Blood Gas Evaluation
Posted: Tuesday, May 19th, 2020 | Updated: Thursday, August 25th, 2022
Posted: Tuesday, May 19th, 2020 | Updated: Thursday, August 25th, 2022
Acidosis
pH is decreased (less than 7.35)
Alkalosis
pH is increased (greater than 7.45)
Respiratory Acidosis
pH is decreased
pCO2 is increased
Possible causes for respiratory acidosis include: neurologic disease, drugs leading to hypoventilation, pleural effusion, pneumothorax, thoracic related trauma, diaphragmatic hernias, brachycephalic syndrome, laryngeal paralysis, tracheal or bronchial collapse, and/or occlusion of an endotracheal tube
Respiratory Alkalosis
pH is increased
pCO2 is decreased
Possible causes for respiratory alkalosis include: hyperventilation leading to hypocapnia, shock, sepsis, fever, and/or hypoxemia
Metabolic Acidosis
pH is decreased
BE and/or HCO3- is decreased
Possible causes of metabolic acidosis include: GI loss of bicarbonate, ingestion of ethylene glycol or medications containing aspirin, the inability to excrete acids as with Addison's disease or kidney disease, and patients with DKA, lactic acidosis, and/or uremic acidosis
Metabolic Alkalosis
pH is increased
BE and/or HCO3- is increased
Possible causes for metabolic alkalosis include: vomiting, diarrhea, GI obstruction, loss via the urine, bicarbonate administration, and/or the metabolism of citrate
Venous Blood
Used to assess ventilation and acid/base status
Arterial Blood
Used to assess oxygenation, ventilation, and acid/base status
PaO2
Normal 90 to 100 mmHg (interpreted from arterial blood samples). Healthy patients will have a PaO2 of 95 mmHg or greater.
If the patient is receiving supplemental oxygen the PaO2 will equal about 5 times the FiO2
Normal PaO2:FiO2 ratio is 5:1
PaO2 is typically 100 mmHg
FiO2 is typically 21% (room air)
Example: If the patient is on 100% oxygen (FiO2), the expected PaO2 would be 500 mmHg
Hypoxemia if below 80 mmHg
Life threatening hypoxemia if below 60 mmHg (Mechanical ventilation should be considered for this level of hypoxemia)
Cyanosis is seen with levels at or below 40 to 50 mmHg
PvO2
For a reference. The normal PvO2 is approximately 40 mmHg to 55 mmHg in dogs and closer to 100 mmHg in cats
pH is decreased (less than 7.35)
Alkalosis
pH is increased (greater than 7.45)
Respiratory Acidosis
pH is decreased
pCO2 is increased
Possible causes for respiratory acidosis include: neurologic disease, drugs leading to hypoventilation, pleural effusion, pneumothorax, thoracic related trauma, diaphragmatic hernias, brachycephalic syndrome, laryngeal paralysis, tracheal or bronchial collapse, and/or occlusion of an endotracheal tube
Respiratory Alkalosis
pH is increased
pCO2 is decreased
Possible causes for respiratory alkalosis include: hyperventilation leading to hypocapnia, shock, sepsis, fever, and/or hypoxemia
Metabolic Acidosis
pH is decreased
BE and/or HCO3- is decreased
Possible causes of metabolic acidosis include: GI loss of bicarbonate, ingestion of ethylene glycol or medications containing aspirin, the inability to excrete acids as with Addison's disease or kidney disease, and patients with DKA, lactic acidosis, and/or uremic acidosis
Metabolic Alkalosis
pH is increased
BE and/or HCO3- is increased
Possible causes for metabolic alkalosis include: vomiting, diarrhea, GI obstruction, loss via the urine, bicarbonate administration, and/or the metabolism of citrate
Venous Blood
Used to assess ventilation and acid/base status
Arterial Blood
Used to assess oxygenation, ventilation, and acid/base status
PaO2
Normal 90 to 100 mmHg (interpreted from arterial blood samples). Healthy patients will have a PaO2 of 95 mmHg or greater.
