Blood Gas Analysis/ABG Interpretation

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Basic understanding

pH is measured in terms of hydrogen (H+) ion concentration. An increase in H+ ion concentration decreases pH and vice versa. Changes in H+ ion concentration can be stabilized through several buffering systems: bicarbonate-carbonic acid, proteins, hemoglobin, and phosphates.

Acidosis, therefore, can be described as a physiologic condition caused by the body's inability to buffer excess H+ ions. At the other end, alkalosis results from a deficiency in H+ ion concentration. Acidemia and alkalemia refer to the process of acidosis or alkalosis, respectively, occurring in arterial blood.

Body acids are formed as end products of cellular metabolism. Under normal physiologic conditions, a person generates 50 to 100 mEq/day of acid from metabolism of carbohydrates, proteins, and fats. In addition, the body loses base in the stool. In order to maintain acid-base homeostasis, acid production must balance the neutralization or excretion. The lungs and kidneys are the main regulators of acid-base homeostasis. The lungs release CO2, an end product of carbonic acid (H2CO3). The renal tubules, with the regulation of bicarbonate (HCO3-), excrete other acids produced from the metabolism of proteins, carbohydrates, and fats.

Normal values

pH: 7.35 to 7.45

Bicarbonate (HCO₃): 22 to 26 mEq/L

pCO₂: 35 to 45 mmHg

pO₂: 75 to 100 mmHg

Acid base balance is regulated by

1. Buffer mechanism – rapid first line of defence (takes seconds)

Physiologic buffers, consisting of a weak acid (which can easily be broken down) and its base salt or of a weak base and its acid salt. These buffers are the bicarbonate-carbonic acid buffering system, intracellular protein buffers, and phosphate buffers in the bone.

2. Respiratory mechanism – second line of defence (takes minutes)

Pulmonary compensation, in which changes in ventilation work to change the partial pressure of arterial carbon dioxide (PaCO2) and drive the pH toward the normal range. A drop in pH, for example, results in increased ventilation to blow off excess CO2. An increase in pH decreases ventilatory effort, which increases PaCO2 and lowers the pH back toward normal

3. Renal mechanism – third line of defence (takes hours to days)

In acidosis the kidneys excrete H+ in urine and retain HCO3-. In alkalosis, the kidneys excrete bicarbonate and retain H+ in the form of organic acids, resulting in near-normalization of pH.2,3 Lastly, bone may also serve as a buffer because it contains a large reservoir of bicarbonate and phosphate and can buffer a significant acute acid load.

Acid Base Imbalance

pH < 7.35 is acidosis (respiratory or metabolic)

pH > 7.45 is alkalosis (respiratory or metabolic)

Respiratory Acidosis

1. pH < 7.35

2. Excess carbonic acid

3. Increased retention of CO₂>45 mmHg (Hypercapnia)

4. pO2 < 80 mmHg

5. Causes

ü Alveolar hypoventilation - Asphyxia (Birth asphyxia, foreign body obstruction, laryngeal edema)

ü Respiratory obstruction – mechanical (ex. Tumor), functional( ex. Asthma)

ü Impaired gas exchange in the alveoli (Emphysema)

ü Neuromuscular impairment (Spinal cord injury, snake bite, Guillian Barre syndrome, OP poisoning)

ü Respiratory centre depression (Sedative overdose, MgSO₄, CNS infections, CVA)

6. Signs of respiratory acidosis

v Dyspnoea

v Irritability

v Disorientation

v Tachycardia

v Cyanosis

v Coma

7. Compensatory mechanisms

The urinary system excretes increased hydrogen ions to compensate

The urinary system retains sodium to facilitate the body’s attempt to increase NaHCO₃.

The rate and depth of respirations increases as elevated CO₂ levels stimulate breathing centre.

Respiratory alkalosis

1. pH > 7.45

2. Carbonic acid deficit

3. pCO₂ < 35 mmHg

4. hyperventilation blows off excessive CO₂

5. causes

· Hyperventilation related to anxiety & panic

· Mechanical overventilation

· Hepatic disease

· Pregnancy

· Septicaemia

· Salicylate toxicity

6. Signs of respiratory alkalosis

o Deep & rapid breathing

o Light-headedness

o Tingling & numbness

o Tinnitus

o Loss of concentration

o Unconsciousness

7. Compensatory mechanisms

Urinary system retains hydrogen ions & excretes bicarbonate

Metabolic acidosis

1. pH < 7.35

2. Base bicarbonate deficit

3. Excess acid other than carbonic acid

4. HCO3 < 22 mEq/L

5. Causes

v Cellular breakdown with increased ketones (starvation, terminal cancer, dieting, diabetic ketoacidosis)

v Lactic acidosis from anaerobic metabolism

v Direct loss of bicarbonate through severe diarrhea

v Renal insufficiency (acute or chronic renal failure)

6. Signs of metabolic acidosis

ü Weakness

ü Headache

ü Disorientation

ü Deep & rapid breathing (Kussamaul’s respirations)

ü Fruity odor to breath

ü Nausea & vomiting

7. Compensatory mechanisms

o Respiratory system compensates by hyperventilation in an attempt to blow off CO₂ & raise the pH.

o Urinary system excretes hydrogen ions* & retains bicarbonate.

