NSG 170 Arterial Blood Gas (ABG) Interpretation

Comprehensive vocabulary flashcards covering arterial blood gas (ABG) components, reference ranges, physiology, the 4-step interpretation method, compensatory patterns, disorders, and associated nursing responsibilities from NSG 170.

Arterial Blood Gas (ABG)

A diagnostic blood specimen drawn from an artery that evaluates three primary clinical parameters: oxygenation, ventilation, and acid–base status.

Oxygenation (ABG Component)

The clinical assessment evaluated via $PaO_2$ and $SaO_2$ to determine if arterial oxygen content is adequate relative to delivered $FiO_2$.

Ventilation (ABG Component)

The clinical assessment evaluated via $PaCO_2$, reflecting the adequacy of alveolar gas exchange and carbon dioxide elimination by the lungs.

Acid–Base Status (ABG Component)

The clinical assessment evaluated via arterial $pH$ and bicarbonate ($HCO_3^-$) to identify systemic acidity, alkalinity, or buffering balance.

Carbonic Acid–Bicarbonate Buffer Equation

The physiological equilibrium reaction represented as $CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$, in which $CO_2$ behaves as an acid and $HCO_3^-$ behaves as a base.

Respiratory Regulation of Acid–Base Balance

The rapid compensatory mechanism by which the lungs alter $PaCO_2$ within minutes to hours through changes in the rate and depth of ventilation.

Renal Regulation of Acid–Base Balance

The slow compensatory mechanism by which the kidneys adjust systemic bicarbonate ($HCO_3^-$) concentration over approximately $2\text{ to }5\,\text{days}$.

Acidemia

A state defined by a measured arterial blood $pH < 7.35$.

Alkalemia

A state defined by a measured arterial blood $pH > 7.45$.

Acidosis

An underlying pathophysiological or physiological process that tends to lower arterial blood $pH$ by accumulating acid or losing base.

Alkalosis

An underlying pathophysiological or physiological process that tends to raise arterial blood $pH$ by accumulating base or losing acid.

Normal Adult Reference Range: pH

$$7.35\text{--}7.45$$

Normal Adult Reference Range: PaCO₂

$$35\text{--}45\,\text{mm}\,\text{Hg}$$

Normal Adult Reference Range: HCO₃⁻

$$22\text{--}26\,\text{mmol/L}$$

Normal Adult Reference Range: PaO₂ (Room Air)

$$80\text{--}100\,\text{mm}\,\text{Hg}$$

Normal Adult Reference Range: SaO₂

$$95\text{--}100\%$$

Normal Adult Reference Range: Base Excess

$$-2\text{ to }+2\,\text{mmol/L}$$

Normal Adult Reference Range: Lactate

$$\le 2\,\text{mmol/L}$$

Four-Step ABG Method

A systematic diagnostic framework consisting of: Step $1$: Assess oxygenation ($PaO_2$, $SaO_2$); Step $2$: Look at $pH$; Step $3$: Identify the primary driver; Step $4$: Assess compensation.

Hypoxemia

An abnormally low arterial oxygen tension defined by a $PaO_2 < 80\,\text{mm}\,\text{Hg}$ on room air.

Primary Driver

The component ($PaCO_2$ for respiratory or $HCO_3^-$ for metabolic) whose directional change corresponds to and explains the direction of the measured $pH$.

ROME Mnemonic

A clinical memory aid meaning Respiratory Opposite, Metabolic Equal; indicates that in respiratory disorders $pH$ and $PaCO_2$ move in opposite directions, whereas in metabolic disorders $pH$ and $HCO_3^-$ move in the same direction.

Uncompensated Disorder

An acid–base abnormality in which the primary driver and $pH$ are abnormal, while the opposing buffer system parameter remains within normal reference limits.

Partially Compensated Disorder

An acid–base abnormality in which the opposing organ system has responded (both $PaCO_2$ and $HCO_3^-$ are abnormal), but arterial $pH$ remains outside the normal range ($< 7.35$ or $> 7.45$).

Fully Compensated Disorder

An acid–base abnormality where both $PaCO_2$ and $HCO_3^-$ are abnormal, and physiological compensation has returned the arterial $pH$ to the normal range ($7.35\text{--}7.45$).

Compensation Rule of Overcorrection

The physiological principle stating that secondary compensatory mechanisms move the blood $pH$ toward normal but never overcorrect past normal reference limits.

Mixed Acid–Base Disorder

The simultaneous presence of two or more independent acid–base disturbances, suspected when secondary compensation moves in an unexpected direction or magnitude, or when an abnormal $PaCO_2$ and $HCO_3^-$ coexist with a normal $pH$.

Respiratory Acidosis

An acid–base disorder defined by $\uparrow PaCO_2$ and $\downarrow pH$ ($< 7.35$), caused by hypoventilation conditions such as opioid/sedative overdose, COPD exacerbation, atelectasis, neuromuscular weakness, or obesity hypoventilation.

Respiratory Alkalosis

An acid–base disorder defined by $\downarrow PaCO_2$ and $\uparrow pH$ ($> 7.45$), caused by hyperventilation conditions including anxiety/pain, fever, sepsis, pulmonary embolism, hypoxemia, or mechanical over-ventilation.

Metabolic Acidosis

An acid–base disorder defined by $\downarrow HCO_3^-$ and $\downarrow pH$ ($< 7.35$), caused by excess organic acid or bicarbonate loss such as DKA, lactic acidosis/shock, renal failure, severe diarrhea, or salicylate toxicity.

Metabolic Alkalosis

An acid–base disorder defined by $\uparrow HCO_3^-$ and $\uparrow pH$ ($> 7.45$), caused by acid loss or base excess such as vomiting, nasogastric (NG) suction, diuretic therapy, or excess alkali ingestion.

Kussmaul Respirations

Deep, sighing, rapid compensatory respirations seen clinically in metabolic acidosis (such as DKA) to eliminate carbon dioxide and mitigate systemic acidemia.

Pulse Oximetry ($SpO_2$) Clinical Limitations

Noninvasive monitoring that assesses arterial oxygen saturation without providing data on $PaCO_2$ or $pH$, with reduced reliability during shock, nail clubbing, presence of nail polish/artificial nails, and in certain skin tones.

ABG Nursing Priority Sequence

The five-step clinical action continuum for managing ABG abnormalities: Recognize $\rightarrow$ Assess $\rightarrow$ Intervene/Escalate $\rightarrow$ Reassess $\rightarrow$ Communicate.

Arterial Specimen Documentation

The nursing requirement to document specific oxygen therapy details at the exact time of ABG puncture, including the delivery device and delivered $FiO_2$.

Post-Arterial Puncture Safety Monitoring

The immediate nursing action of applying sustained pressure to the arterial puncture site per agency policy, monitoring for hematoma or bleeding, and evaluating distal neurovascular circulation.