877 ABG

ABG stands for what and what does it measure?

Arterial Blood Gas; measures pH, PaO₂, PaCO₂, SaO₂, and HCO₃⁻

What are the most common arterial sites used for ABG collection?

Radial, brachial, axillary, and femoral arteries

Why are ABGs considered invasive?

Blood is obtained directly from an artery

What are common risks of ABG collection?

Bleeding, infection, pain, and nerve injury

When are ABGs commonly drawn after intubation?

Within 30 minutes

Why are ABGs frequently obtained in ICU patients?

To monitor ventilation, oxygenation, acid-base status, and response to interventions

Which patient population commonly requires serial ABGs due to CO₂ retention?

COPD patients

What PT considerations should be assessed after a radial artery ABG draw?

Time since draw, dressing integrity, bleeding, pain, numbness/tingling, sensation, and hand strength

Why should a PT be cautious with walker use after a recent radial artery ABG draw?

Weight-bearing may irritate the puncture site and increase pain

Why is trending ABG values often more important than a single value?

Trends indicate whether the patient is improving or deteriorating

What FiO₂ corresponds to room air?

21% or 0.21

What does FiO₂ represent?

Fraction of inspired oxygen

Normal PaO₂ value?

80–100 mmHg

Normal PAO₂ value?

~100 mmHg

Normal mixed venous oxygen tension (PvO₂)?

~40 mmHg

What is PaO₂?

Partial pressure of oxygen in arterial blood

What is PAO₂?

Partial pressure of oxygen in alveoli

What is SpO₂?

Peripheral oxygen saturation measured by pulse oximetry

What is SaO₂?

Arterial oxyhemoglobin saturation

What is hypoxia?

Low oxygen in tissues

What is hypoxemia?

Low oxygen in blood

What PaO₂ defines hypoxemia?

PaO₂ <80 mmHg

What PaO₂ range indicates mild hypoxemia?

60–80 mmHg

What PaO₂ range indicates moderate hypoxemia?

40–60 mmHg

What PaO₂ range indicates severe hypoxemia?

What are classic signs of hypoxemia?

Cyanosis, tachypnea, tachycardia, diaphoresis, confusion, dizziness, headache

Why might a patient become confused during ambulation if hypoxemia develops?

Reduced oxygen delivery to the brain impairs cognition

A patient becomes pale, diaphoretic, confused, and tachypneic during gait training. What should the PT suspect first?

Acute hypoxemia

What are the five major causes of hypoxemia?

Low inspired oxygen, hypoventilation, shunt, diffusion defect, and V/Q mismatch

A patient at high altitude develops hypoxemia. Which mechanism is most likely responsible?

Low inspired oxygen

A patient with pneumonia develops hypoxemia. Which mechanism is most likely responsible?

Shunt and/or diffusion impairment

A patient with COPD develops hypoxemia. Which mechanism is most likely responsible?

V/Q mismatch

What is the A-a gradient?

Difference between alveolar and arterial oxygen pressures

Formula for A-a gradient?

PAO₂ − PaO₂

Estimated normal A-a gradient?

(Age + 10) ÷ 4

What does an elevated A-a gradient suggest?

Lung pathology impairing oxygen transfer

What conditions commonly elevate the A-a gradient?

V/Q mismatch, shunt, diffusion defects, pneumonia, CHF, PE

What does a normal A-a gradient with hypoxemia suggest?

High altitude or hypoventilation

What percentage of oxygen is transported bound to hemoglobin?

~98%

What percentage of oxygen is dissolved in plasma?

~2%

What is oxyhemoglobin?

Hemoglobin bound to oxygen

What is deoxyhemoglobin?

Hemoglobin that has released oxygen

What is methemoglobin?

Oxidized hemoglobin that cannot carry oxygen

What is carboxyhemoglobin?

Hemoglobin bound to carbon monoxide

How much stronger does carbon monoxide bind hemoglobin than oxygen?

~210 times stronger

Why is carbon monoxide poisoning dangerous even with normal lungs?

Hemoglobin cannot effectively carry oxygen

What does a rightward shift of the oxygen dissociation curve indicate?

Increased oxygen unloading to tissues

Memory aid for right shift?

RIGHT = RELEASE

Causes of a right shift?

↑H⁺, ↑CO₂, ↑temperature, exercise, ↑altitude, ↑2,3-DPG

Why does exercise cause a right shift?

