The document provides an overview of blood gas analysis and interpretation. It discusses oxygen transport and acid-base balance regulation in the body. Normal blood gas values are outlined as well as the causes and features of respiratory and metabolic acidosis and alkalosis. The compensation mechanisms between the respiratory and metabolic systems are described. Methods for classifying blood gas results and interpreting them based on the patient's condition are also covered. Sample blood gas results are presented and analyzed.

1. Respiratory Care
Blood Gas Theory
& the Art
of ABG Interpretation
Saleh Al -Qarni

2. Introduction to Blood Gases
 O2 Transport:
 Majority of O2 transported by hemoglobin (O2Hb)
 Minor amount dissolved in the plasma
 Ct = (gmHb x SxO2 x 1.39) + (PxO2 x .003)
 CO2 Transport:
 Majority of CO2 transported as HCO3
- (90%)
 5% dissolved in the plasma
 Dissolved gases exert a measurable pressure
 Thus, Blood “Gas” measurement
 ie. PaO2: Partial Pressure of Arterial Oxygen

3. Acid-Base Balance
 Human body operates in narrow pH range
 Normal Range: 7.35 – 7.45
 < 7.40: Acidotic >7.40 Alkalotic
 Balance dependent on reduction of CO2 in the blood
 2 Systems to regulate this balance
 Respiratory: CO2 regulated by the lungs via ventilation
 Metabolic: CO2 regulated by the kidneys via excretion in urine
 One system balanced by the other
 ABGs show the status of this balance
 Can determine system imbalances via ABG results

4. Normal Values
 pH: 7.35 – 7.45
 7.20-7.50 (high survival probability)
 PaCO2: 35-45 mmHg
 PaO2: 80-100 mmHg
 decreases with age approx 102-.33(age)
 HCO3
-: 22-26 mEq/L BE: -2 to +2
 SaO2: 95-98% SvO2: 75% +/-
 Hb: 15 g/L CaO2: 19.8 vol%

5. Respiratory Acidosis
 Feature: Abnormally high CO2
 Cause(s): Hypoventilation
 CNS Depression: Drug overdose, anaesthesia
 Neuromuscular: Drugs, botulism, Guilliane-Barre syndrome, etc
 Intrathoracic: Pneumothorax, hemothorax
 Pulmonary: Obstruction, asthma, COPD, pneumonia, secretions
 Obesity, Pregnancy, Hypoxia (most common)
 Compensation: Kidneys conserve HCO3
-
 A slow process

6. Respiratory Alkalosis
 Feature: Abnormally low CO2
 Cause(s): Hyperventilation
 Drug overdose: ie Aspirin
 Anxiety, Pain, Fear
 Central: CVA
 Infection: Increased metabolic rate
 Compensation: Kidneys excrete HCO3
-
 May occur more quickly than conservation of HCO3
-
 Interim?: Deadspace added to breathing circuit to incr. CO2

7. Metabolic Acidosis
 Feature: Abnormally low HCO3-
 Cause(s):
 Diarrhea (lower GI), starvation (muscle breakdown)
 Cardiac arrest (lactic acidosis)
 Liver failure
 Ingestion of certain substances: ie Methanol, Ethylene glycol, late-
stage ASA overdose.
 Compensation: Hyperventilation
 Very rapid compensation, but self-limiting with patient ability

8. Metabolic Alkalosis
 Feature: Abnormally high HCO3
-
 Cause(s): Conservation of HCO3
- by kidney
 Vomiting / NG Suctioning: (upper GI), loss of acid from stomach
 Poor diet, ingestion of a base
 Hypochloremia (Cl-), Hypokalemia (K+), Hypernatremia (Na+)
 Steroid Therapy
 Massive blood replacement
 Diuresis (excretion of electrolytes, ie via Lasix)
 Compensation: Hypoventilation
 Ventilation decreases in proportion to HCO3
- conservation

9. Compensation: One Follows The Other
Condition Cause Cause To Compensate
CO2 HCO3
- CO2 HCO3
-
Resp. Acidosis  
Resp. Alkalosis  
Met. Acidosis  
Met. Alkalosis  

10. Hypoxia: A Lack of Oxygen
 4 Types of Hypoxia:
 Anemic: Decrease in amount of functional Hb
 Circulatory (Stagnant): Decreased circulation of blood
 Histotoxic: Poisoning ie Cyanide
 Hypoxic:
 V/Q Mismatch: most common
 Shunt: Blood flow past non-ventilated alveoli
 Diffusion Defect: ie Fibrosis of the lung
 Ambient: ie High altitude
 Hypoventilation

11. Oxygen Cascade
 Flow requires a pressure gradient
 A natural pressure gradient occurs in the body
 Oxygen is transported along this gradient
 Oxygen Cascade: PO2
150 (ambient)  110 (lungs)  99 (arterial) 
40 (capillary)  3-22 (mitochondria)
 Disruption of any one component affects all values
downstream

12. Blood Gas Classification
 Must be methodical
 Establish: Arterial Blood?
 Based on PO2 and PCO2 values
 Establish: Hypoxemia or adequately oxygenated?
 Establish: Acidotic or Alkalotic
 Establish: Metabolic or Respiratory in origin
 Combined?
 Establish: Acute or Chronic
 Based on degree of compensation
 NOTE: The body rarely OVERcompensates

13. Blood Gas Interpretation
 Contrast: Classification vs Patient History
 Does Classification match history?
 Results vs patient presentation at time of ABG
 Classic Misinterpretations:
 “Normal” ABG from asthmatic still clearly distressed
 Patient is tiring. CO2 is rising. A good time to intubate.
 Manually-ventilated COPD patient
 7.56 46 310 42 +16
 Classification: Acute Metabolic Alkalosis
 Interpretation: Induced Respiratory Alkalosis

14. Blood Gas Analysis
 Question samples when:
 Air bubbles present in sample
 Clots present in sample
 Values incompatible with stated variables
 ie PaO2 260 on 1 liter N/C
 Arterial vs Venous source cannot be determined.
 Can still use pH values
 Analyzers: Frequent QC

15. Sample ABGs
 Format: pH PCO2 PO2 HCO3
- BE(optional)
 #1: 7.47 38 94 27.5
 #2: 7.34 38 143 19.6
 #3: 7.38 30 115 17.7
 #4: 7.12 97 57 29.9
 #5: 7.30 29 62 23.1
 #6: 7.08 30 75 8.3 -20.6

16. Answers:
 #1: Mild Met. Alkalosis, adequate oxygenation
 #2: Mild Met. Acidosis, oxygenation fine
 #3: Fully-compensated Met. Acidosis, O2 fine
 #4: Acute Resp. Acidosis (minor compensation)
with severe hypoxemia
 #5: Sample error: Values do not jive (Air bubble?)
 #6: Acute Metabolic Acidosis (mild compensation) with
moderate hypoxemia
 Worry about this patient arresting in the near future