Blood gases Flashcards
What does blood gas analysis assess?
Acid-base balance and respiratory function (oxygenation and ventilation)
Abnormalities in these parameters are common in emergency and hospitalized patients.
What are the common parameters measured in blood gas analysis?
- pH
- CO2
- BE
- HCO3
- O2
- Electrolytes (Na, K, Ca, Cl)
- Metabolic disturbances (glucose, lactate, creatinine)
Other cartridges may also measure ACT, BUN among others.
What are the consequences of air exposure in blood gas samples?
It can lead to erroneous results, with lower PCO2 and increased PO2 readings
This occurs if the sample is left uncapped or collected into a tube before analysis.
What is the difference in pH and CO2 levels between arterial and venous blood samples?
Venous samples have a slightly lower pH and a higher PCO2
PvCO2 is typically around 5 mmHg higher than PaCO2.
Which artery is most commonly used for arterial blood sampling?
Dorsal metatarsal (DMT) artery
It is easily palpable and can be compressed after sampling.
What is the definition of PaO2?
Partial pressure of oxygen dissolved in arterial blood
It measures oxygenation, not ventilation.
What is considered hypoxaemia in terms of PaO2?
PaO2 < 80 mmHg
SpO2 < 95% indicates hypoxaemia.
What are the five main causes of hypoxaemia?
- Low inspired oxygen
- Hypoventilation
- V/Q mismatch
- Shunt (cardiac and non-cardiac causes)
- Diffusion impairment
These causes should be assessed to determine the underlying issue.
What is the A-a gradient?
The difference in partial pressure of oxygen between the alveoli and arterial blood
It assesses lung function by removing the effect of changes in minute ventilation.
How is A-a gradient calculated and what is normal
use the sea level equation
PAO2 = 150 - (1.1xpaCO2)
A-a = PAO2 - PaO2
Normal 5-15
>20 is pulmonary dysfunction
Hypoxaemia with normal A-a is hypoventilation
.
What does the 120 rule indicate?
The sum of PaO2 and PaCO2 should equal 120 mmHg in normal animals
- A sum < 120 mmHg suggests pulmonary dysfunction. whereas >120mmhg in a hypoaemic animal indicates hypoventilation
What does the PaO2:FiO2 ratio assess?
Lung function of patients on supplemental oxygen
- Normal is 500
- 300 - 500 is mild dysfuction
- 200 - 300 moderate dysfuction
- <200 is severe, ARDS
What is an advantage and disavantage of PaO2:FIO2
- The value can be calculated and compaired while the patient is on/off/different O2 supplementation to track progress/intervention effectiveness.
- It does not distinguish between lung function and hypoventilation
What is the definition of PaCO2?
Partial pressure of carbon dioxide dissolved in arterial blood
It measures the patient’s ability to ventilate.
What are some causes of hypoventilation?
- Airway obstruction
- Depression of respiratory centres
- Drugs (anesthetics, sedatives)
- Neurologic disease
- Neuromuscular disease
- Severe restrictive disease
- Severe pulmonary disease
Treatment should focus on correcting the underlying problem.
What is neuromuscular disease?
A condition affecting the nerves that control voluntary muscles.
What does severe restrictive disease prevent?
Lung expansion.
leading to hypoventilation eg pleural effusion
What is a late finding in severe pulmonary disease?
Hypoventilation.
Hypercapnia in a patient with lung disease, worsening,m may require emergency intubation as it indicates respiratory fatigue and decompensation
CO2 much more diffusable than O2 so if increased due to lung dx = BAD
What is the primary aim of treating hypoventilation?
Correcting the underlying problem.
What may be required in severe cases of hypoventilation?
Intubation and support with positive pressure ventilation.
What should be considered for patients requiring hypoventilation treatment?
Are any drugs reversable eg sedation and analgesia plan, can alternative drugs be provided if causing resp depression
What does hypercapnia indicate in patients with severe respiratory disease?
Respiratory fatigue and impending respiratory arrest.
What compensatory mechanism occurs in patients with severe respiratory disease?
Hyperventilation to compensate for hypoxemia.
(so if compensating would expect hypo rather than hypercapnia)
What are some causes of hyperventilation? (List at least three)
- Fear or anxiety
- Pain
- Hyperthermia
- Drug effect – corticosteroids
- Marked decrease in arterial blood oxygen content or delivery
- Severe anemia, hypoperfusion, hypoxemia
- Neurologic disease – centrally mediated
- Compensation for a metabolic acidosis.
What can the degree of hypocapnia indicate in hypoxemic patients?
How hard a patient may be working to achieve their level of oxygenation.
What are some advantages/utilities of running a blood gas?
- Only small volume of blood
- rapid results
-May help triage/point to the most critical patients - signs of acid/base disturbance are hard to detect on clinical exam
- acid/base/metabolic disturbances often no change on biochem
- to help guid and track success of treatment for metabolic and respiratory disorders
- machines are low maintanence