Working tissues require increased oxygen delivery

What does a leftward shift indicate?

Hemoglobin holds oxygen more tightly

Memory aid for left shift?

LEFT = tissues LEFT behind

Causes of a left shift?

↓H⁺, ↓CO₂, ↓temperature, ↓altitude, ↓2,3-DPG

Why does hypothermia cause a left shift?

The body centralizes blood flow and conserves oxygen

A hypothermic patient develops dark fingertips. Explain physiologically.

Left shift reduces peripheral oxygen delivery as blood is centralized

A runner’s SpO₂ drops slightly during a hard workout. What physiologic adaptation should occur?

Rightward shift to unload more oxygen to tissues

Normal blood pH range?

7.35–7.45

Average blood pH?

7.40

Define acidemia.

pH <7.35

Define alkalemia.

pH >7.45

What is the relationship between H⁺ concentration and pH?

Inverse relationship; ↑H⁺ = ↓pH

Why is pH considered logarithmic?

Small numerical changes reflect large physiologic changes

What is an acid?

Substance that donates H⁺

What is a base?

Substance that accepts H⁺

Major physiologic sources of acid production?

Anaerobic metabolism, CO₂ production, fatty acid oxidation, ketone production, phospholipid metabolism

Which buffer system is most important in the body?

Carbonic acid-bicarbonate buffer system

Major equation of acid-base regulation?

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

Which organ primarily regulates CO₂?

Lungs

Which organ primarily regulates HCO₃⁻?

Kidneys

What happens to the buffer equation during acidosis?

Shifts left to consume H⁺

What happens to the buffer equation during alkalosis?

Shifts right to generate H⁺

What does the suffix “-emia” mean?

State of the blood

What does the suffix “-osis” mean?

Pathologic process

Difference between acidemia and acidosis?

Acidemia is low blood pH; acidosis is the process causing it

Difference between alkalemia and alkalosis?

Alkalemia is high blood pH; alkalosis is the process causing it

What primarily determines a metabolic disorder?

HCO₃⁻ abnormality

What primarily determines a respiratory disorder?

PaCO₂ abnormality

What is compensation?

Physiologic attempt to normalize pH

Can the body overcompensate?

No

ABG: pH 7.28, HCO₃⁻ 15. Primary disturbance?

Metabolic acidosis

ABG: pH 7.52, HCO₃⁻ 34. Primary disturbance?

Metabolic alkalosis

ABG: pH 7.28, PaCO₂ 60. Primary disturbance?

Respiratory acidosis

ABG: pH 7.52, PaCO₂ 28. Primary disturbance?

Respiratory alkalosis

What are common causes of metabolic acidosis?

DKA, renal failure, diarrhea, lactic acidosis, liver failure

Why does DKA cause metabolic acidosis?

Ketone accumulation increases acid load

What classic breath odor is associated with DKA?

Fruity or sweet breath

A patient has glucose of 900 mg/dL and fruity breath. What acid-base disorder should you suspect?

Metabolic acidosis from DKA

Symptoms of metabolic acidosis?

Tachycardia, confusion, fatigue, headache, nausea, vomiting, deep rapid breathing

What are common causes of metabolic alkalosis?

Vomiting, dehydration, medications, kidney/heart/liver failure

Why does prolonged vomiting cause metabolic alkalosis?

Loss of gastric acid

Symptoms of metabolic alkalosis?

Paresthesias, muscle cramps, confusion, agitation, seizures

What is the primary cause of respiratory acidosis?

Hypoventilation

Common causes of respiratory acidosis?

COPD, CNS depressants, ALS, asthma, pneumonia, pulmonary edema

Why does hypoventilation produce respiratory acidosis?

CO₂ retention increases carbonic acid

Symptoms of respiratory acidosis?

Dyspnea, wheezing, headache, confusion, cyanosis, fatigue

A COPD patient becomes increasingly confused and lethargic. What acid-base disorder should be suspected?

Respiratory acidosis due to CO₂ retention

What is the primary cause of respiratory alkalosis?

Hyperventilation

Common causes of respiratory alkalosis?

Anxiety, fever, pain, sepsis, hypoxia, PE, hyperthyroidism

Why does hyperventilation produce respiratory alkalosis?

Excessive CO₂ elimination

Symptoms of respiratory alkalosis?

Dizziness, paresthesias, chest tightness, lightheadedness