hypercalciuria Myocardial failure

Anion GapAnion Gap

Numerical difference between Na+ and HCO3, Cl-

Helpful tool in suggesting the presence and clarifying the differential diagnosis of metabolic acidosis

Anion Gap = [Na+] – [HCO3 + Cl-]

N0 value = 12 2 meq/L

Normal vs. Elevated Anion GapNormal vs. Elevated Anion Gap

Normal Anion Gap Reduced HCO3 is counterbalanced by a

measurable anion GI disorders (diarrhea, pancreatic fistulas) Uterosigmoidoscopy, ileostomy Ingestion of acids, parenteral

hyperalimentation Carbonic anhydrase inhibitors Renal acidification defects

Normal vs. Elevated Anion GapNormal vs. Elevated Anion Gap

Elevated Anion Gap Reduced HCO3 is replaced by an

unmeasurable organic anion Ketoacidosis (starvation, alcohol-induced) Lactic acidosis Chronic renal failure Methyl alcohol / ethyl alcohol ingestion Paraldehyde ingestion Salicylate overdose


Compensation

Expected pCO2 = HCO3 x 1.5 + 8.4

Limit = 10 mmHg


Management Sustain normality of blood acid base

parameters Maintain serum HCO3 = 10 to 15 meq/L

HCO3 administration for pH < 7.2 Treat the underlying cause

HCO3 = (desired – actual HCO3) x 0.4 x wt (kg)

HCO3 = BE x 0.3 x wt (kg) 2

NaHCONaHCO33 Deficit Computation Deficit Computation

CaseCase Urine output improved after fluid

challenge Few hours later tachypneic PEEP increased to 8 cmH2O

pH 7.36

PaCO2 34 mmHg

PaO2 89 mmHg

HCO3 18 meq/L

BE - 6.1 meq/L

SaO2 96.6%

CaseCase

Furosemide drip started at 10mg/hr Laboratories:

sodium 140 mmol/L

potassium 2 mmol/L

chloride 100 mmol/L

creatinine 0.34 mmol/L

albumin 26 g/dL

CaseCase Repeat ABG at 40% FiO2, VT 400ml,

BUR 20/min, PEEP 8cmH2O

pH 7.53

PaCO2 47 mmHg

PaO2 109 mmHg

HCO3 36 meq/L

BE 11.3 meq/L

SaO2 98.5%




D. Metabolic acidosisC. Metabolic alkalosis

Metabolic AlkalosisMetabolic Alkalosis

Hallmark

Compensation PaCO2 (hypoventilation)

pH, HCO3


Pathway

HCO3PaCO2

HCO3

ratio H+ conc

Alkalinization of ECF PaCO2 with mild hypoxemia

Normalization of pH


Compensation

Every 1 mEq increase in HCO3 will increase PaCO2 by 0.5-1 mmHg

Expected PaCO2= ( O.8 x HCO3 ) + 16 4

Causes of Metabolic Alkalosis

Hypokalemia*Ingestion of large amounts of alkali or licoriceGastric fluid loss: Vomiting, NG suctioning*Hyperaldosteronism 20 to nonadrenal factors Bartter’s syndrome Inadequate renal perfusion diuretics (inhibiting NaCl reabsorption)*Bicarbonate administration Sodium bicarbonate overcorrection Blood transfusionAdrenocortical hypersecretion (e.g tumor)Steroids*Eucapnic ventilation posthypercapnia

* Common in the ICU

Treatment of Treatment of Metabolic AlkalosisMetabolic Alkalosis

Replace fluids (with Normal Saline) Replace electrolyte deficit

K supplementation (K > 4.5 mEq/L) Ammonium chloride Hydrochloric acid

Acetazolamide (carbonic anhydrase inhibitor) -promotes renal excretion of HCO3

-

Sample problem:Sample problem:

68 y.o., male, w/ increasing SOB, admitted at ER, initial ABG: (32% fio2)

Ph: 7.25, Pco2: 75, Po2= 95,HCO3: 32, O2 sat: 94%

Role of NursesRole of Nurses

Evaluation of symptoms

When to refer to MD

Administration of medications

Awareness of potential

complications of medications

Evaluation of Evaluation of

SymptomsSymptoms Respiratory acidosis

Alteration of state of consciousness

Confusion

Stupor

Obtundation

Coma


SymptomsSymptoms Respiratory alkalosis

Cerebral vasoconstriction

Nausea, vomiting, lightheadedness

Carpopedal spasm

Circumoral, digital paresthesias


SymptomsSymptoms Metabolic alkalosis

Increase neuromuscular activity

Chvostek, Trousseau sign

Twitching, tetany

Arrhythmias


SymptomsSymptoms Metabolic acidosis

Kussmaul’s respiration

Hyperpnea, tachypnea

Hypotension

Arrhythmias

Administration of Administration of

MedicationsMedications Sodium Bicarbonate

Given for correction of metabolic

acidosis

Bicarbonate deficit

HCO3 = BE x 0.3 x wt (kg) 2

HCO3 = (desired – actual HCO3) x 0.4 x wt (kg)

Potential Complications of Potential Complications of

MedicationsMedications

Local site complication

Patent IV site

Bicarbonate overcorrection

Hypokalemia

Fluid overload or hypernatremia

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