ABGs

What is an ABG

o Arterial Blood Gas --- Drawn from artery

o It is an invasive procedure.

o Caution must be taken with patient on anticoagulants.

o Arterial blood gas analysis is an essential part of diagnosing and managing the patient’s oxygenation status, ventilation failure and acid base balance.

What are components of ABG?

The Components pH / PaCO2 / PaO2 / HCO3 / O2sat / BE

Desired Ranges pH - 7.35 - 7.45 PaCO2 - 35-45 mmHg PaO2 - 80-100 mmHg HCO3 - 21-27 O2sat - 95-100% Base Excess - +/-2 mEq/L

Why Order an ABG?

Aids in establishing a diagnosis Helps guide treatment plan Aids in ventilator management Improvement in acid/base

management allows for optimal function of medications

Acid/base status may alter electrolyte levels critical to patient status/care

Logistics

When to order an arterial line -- Need for continuous BP monitoring Need for multiple ABGs

Where to place -- the options Radial Femoral Brachial Dorsalis Pedis Axillary

Acid Base Balance

The body produces acids daily 15,000 mmol CO2

50-100 mEq Nonvolatile acids

The lungs and kidneys attempt to maintain balance

Acid Base Balance

Assessment of status via bicarbonate-carbon dioxide buffer system

CO2 + H2O <--> H2CO3 <--> HCO3- + H+

ph = 6.10 + log ([HCO3] / [0.03 x PCO2])

pH

Normal pH is 7.35-7.45 Value <7.35 is acidotic Value >7.45 is alkalotic Acidosis & Alkalosis can be caused

by a problem with the respiratory system or a metabolic cause

Can also have combined respiratory/metabolic states

The Terms & Types

ACIDS Acidemia Acidosis

Respiratory CO2

Metabolic HCO3

BASES Alkalemia Alkalosis

Respiratory CO2

Metabolic HCO3

Is it Respiratory or Metabolic?

1. Respiratory Acidosis

2. Respiratory Alkalosis

3. Metabolic Acidosis

4. Metabolic Alkalosis

Increased pCO2 >50

Decreased pCO2<30

Decreased HCO3 <18

Increased HCO3 >30

Respiratory Acidosis

ph, CO2, Ventilation Causes

CNS depression Pleural disease COPD/ARDS Musculoskeletal disorders Compensation for metabolic alkalosis

Respiratory Acidosis

Acute vs Chronic Acute - little kidney involvement.

Buffering via titration via Hb for example

pH by 0.08 for 10mmHg in CO2

Chronic - Renal compensation via synthesis and retention of HCO3 (Cl to balance charges hypochloremia)

pH by 0.03 for 10mmHg in CO2

Assessing Oxygenation

Normal value for arterial blood gas 80-100mmHg

Normal value for venous blood gas 40mmHg

Normal SaO2 Arterial: 97% Venous: 75%

Important points for assessing tissue oxygenation

This is the O2 that’s really available at the tissue level.

Is the THb normal? Low THb means the ability of the blood

to carry the O2 to the tissues is decreased

Is perfusion normal? Low perfusion means the blood isn’t

even getting to the tissues

Signs & symptoms of Respiratory Acidosis

Respiratory : Dyspnoea, respiratory distress and/or shallow respiration.

Nervous: Headache, restlessness and confusion. If co2 level extremely high drowsiness and unresponsiveness may be noted.

CVS: Tacycardia and dysrhythmias

Management Increase the ventilation. Causes can be treated rapidly

include pneumothorax, pain and CNS depression r/t medication.

If the cause can not be readily resolved, mechanical ventilation.

Caution for given O2 in chronic respiratory failure

Respiratory Alkalosis

pH, CO2, Ventilation CO2 HCO3 (Cl to balance charges

hyperchloremia) Causes

Intracerebral hemorrhage Salicylate and Progesterone drug usage Anxiety lung compliance Cirrhosis of the liver Sepsis

Respiratory Alkalosis

Acute vs. Chronic Acute - HCO3 by 2 mEq/L for every

10mmHg in PCO2

Chronic - Ratio increases to 4 mEq/L of HCO3 for every 10mmHg in PCO2

Decreased bicarb reabsorption and decreased ammonium excretion to normalize pH

Signs & symptoms CNS: Light Headedness, numbness,

tingling, confusion, inability to concentrate and blurred vision.

Dysrhythmias and palpitations Dry mouth, diaphoresis and tetanic

spasms of the arms and legs.

