ABG Diagnostics

Question 1

What is the numerical value that serves as the clinical cut-off for determining acidosis versus alkalosis based on pH?

Answer 1

7.4. This is the clinical cut-off point used to distinguish between acidosis (pH below 7.4) and alkalosis (pH above 7.4).

Question 2

Name the most commonly used artery for obtaining an ABG sample, and explain why it’s preferred.

Answer 2

Radial artery. It’s preferred due to its superficial location, easy palpation, and collateral circulation provided by the ulnar artery.

Question 3

What is the calculated concentration in arterial blood that reflects the kidneys’ effort to neutralize acid, and what is its normal range?

Answer 3

HCO3 (Bicarbonate), normal range is 22-26 mEq/L. This reflects the metabolic component of acid-base balance.

Question 4

Besides pH, identify the two other ABG components that must be analyzed to determine the primary acid-base disorder.

Answer 4

PaCO2 (reflecting the respiratory component) and HCO3 (reflecting the metabolic component). These help determine whether the primary disturbance is respiratory or metabolic.

Question 5

Describe the expected compensatory mechanism for metabolic acidosis, including the specific physiological changes involved.

Answer 5

The body compensates by increasing minute ventilation (hyperventilation), leading to a decrease in PaCO2, which helps increase pH toward the normal range.

Question 6

What is the term for an acid-base disturbance where the expected compensation occurs but the pH remains outside the normal range?

Answer 6

Partially compensated. Compensation is occurring, but it’s insufficient to fully correct the pH imbalance.

Question 7

State Winter’s formula and explain how it’s used to identify a concomitant acid-base disorder in metabolic acidosis.

Answer 7

Winter’s formula: PaCO2 = (1.5 x HCO3) + 8. In metabolic acidosis, if the actual PaCO2 is lower than the calculated value using this formula, it suggests a concomitant respiratory alkalosis.

Question 8

List the three components used to calculate the anion gap, and provide the formula.

Answer 8

The three components are sodium (Na), chloride (Cl), and bicarbonate (HCO3). The formula is: AG = (Na - (Cl + HCO3)).

Question 9

What are the two most common causes of high anion gap metabolic acidosis (HAGMA)?

Answer 9

Lactic acidosis (due to tissue ischemia or altered cellular metabolism) and diabetic ketoacidosis. These are frequently encountered in clinical practice.

Question 10

Provide three specific examples of gastrointestinal conditions that can lead to non-anion gap metabolic acidosis (NAGMA).

Answer 10

Diarrhea, ileostomy, and proximal colostomy. These conditions lead to loss of bicarbonate-rich fluids from the GI tract.

Question 11

Explain how the ratio of change in anion gap to change in bicarbonate is used to determine the presence of concurrent acid-base disorders in HAGMA.

Answer 11

A ratio between 1.0 and 2.0 indicates uncomplicated anion gap metabolic acidosis. A ratio less than 1 suggests a concurrent NAGMA, while a ratio greater than 2 suggests a concurrent metabolic alkalosis.

Question 12

State the formula for calculating the desired PaO2 based on age.

Answer 12

Expected PaO2 for age = 104 - (age x 0.43). This helps determine if a patient’s oxygenation is adequate for their age.

Question 13

A patient presents with a pH of 7.32, PaCO2 of 55 mmHg, and HCO3 of 28 mEq/L. Identify the primary acid-base disorder and determine if compensation is present.

Answer 13

The patient has respiratory acidosis. The elevated PaCO2 indicates a respiratory origin, and the HCO3 is within the normal range, suggesting no compensation has occurred yet.

Question 14

A patient with a history of COPD has an ABG showing pH of 7.36, PaCO2 of 50 mmHg, and HCO3 of 32 mEq/L. What does this ABG reveal about the patient’s acid-base status?

Answer 14

The patient has compensated respiratory acidosis. The elevated PaCO2 indicates respiratory acidosis, but the elevated HCO3 shows the kidneys are compensating to normalize the pH.

Question 15

A patient experiencing an anxiety attack has an ABG with a pH of 7.50, PaCO2 of 30 mmHg, and HCO3 of 23 mEq/L. Interpret these findings.