If the patient is receiving supplemental oxygen the PaO2 will equal about 5 times the FiO2
Normal PaO2:FiO2 ratio is 5:1
PaO2 is typically 100 mmHg
FiO2 is typically 21% (room air)
Example: If the patient is on 100% oxygen (FiO2), the expected PaO2 would be 500 mmHg
Hypoxemia if below 80 mmHg
Life threatening hypoxemia if below 60 mmHg (Mechanical ventilation should be considered for this level of hypoxemia)
Cyanosis is seen with levels at or below 40 to 50 mmHg
PvO2
For a reference. The normal PvO2 is approximately 40 mmHg to 55 mmHg in dogs and closer to 100 mmHg in cats
SaO2
When evaluating blood gases, the SaO2 or oxygen saturation of arterial blood samples should be greater than 90% in most patients. This correlates to having a PaO2 of greater than 60 mmHg. Some patients who are severely hypoxemic may have values as low as 80%
When evaluating blood gases, the SaO2 or oxygen saturation of arterial blood samples should be greater than 90% in most patients. This correlates to having a PaO2 of greater than 60 mmHg. Some patients who are severely hypoxemic may have values as low as 80%
PaCO2
When levels trend near or above 60 mmHg, also consider mechanical ventilation
When levels trend near or above 60 mmHg, also consider mechanical ventilation
Summary
Measuring blood gases allows us to gain knowledge about our patient's oxygenation, ventilation, and acid-base status. Venous blood gas samples, if requested, are best obtained from a central line or a jugular vein. Arterial samples are mostly obtained from the dorsal pedal artery or femoral artery. Normal pH values are generally between 7.35 and 7.45 while bicarbonate values hover anywhere from 18 to 26 mEq/L in dogs and 17 to 23 mEq/L in cats. pCO2 values range anywhere from 32 to 44 mmHg in dogs and up to 50 mmHg in venous blood samples. In cats, pCO2 values typically range from 28 to 32 mmHg and up to 45 mmHg in venous blood samples. pH values greater than 7.45 indicate alkalosis and values less than 7.35 indicate acidosis. High bicarbonate levels trend toward alkalosis and low bicarbonate levels trend toward acidosis. High pCO2 levels trend toward acidosis and low CO2 levels trend toward alkalosis
It's imperative that acidemia be treated when the pH is less than 7.20, bicarbonate is less than 12 mEq/L, and/or base deficit is less than -10 mEq/L. Values causing acidemia may have a negative impact on normal cellular protein structure and function
Acidosis and alkalosis may be compensated or uncompensated and metabolic or respiratory in origin
Measuring blood gases allows us to gain knowledge about our patient's oxygenation, ventilation, and acid-base status. Venous blood gas samples, if requested, are best obtained from a central line or a jugular vein. Arterial samples are mostly obtained from the dorsal pedal artery or femoral artery. Normal pH values are generally between 7.35 and 7.45 while bicarbonate values hover anywhere from 18 to 26 mEq/L in dogs and 17 to 23 mEq/L in cats. pCO2 values range anywhere from 32 to 44 mmHg in dogs and up to 50 mmHg in venous blood samples. In cats, pCO2 values typically range from 28 to 32 mmHg and up to 45 mmHg in venous blood samples. pH values greater than 7.45 indicate alkalosis and values less than 7.35 indicate acidosis. High bicarbonate levels trend toward alkalosis and low bicarbonate levels trend toward acidosis. High pCO2 levels trend toward acidosis and low CO2 levels trend toward alkalosis
It's imperative that acidemia be treated when the pH is less than 7.20, bicarbonate is less than 12 mEq/L, and/or base deficit is less than -10 mEq/L. Values causing acidemia may have a negative impact on normal cellular protein structure and function
Acidosis and alkalosis may be compensated or uncompensated and metabolic or respiratory in origin
Sources:
Ford, Richard B., and Elisa M. Mazzaferro. Kirk and Bistner's Handbook of Veterinary Procedures and Emergency Treatment. 8th ed., Saunders Elsevier, 2006.
Sidari, Heather Ann. "The Veterinary Nurse's Role in Reading Blood Gases." Today's Veterinary Nurse. Summer 2021.
Waddell, Lori S. "The Practioner's Acid-Base Primer: Obtaining & Interpreting Blood Gases" Today's Veterinary Practice. May/June 2013. www.todaysveterinarypractice.com
Ford, Richard B., and Elisa M. Mazzaferro. Kirk and Bistner's Handbook of Veterinary Procedures and Emergency Treatment. 8th ed., Saunders Elsevier, 2006.
Sidari, Heather Ann. "The Veterinary Nurse's Role in Reading Blood Gases." Today's Veterinary Nurse. Summer 2021.
Waddell, Lori S. "The Practioner's Acid-Base Primer: Obtaining & Interpreting Blood Gases" Today's Veterinary Practice. May/June 2013. www.todaysveterinarypractice.com