INTERPRETING ABGS – AS EASY AS 1-2-3

CHRIS WEIR, RRT, CPFT NEONATAL RESPIRATORY THERAPIST WOMEN’S AND CHILDREN’S HOSPITAL AT CENTENNIAL

Basic Concepts

 H+ ion regulation  The more hydrogen ions, the lower the pH and the more acidic the solution  The less hydrogen ions, the higher the pH and the more alkaline the solution  Acid – a substance that can contribute free H+ ions to a solution. A strong acid will contribute the free H+ ion more readily than a weak acid.  Base – a substance that can combine with/accept a free H+ ion to remove it from a solution.

pH

 Refers to the concentration of the free hydrogen ions in the produced by acid and base reactions. Thus, pH tells you about the acidity or alkalinity of the blood.  Reflects the ratio of HCO3 to CO2  pH will change if there is a change in the HCO3 not balanced by the CO2  Normal range: 7.35 - 7.45

1 pH

 pH < 7.35 = acidosis (overabundance of acid)

 pH > 7.45 = (overabundance of base)

PaCO2

 of CO2 in the blood  Respiratory component of the blood gas  Indication of alveolar ventilation  Normal range: 35 - 45 mmHg

PaCO2

 > 45 mmHg = acidity CO2 being retained by lungs

 < 35 mmHg = alkalinity CO2 being blown off by lungs

2 PaO2

 Refers to the partial pressure of O2 in the blood  Normal values: Term 50-80 mmHg (on RA)

PaO2

 Hypoxemia defined as PaO2 < 50 mmHg  Hyperoxemia defined as PaO2 > 80 mmHg

HCO3

 Chief base found in the blood.  Normal range: 20 - 26 mEq/l  Renal component of the blood gas  Decreased amounts of HCO3 =  Increased amounts of HCO3 = alkalosis  May also combine with other ions (potassium, calcium, magnesium) to form additional alkaline substances.

3 Base Deficit/Excess

 Normal range: -4 to +4 mEq/l

 Reflects the concentration of buffer or base available in the blood

 A deficit reflects an excess of acid or a diminished amount of base available. An excess reflects an excess of base in the blood and a deficit in the amount of acid in the blood.

Base Deficit/Excess

 Base deficit >>>> metabolic acidosis

 Base excess >>>> metabolic alkalosis

Normal Blood Gas Values

 Arterial  Capillary  pH: 7.35-7.45  pH: 7.35-7.45  pCO2: 35-45  pCO2: 35-50  p02: 50-80  pO2: not reliable  HCO3: 20-26  HCO3: 20-26  Base: -4 to 4  Base: -4 to 4

 First 48 hours of life:  pH: 7.30 – 7.45  HCO3: 19 - 22

4 STEP 1:

 Look at each component separately:  pH  pCO2  pO2  HCO3  Base

STEP 2:

 Acidosis or alkalosis?  If the pH is normal, you either have a normal blood gas or a compensated blood gas.  Normal for a baby to be a little more acidotic in the first 48 hours of life.

STEP 3:

 Is this a respiratory or metabolic problem?  Acidosis = Increased pCO2 and decreased HCO3  Alkalosis = Decreased pCO2 and increased HCO3

5 System At Fault:

 Primary abnormality in the pCO2 is respiratory in origin  Primary abnormality in the HCO3 is metabolic in origin

STEP 4:

 Hypoxemia or Hyperoxemia?

STEP 5:

 Is there any compensation?  To what degree?

6 TO DETERMINE COMPENSATION:

 Look at the system not at fault

 Absent compensation - system not at fault WNL

 Partial compensation - opposite system not WNL & pH not WNL

 Complete compensation - opposite system not WNL & pH WNL

Causes of Blood Gas Abnormalities:

   Metabolic Acidosis  Metabolic Alkalosis  Mixed Acidosis  Mixed Alkalosis

Respiratory Acidosis:

 Results from the formation of excess carbonic acid because of increased PCO2  Caused by insufficient alveolar ventilation  Blood gas findings: decreased pH, increased pCO2, normal HCO3

7 Respiratory Acidosis:

 Hypoventilation  Sedation  PPH  HMD  Upper Airway Not Patent  Pulmonary Hypoplasia  Recurrent Apnea  Central Depression  Pneumothorax

How the body compensates for respiratory acidosis:

 Over 3-4 days, the kidneys increase the rate of H+ secretion and reabsorption.  Blood gas findings: low normal pH, increased pCO2 and bicarbonate

Respiratory Alkalosis:

 Results from alveolar hyperventilation leading to a deficiency of carbonic acid  Caused by hyperventilation, usually iatrogenic  Blood gas findings: increased pH, decreased pCO2, normal HCO3

8 How the body compensates for respiratory alkalosis :

 The kidneys decrease H+ secretion by retaining chloride and excreting fewer acid salts. Bicarbonate reabsorption is also decreased.  Blood gas findings: pH high normal, low pCO2 and bicarbonate levels

Metabolic Acidosis:

