ABG Interpretation: Metabolic Components, Respiratory Disturbances, and the Anion Gap

Metabolic and Respiratory Acid-Base Factors: Bicarbonate and Base Excess

• Bicarbonate ($HCO_3^-$)

  • Bicarbonate is the primary indicator of the metabolic system or the kidneys' role in acid-base balance.

  • Units: Measured in milliequivalents per liter ($mEq/L$).

  • Normal Range: The standard normal range is $22$ to $26 \text{ mEq/L}$.

  • Mean Value: Alternatively expressed as $24 \text{ mEq/L} \times / - 2$.

• Base Excess (BE)

  • Definition: Base excess is an evaluation of all the base in the system, representing the total sum of all bases, not just bicarbonate.

  • Relationship with Bicarbonate: Base excess and bicarbonate typically move in a parallel direction (they increase or decrease together).

  • Units: Measured in milliequivalents per liter ($mEq/L$).

  • Normal Range: The normal range is $-2$ to $+2 \text{ mEq/L}$ (or $\text{plus or minus } 2$).

  • Clinical Utility in Arterial Blood Gas (ABG) Interpretation:

  • In routine ABG interpretation, Base Excess is often not the primary focus.

  • It serves as a technical clue when a blood gas profile looks "wonky"—specifically in metabolic situations where an excess or deficit of total base is present.

  • It can explain a $PH$ outside of the normal range when both $PaCO_2$ and $HCO_3^-$ appear to be within (or just on the edge of) normal limits.

Classification of Respiratory Acid-Base Disturbances

• Respiratory Acidosis

  • Occurs due to hypoventilation, leading to hypercapnia (increased $CO_2$ levels).

  • Sub-classifications:

  • Acute Ventilatory Failure: A sudden failure to ventilate, causing a rapid drop in $PH$ and an increase in $CO_2$.

  • Acute Ventilatory Failure with Partial Renal Compensation: Occurs when the kidneys begin to retain bicarbonate to offset the acidity, typically beginning within $24$ to $48$ hours.

  • Chronic Ventilatory Failure with Complete Renal Compensation: Observed in patients with chronic conditions like severe emphysema. These patients experience gas trapping and physiologically adapt to living at a higher $PaCO_2$ level than non-diseased individuals.

• Respiratory Alkalosis

  • Occurs due to alveolar hyperventilation, leading to hypocapnia (decreased $CO_2$ levels).

  • Sub-classifications:

  • Acute Alveolar Hyperventilation: Sudden over-breathing causing an increase in $PH$ and a decrease in $CO_2$.

  • Acute Alveolar Hyperventilation with Partial Renal Compensation: The kidneys start to excrete bicarbonate to lower the $PH$ back toward normal.

  • Chronic Alveolar Hyperventilation: Long-term hyperventilation with full or partial renal adjustment.

Classification of Metabolic and Mixed Disturbances

• Metabolic Acidosis or Alkalosis

  • These conditions can occur with Partial or Complete Respiratory Compensation, where the lungs adjust the rate of breathing to normalize $PH$.

• Mixed Acid-Base Disturbances

  • A patient may experience a mixed alkalosis or mixed acidosis, where both respiratory and metabolic factors contribute to the deviation in $PH$.

Real-World Examples of Ventilatory Failure (Respiratory Acidosis)

• COPD Exacerbation: Often presents as "acute on chronic" respiratory acidosis.

• Drug Overdose: Specifically narcotics or drugs that cause sedation and suppress the respiratory drive.

  • Examples: Oxycodone, Fentanyl, Barbiturates (e.g., Phenobarbital).

• General Anesthesia: Anesthetic drugs can cause a cessation of breathing, necessitating airway intervention.

  • Propofol: Commonly referred to in hospitals as the "Milk of Amnesia."

  • Ketamine: Another example of an anesthetic that impacts ventilation.

• Head Trauma: Impact varies depending on the specific area of the brain injured.

• Neurologic Disorders:

  • Spinal Cord Injuries: Specifically high-level injuries that disrupt the neural drive to breathe.

  • Guillain-Barré Syndrome (GBS): Described as "Ground to the Brain" (G to B), where musculoskeletal impacts start at the feet and move upward. If it reaches the diaphragm, it inhibits the muscle's ability to contract and lower, resulting in ventilatory failure.

  • Myasthenia Gravis (MG): Described as "Mind to the Ground" (M to G), working in the opposite direction but also potentially impacting the diaphragm.

Real-World Examples of Hyperventilation (Respiratory Alkalosis)

• Hypoxia and Hypoxemia

  • Hypoxemia: A reduction of oxygen levels in the blood.