Answer 15

The patient has respiratory alkalosis. The low PaCO2 and elevated pH point to a respiratory origin. The near-normal HCO3 suggests minimal compensation.

Question 16

Describe the two main categories of causes of metabolic alkalosis, providing examples of conditions for each category.

Answer 16

The two main categories are hypovolemia with chloride depletion (e.g., vomiting, gastric suction) and hypervolemia and chloride expansion (e.g., heart failure, hyperaldosteronism).

Question 17

List five distinct causes of respiratory acidosis, ensuring they represent different underlying mechanisms.

Answer 17

Airway obstruction (e.g., COPD), CNS depression (e.g., opioid overdose), neuromuscular impairment (e.g., Guillain-Barré syndrome), ventilatory restriction (e.g., chest wall deformity), and increased CO2 production (e.g., malignant hyperthermia).

Question 18

Explain why a patient with myasthenia gravis might develop respiratory acidosis.

Answer 18

Myasthenia gravis can cause muscle weakness, including the muscles involved in breathing (diaphragm, intercostal muscles), leading to hypoventilation and CO2 retention.

Question 19

Identify one key cause of metabolic acidosis in chronic kidney disease (CKD).

Answer 19

Loss of renal function impairs acid excretion, leading to an accumulation of metabolic acids.

Question 20

Which electrolyte abnormality commonly accompanies metabolic alkalosis?

Answer 20

Hypokalemia. It often co-occurs due to intracellular shifting of potassium or renal losses.

Question 21

What is the classic triad of symptoms seen in diabetic ketoacidosis (DKA)?

Answer 21

Polyuria, polydipsia, and polyphagia.

Question 22

How does hyperventilation affect blood pH, and what is this condition called?

Answer 22

Hyperventilation decreases PaCO2, which increases blood pH, leading to respiratory alkalosis.

Question 23

Explain how renal tubular acidosis (RTA) can result in metabolic acidosis.

Answer 23

RTA involves impaired bicarbonate reabsorption or hydrogen ion excretion, resulting in a net acid gain and a decrease in bicarbonate levels.

Question 24

Differentiate between type I and type II renal tubular acidosis in terms of their primary defect.

Answer 24

Type I RTA involves a defect in distal H+ excretion, while type II RTA involves a defect in proximal HCO3 reabsorption.

Question 25

Explain why vomiting can lead to metabolic alkalosis.

Answer 25

Vomiting leads to a loss of gastric acid (HCl), reducing the acid load and increasing the serum bicarbonate concentration.

Question 26

Name a condition that can cause a mixed acid-base disorder, where both metabolic acidosis and respiratory acidosis are present.

Answer 26

Severe asthma exacerbation can lead to both metabolic acidosis (due to hypoxia and lactic acidosis) and respiratory acidosis (due to hypoventilation).

Question 27

Explain why hyperaldosteronism can cause metabolic alkalosis.

Answer 27

Hyperaldosteronism leads to increased sodium reabsorption and potassium and hydrogen ion excretion, resulting in an increase in bicarbonate levels and a shift toward alkalosis.

Question 28

What is the effect of acute kidney injury (AKI) on acid-base status, and what disorder is typically seen?

Answer 28

AKI can impair the kidney’s ability to excrete acid, leading to the development of metabolic acidosis, often characterized by a low bicarbonate level.

Question 29

State two common causes of respiratory alkalosis.

Answer 29

Anxiety (due to hyperventilation) and hypoxia (such as in pulmonary embolism or high altitude).

Question 30

Describe the classic ABG findings in a patient with chronic obstructive pulmonary disease (COPD) exacerbation.

Answer 30

Patients often present with respiratory acidosis (elevated PaCO2) and a compensatory increase in bicarbonate (HCO3), along with a reduced pH.

Question 31

Explain the term ‘compensated metabolic alkalosis’.

Answer 31

Compensated metabolic alkalosis occurs when the respiratory system compensates for the metabolic alkalosis by hypoventilating (increasing PaCO2) to increase the acidic load and balance the elevated bicarbonate levels.

Question 32

What acid-base disorder might you expect in a patient with severe sepsis and why?

Answer 32

Metabolic acidosis, typically due to lactic acidosis. Sepsis often leads to tissue hypoxia and increased lactate production.