 A deficiency in the concentration of bicarbonate in the ECF.  Caused by any systemic disease that increases acid production or retention, or problems leading to excessive base losses. Examples – hypoxia leading to lactic acid production, renal disease, and loss of base secondary to diarrhea.  Blood gas findings: decreased pH, decreased HCO3, normal pCO2

Metabolic Acidosis:

 Hyperalimentation  Hypoperfusion / hypovolemia  PDA  Renal Tubular Acidosis  Cold Stress  Renal Failure  Inborn Error of Metabolism  Sepsis

9 How the body compensates for metabolic acidosis:

 Healthy lungs will blow off additional CO2 through hyperventilation. If renal disease is not a problem, the kidneys will respond by increasing the excretion of acid salts and the reabsorption of HCO3.  Blood gas findings: pH low normal, pCO2 and HCO3 low

Metabolic Alkalosis:

 Results from an excess concentration of HCO3 in the ECF  Caused by problems leading to increased loss of acid. Examples – severe vomiting, gastric suction, and increased retention or intake of bases, such as occurs with excessive NaHCO3 administration. Also, diuretic therapy, chronic R Acidosis compensation, hypokalemia/hypochloremia.  Blood gas findings: high pH, high HCO3, normal pCO2

How the body compensates for metabolic alkalosis:

 The lungs compensate by retaining through hypoventilation.  Blood gas findings: pH high normal, HCO3 and pCO2 high

10 Interventions For Each Interpretation :

Respiratory Acidosis:

 In ventilated infants, increase the tidal volume.  Unventilated infants may require more ventilatory support depending on their WOB and degree of acidosis.  Correct the cause.

Respiratory Alkalosis:

 Caused by alveolar hyperventilation - WEAN !  Decrease the TV

11 Metabolic Acidosis:

 Identify and treat the cause  May want to consider volume expansion or bicarbonate administration depending on the specific clinical situation  Does the baby need additional buffer added to the IV fluids/TPN?

Metabolic Alkalosis:

 Identify and treat the cause  May also want to remove acetate from IV fluids, reduce diuretic doses, treat hyponatremia, hypokalemia, and hypochloremia

Blood Gas Sampling

 Infection control  Bleeding disorders  Steady state

12 Error in Blood Gas Measurement

  Hemoglobin  Dilution  Air bubbles

Arterial Sampling

 Sites

 Peripheral

 Central

Capillary Sampling

 Technique

 Values

13 Practice Problem #1

 A 31 week old infant is one hour old. CXR shows diffuse atelectasis with air bronchograms.  CBG – 7.29/59/42/26

Answer – Problem #1

 Acidosis  pCO2 is high indicating a respiratory problem leading to the acidosis  Capillary specimen  No compensation  Uncompensated respiratory acidosis  Treatment: NCPAP or MV

Practice Problem #2

 A 33 week infant is receiving mechanical ventilation for severe TTN. Settings: IMV 25, PIP 18, PEEP 4, .30.  ABG: 7.49/26/95/22

14 Answer – Problem #2

 Alkalemia  The PaCO2 is low indicating a respiratory alkalosis  Pa02 is high  No compensation  Uncompensated Respiratory Alkalosis  Treatment: wean the PIP or rate along with Fi02

Practice Problem #3

 26 week infant has been on the ventilator for 2 weeks for RDS. PIE is present.  CBG: 7.37/55/65/29

Answer - #3

 pH normal  pCO2 is high indicating a respiratory problem, which could lead to acidosis  Capillary specimen  Compensation present – pH normal with abnormal HCO3 and pC02. pH closer to acidosis  Compensated respiratory acidosis  Treatment: no action needed. Further increases in the pCO2 could result in decompensation.

15 Practice Problem #4

 26 week old infant on the ventilator for RDS. Settings: IMV 30, 19/5, and .40. Infant has lost 30 gms in the past 12 hours with a Na of 148.  ABG: 7.29/53/55/17

Answer - #4

 Acidemia  The PaCO2 is high indicating a respiratory acidosis and the HCO3 is low indicating a metabolic acidosis.  Oxygen level adequate.  No compensation – pH not normal  Uncompensated mixed acidosis.  Treatment: increase alveolar ventilation and consider giving volume to correct the hypovolemia

Practice Problem #5

 Term infant with tight nuchal cord. Infant pale, grunting, with cap refill of 8 seconds.  ABG: 7.15/40/75/15/-15

16 Answer - #5

 Acidosis  Metabolic in origin – decreased HCO3 with normal pCO2  Oxygen level adequate  No compensation – pCO2 normal  Uncompensated Metabolic Acidosis  Treatment: consider volume or HCO3 depending on respiratory assessment.

References

Gleson, C., & Devaskar, S. (2011). Avery’s Diseases of the Newborn (9 th Ed.). Philadelphia: W.B. Saunders. ISBN: 978-1437701340.

Gomella, T.L. (2009). Neonatology: Management, Procedures, On-Call Problems, Diseases and Drugs (6 th Edition). Norwalk, Conn.: Appleton & Lang. ISBN: 9780071544313

Karlsen, K.A. (2012). The S.T.A.B.L.E. Program. Park City: The S.T.A. B.L.E. Program.

17