  • Hypoxia: Oxygen starvation at the tissue level.

  • Mechanism: When tissues are starved of oxygen, they signal the body to increase breathing, which "blows off" $CO_2$ and leads to respiratory alkalosis.

• Lung Diseases (Impact on Fick’s Law):

  • Pneumonia, Pleural Effusions, Atelectasis, and Pulmonary Fibrosis.

• Other Causes:

  • High Altitude: Low atmospheric pressure leads to reduced oxygen intake.

  • Heart Disease: Reduced contractility leads to lower cardiac output, which direct results in poor oxygen delivery to tissues.

  • Psychological/Physiological Stress: Pain, anxiety, and fever.

  • Traumatic Brain Injury (TBI).

  • Stimulants: Drugs like amphetamines (e.g., ADHD medications) which can cause patients to over-ventilate and feel palpitations.

Metabolic Acidosis: Types and Etiologies

• Lactic Acidosis

  • A product of anaerobic metabolism.

  • Occurs when the oxygen demands of the tissues are not met, causing the body to switch from aerobic to anaerobic glucose metabolism.

  • Lactic acid is the resulting byproduct.

• Ketoacidosis

  • Occurs when blood insulin levels are low; glucose cannot enter the cells to be metabolized.

  • The body produces ketones for energy, which accumulate in the blood.

• Salicylate Intoxication

  • Caused by an aspirin overdose, leading to the accumulation of salicylic acids in the system.

• Renal Failure

  • The kidneys fail to maintain the balance of bicarbonate and hydrogen ions.

  • Characterized by a decrease in $HCO_3^-$ and an increase in $H^+$ ions.

• Uncontrolled Diarrhea

  • Leads to a significant loss of base ($HCO_3^-$) from the digestive tract, resulting in metabolic acidosis.

The Anion Gap

• Definition and Purpose

  • The anion gap is used to determine if metabolic acidosis is caused by the accumulation of fixed acids or the excessive loss of bicarbonate ($HCO_3^-$).

  • This calculation assists physicians in determining the appropriate treatment.

• Law of Electroneutrality

  • The total number of positive ions (cations) must equal the total number of negative ions (anions) in body fluids.

• Key Electrolytes Involved:

  • Cation: Sodium ($Na^+$).

  • Anions: Chloride ($Cl^-$) and Bicarbonate ($HCO_3^-$).

• Anion Gap Formula:

  • $Anion Gap = [Na^+] - ([Cl^-] + [HCO_3^-])$

  • Normal Range: $9$ to $14 \text{ mEq/L}$.

• Interpretation of Results:

  • Gap > 14 mEq/L: Generally indicates an accumulation of fixed acids in the blood (e.g., lactic acid, ketones, or salicylic acid). Fixed acids are non-volatile acids that do not dissociate easily.

  • Normal Gap (within 9-14 mEq/L) in Acidosis: Associated with conditions involving the loss of bicarbonate (e.g., excessive diarrhea).

• Sources for Data: Bicarbonate values can be obtained from an ABG, while sodium and chloride are gathered from an electrolyte/serum blood panel.

Questions & Discussion

Is Base Excess relevant to what we are doing right now? It is more technical than the current focus. It is important to note because you may see cases where $PH$ is abnormal but $PaCO_2$ and $HCO_3^-$ appear normal; Base Excess explains that difference.

In acute ventilatory failure, why is it acidosis? Ventilatory failure implies hypoventilation (breathing less than required). This leads to a buildup of $CO_2$ (acid) in the system, which shifts the $PH$ down, creating an acidosis.

Does hypoxia start at the tissue level for hyperventilation? Not exactly. Hypoxemia (low blood oxygen) often precedes tissue hypoxia, but it is the tissue's starvation that signals the increased respiratory drive to blow off $CO_2$.

Why do stimulants cause hyperventilation and palpitations? Stimulants like amphetamines raise the body's temperature and "rev" the system up. In medical settings, if a patient has stable tachycardia, we might have them "bear down" to try and break the rhythm.

What is the definition of a fixed acid? They are non-volatile acids, such as ketones or lactic acid, that accumulate in the blood and do not dissociate like volatile acids (e.g., carbonic acid).

Calculation Example for Anion Gap:

• Given Labs:

  • Sodium ($Na^+$): $140 \text{ mEq/L}$

  • Chloride ($Cl^-$): $110 \text{ mEq/L}$

  • Bicarbonate ($HCO_3^-$): $20 \text{ mEq/L}$

• Calculation:

  • $140 - (110 + 20) = 140 - 130 = 10 \text{ mEq/L}$

• Result: The anion gap is $10$, which falls within the normal range of $9$ to $14$.