Management Resolve the underlying problem Monitor for respiratory muscle

fatigue When the respiratory muscle

become exhausted, acute respiratory failure may ensue

Metabolic Acidosis

pH, HCO3

12-24 hours for complete activation of respiratory compensation

PCO2 by 1.2mmHg for every 1 mEq/L HCO3

The degree of compensation is assessed via the Winter’s Formula

PCO2 = 1.5(HCO3) +8 2

Anion GAP

Calculation of AG is useful approach to analyse metabolic acidosis

AG = (Na+ + K+) – (cl- + Hco3-) * A change in the pH of 0.08 for each

10 mm Hg indicates an ACUTE condition.* A change in the pH of 0.03 for each 10 mm Hg indicates a CHRONIC condition.

The Causes

Metabolic Gap Acidosis M - Methanol U - Uremia D - DKA P - Paraldehyde I - INH L - Lactic Acidosis E - Ehylene Glycol S - Salicylate

Non Gap Metabolic Acidosis Hyperalimentation Acetazolamide RTA (Calculate

urine anion gap) Diarrhea Pancreatic Fistula

Sign & symptoms

CNS: Headache, confusion and restlessness progressing to lethargy, then stupor or coma.

CVS: Dysrhythmias Kussmaul’s respirations Warm, flushed skin as well as

nausea and vomiting

Management

Treat the cause Hypoxia of any tissue bed will produce metabolic

acids as a result of anaerobic metabolism even if the pao2 is normal

Restore tissue perfusion to the hypoxic tissues The use of bicarbonate is indicated for known

bicarbonate - responsive acidosis such as seen with renal failure

Metabolic Alkalosis

pH, HCO3

PCO2 by 0.7 for every 1mEq/L in HCO3

Causes Vomiting Diuretics Chronic diarrhea Hypokalemia Renal Failure

Signs/symptoms

CNS: Dizziness, lethargy disorientation, siezures & coma.

M/S: weakness, muscle twitching, muscle cramps and tetany.

Nausea, vomiting and respiratory depression.

It is difficult to treat.

Stepwise approach to ABG

Step 1: Acidemic or Alkalemic? Step 2: Is the primary disturbance

respiratory or metabolic? Step 3. Asses to Pa O2. A value below 80mm

Hg indicates Hypoxemia. For a respiratory disturbance, determine whether it is acute or chronic.

Step 4. For a metabolic acidosis, determine whether an anion gap is present.

Step 5. Assess the normal compensation by the respiratory system for a metabolic disturbance

Compensated or Uncompensated—what does this mean?

1. Evaluate pH—is it normal? Yes

2. Next evaluate pCO2 & HCO3

• pH normal + increased pCO2 + increased HCO3 = compensated respiratory acidosis

• pH normal + decreased HCO3 + decreased pCO2 = compensated metabolic acidosis

Compensated vs. Uncompensated

1. Is pH normal? No2. Acidotic vs. Alkalotic3. Respiratory vs. Metabolic

• pH<7.30 + pCO2>50 + normal HCO3 = uncompensated respiratory acidosis

• pH<7.30 + HCO3<18 + normal pCO2 = uncompensated metabolic acidosis

• pH>7.50 + pCO2<30 + normal HCO3 = uncompensated respiratory alkalosis

• pH>7.50 + HCO3>30 + normal pCO2 = uncompensated metabolic alkalosis

Practice

12 year old diabetic presents with Kussmaul breathing

pH : 7.05 pCO2: 12 mmHg pO2: 108 mmHg HCO3: 5 mEq/L BE: -30 mEq/L

Severe partly compensated metabolic acidosis without hypoxemia due to ketoacidosis

17 year old w/severe kyphoscoliosis, admitted for pneumonia

pH: 7.37 pCO2: 25 mmHg pO2: 60 mmHg HCO3: 14 mEq/L BE : -7 mEq/L

Compensated respiratory alkalosis due to chronic hyperventilation secondary to hypoxia

9 year old w/hx of asthma, audibly wheezing x 1 week, has not slept in 2 nights; presents sitting up and using accessory muscles to breath w/audible wheezes

pH: 7.51 pCO2: 25 mmHg pO2 35 mmHg HCO3: 22 mEq/L BE: -2 mEq/L

Uncompensated respiratory alkalosis with severe hypoxia due to asthma exacerbation

7 year old post op presenting with chills, fever and hypotension

pH: 7.25 pCO2: 32 mmHg pO2: 55 mmHg HCO3: 10 mEq/L BE: -15 mEq/L

Uncompensated metabolic acidosis due to low perfusion state and hypoxia causing increased lactic acid