Question 33

Describe the typical ABG changes in a patient with early stages of acute respiratory distress syndrome (ARDS).

Answer 33

In the early stages of ARDS, patients may develop respiratory alkalosis due to hyperventilation from hypoxia, leading to a reduced PaCO2 and elevated pH.

Question 34

Explain how hypovolemic shock can lead to metabolic acidosis.

Answer 34

Hypovolemic shock leads to decreased tissue perfusion, causing anaerobic metabolism and lactic acid accumulation, which results in metabolic acidosis.

Question 35

What is the primary compensatory response in metabolic alkalosis, and how does it affect PaCO2?

Answer 35

The primary compensatory response to metabolic alkalosis is hypoventilation, leading to an increase in PaCO2 as the body retains CO2 to counterbalance the alkalosis.

Question 36

List two common causes of high anion gap metabolic acidosis (HAGMA) that involve toxins or poisons.

Answer 36

Methanol ingestion and ethylene glycol poisoning are common causes of high anion gap metabolic acidosis due to the accumulation of toxic metabolites.

Question 37

How does acute respiratory acidosis affect the kidney’s compensatory response?

Answer 37

In acute respiratory acidosis, the kidneys will attempt to compensate by retaining bicarbonate over a period of hours to days, though compensation may not be sufficient in the acute phase.

Question 38

Explain the concept of ‘corrected bicarbonate’ in the context of mixed acid-base disorders.

Answer 38

Corrected bicarbonate refers to adjusting the bicarbonate level based on a patient’s current PaCO2 to account for the respiratory component in the assessment of mixed acid-base disturbances.

Question 39

Name one condition that can cause a normal anion gap metabolic acidosis but result in a significant shift in chloride levels.

Answer 39

Diarrhea is a common cause of normal anion gap metabolic acidosis, where the loss of bicarbonate is matched by an increase in chloride levels (hyperchloremic acidosis).

Question 40

What compensatory mechanism is seen in respiratory alkalosis, and how does it affect bicarbonate levels?

Answer 40

In respiratory alkalosis, the kidneys compensate by excreting bicarbonate, leading to a decrease in bicarbonate levels to help restore pH balance.

Question 41

What are the hallmark ABG findings in a patient with hyperventilation due to anxiety?

Answer 41

In hyperventilation due to anxiety, ABG findings typically include respiratory alkalosis with a decreased PaCO2 and increased pH, along with normal or slightly reduced bicarbonate levels as compensation.

Question 42

Describe the pathophysiology behind lactic acidosis as a cause of metabolic acidosis.

Answer 42

Lactic acidosis results from the accumulation of lactate due to anaerobic metabolism, often triggered by conditions such as shock, sepsis, or severe hypoxia, leading to a decreased pH and a low bicarbonate level.

Question 43

Identify a condition in which both metabolic acidosis and metabolic alkalosis could be present simultaneously.

Answer 43

Chronic kidney disease with vomiting can present with both metabolic acidosis (due to kidney failure) and metabolic alkalosis (due to loss of gastric acid).

Question 44

Explain the concept of ‘buffering capacity’ in acid-base physiology.

Answer 44

Buffering capacity refers to the ability of a system to resist changes in pH when an acid or base is added. The bicarbonate buffer system is the primary extracellular buffer.

Question 45

How does hypercapnia contribute to acidosis?

Answer 45

Hypercapnia (elevated PaCO2) leads to increased carbonic acid concentration in the blood, which dissociates to release hydrogen ions, thus lowering pH and causing respiratory acidosis.

Question 46

Explain the pathophysiological mechanism behind diabetic ketoacidosis (DKA) and its effect on acid-base status.

Answer 46

DKA leads to the production of ketoacids due to the lack of insulin and increased fatty acid metabolism. The accumulation of these acids results in metabolic acidosis.

Question 47

Which acid-base disturbance is most commonly associated with severe asthma attacks?

Answer 47

Respiratory acidosis is commonly seen during severe asthma attacks due to hypercapnia from impaired ventilation.

Question 48

Describe the role of the kidneys in compensating for respiratory alkalosis.

Answer 48

In respiratory alkalosis, the kidneys compensate by decreasing bicarbonate reabsorption and increasing its excretion to reduce the buffering capacity and lower pH.