*In general, the kidneys attempt to preserve sodium by exchanging it for excreted H+ or potassium. In the presence of an H+ load, H+ ions move from the extracellular fluid into the intracellular fluid. For this process to occur, potassium moves outside the cell into the extracellular fluid to maintain electroneutrality. In severe acidosis, significant overall depletion of total body potassium stores can occur despite serum hyperkalemia. This is why I.V. potassium is given to patients with diabetic ketoacidosis early in treatment, despite the often-elevated serum potassium level. External and internal potassium balances are regulated to maintain an extracellular fluid concentration of 3.5 to 5.5 mEq/L and a total body content of about 50 mEq/kg (40 mEq/kg in females).

Metabolic alkalosis

1. Ph > 7.45

2. Base bicarbonate excess

3. Bicarbonate level > 27 mEq/L

4. Causes

Ø Loss of gastric juice (vomiting, nasogastric decompression, lavage)

Ø Secretory adenoma of the colon

Ø Hyperaldosteronism

Ø Cushing's syndrome

Ø Exogenous steroids

Ø Excessive ingestion alkaline drugs (baking soda)

Ø Diuretics**

** Diuretics may precipitate hypokalaemia, in the presence of hypokalemia, kidneys conserve potassium & excrete hydrogen, intracellular potassium moves into interstitial compartment, and hydrogen moves into cells; as a result of these process, the plasma hydrogen level is decreased & base bicarbonate level is increased.

5. Signs of metabolic alkalosis

Tetany (muscle hypertonicity)

Tingling

Tremors

Shallow & slow respirations

Dizziness

Confusion

Coma

6. Compensatory mechanisms

Respiratory system compensates by decreasing the rate & depth of breathing to retain CO₂ & decrease pH.

Urinary system excretes NaHCO₃

Interpretation in primary stage

Condition	pH	pCO₂	HCO₃
Respiratory acidosis	Decreased	Increased	Normal
Respiratory alkalosis	Increased	Decreased	Normal
Metabolic acidosis	Decreased	Normal	Decreased
Metabolic alkalosis	Increased	Normal	Increased

Interpretation in compensatory stage

Condition	pH	pCO₂	HCO₃
Respiratory acidosis comp	Increasing to normal	Increased	Increased
Respiratory alkalosis comp	Decreasing to normal	Decreased	Decreased
Metabolic acidosis comp	Increasing to normal	Decreased	Decreased
Metabolic alkalosis comp	Decreasing to normal	Increased	Increased

Tic Tac Toe method

Acid/Acidosis ￨ Normal ┃ Base/Alkalosis

│ │

Explanation video for Tic Tac Toe method given below

Tic Tac Toe Video

Practice question 1:

pH=7.26 | PaCO2=32 | HCO3=18

Make your tic-tac-toe grid.

pH of 7.26 ABNORMAL and under ACIDOSIS, so we place pH under ACIDOSIS.

PaCO2 of 32 is ABNORMAL and under ALKALOSIS, so we place PaCO2 under ALKALOSIS.

HCO3 of 18 is ABNORMAL and under ACIDOSIS, so we place HCO3 under ACIDOSIS.

pH is under ACIDOSIS, therefore solving for goal #1, we have ACIDOSIS.

pH is on the same column as HCO3, therefore solving for goal #2, we have METABOLIC.

All three values are ABNORMAL, therefore solving for goal #3, we have a PARTIALLY COMPENSATED ABG.

The answer is Metabolic Acidosis, Partially Compensated

Practice question 2:

pH=7.44 | PaCO2=30 | HCO3=21

Make your tic-tac-toe grid.

pH of 7.44 is NORMAL but slightly leaning towards ALKALOSIS, so we place pH under the NORMAL column with an arrow pointing towards the ALKALOSIS column.

PaCO2 of 30 is ABNORMAL and ALKALOSIS, so we place PaCO2 under the ALKALOSIS column.

HCO3 of 21 is ABNORMAL and ACIDOSIS, so we place HCO3 under the ACIDOSIS column.

pH of 7.44 is NORMAL but leaning towards ALKALOSIS, therefore solving for goal #1, we have ALKALOSIS.

pH is NORMAL but is leaning towards ALKALOSIS, therefore under the same column as PaCO2. Solving for goal #2, we have RESPIRATORY.

pH is NORMAL, therefore solving for goal #3, we have a FULLY COMPENSATED ABG.

The answer is Respiratory Alkalosis, Fully Compensated

Practice question 3:

pH=7.1 | PaCO2=40 | HCO3=18

Make your tic-tac-toe grid.

pH of 7.1 is ABNORMAL and ACIDOSIS, therefore, we place pH under the ACIDOSIS column in the tic-tac-toe grid.

PaCO2 of 40 is NORMAL, therefore, place it under the NORMAL column.

HCO3 of 18 is ABNORMAL and ACIDOSIS, so we place HCO3 under the ACIDOSIS column.

pH of 7.1 is ACIDOSIS, therefore, solving for goal #1, we have ACIDOSIS.

pH is under the same column as HCO3, therefore, solving for goal #2, we have determined that it is METABOLIC.

pH is ABNORMAL so as HCO3, but PaCO3 is under the NORMAL column. Solving for goal #3, we can interpret it as UNCOMPENSATED.

The answer is Metabolic Acidosis, Uncompensated

By using tic tac toe method, solve below problems & verify answers

1. Lab values: pH 7.56, paCo2 20, HCO3 20

A. respiratory alkalosis, uncompensated