Arterial Blood Gases and Acid Base Regulation Flashcards
What is in an arterial blood gas measurement?
Why is it used?
An arterial blood gas (ABG) test measures the acidity (pH) and the levels of oxygen, carbon dioxide and plasma bicarbonate in the blood from an artery - PO2, PCO2, HCO3- and pH
This test is used to check how well your lungs are able to move oxygen into the blood and remove carbon dioxide from the blood
What is PO2?
Partial pressure of oxygen
Indicates how much oxygen is dissolved in the arterial blood, and if it is particularly low it can suggest inadequate gas exchange in the lungs
What is PCO2?
Partial pressure of carbon dioxide
Indicates how much CO2 is dissolved in arterial blood, and if it is particularly high it can suggest inadequate gas exchange in the lungs
What is pH?
Concentration of hydrogen
Describes the acidity, neutrality or alkalinity of the blood
The pH of arterial blood is finely tuned and small deviations can affect oxygen transport and delivery
What is HCO3-?
Plasma bicarbonate
Describes the concentration of bicarbonate dissolved in arterial blood
If plasma bicarbonate is higher or lower than normal, this could be evidence of gas exchange imbalance
What is the PO2 when blood is first pumped into the systemic circulation?
And what is the O2 sats, and the PCO2?
O2 diffuses from lung alveoli into the RBCs
The oxygenated RBCs are delivered back to the heart, and pumped out via the left ventricle
The PO2 at this stage is >10kPa
O2 sats = >95%
PCO2 = 4.7-6.0 kPa
What is the PO2 when blood is in the venous system returning to the heart?
PO2 = 4.0-5.3kPa
O2 sats = 75%
PCO2 = 5.3-6.7kPa
What are these the O2 sats and PCO2 for blood in venous return?
O2 sats = 75%
PCO2 = 5.3-6.7kPa
O2 sats = 75% due to oxygen disassociation curve
PCO2 does not increase by much due to conversion of CO2 to HCO3- for storage
What is pulmonary transit time?
What is the pulmonary transit time in healthy adults?
How long the RBCs are close enough to the respiratory exchange system to exchange gases
0.75s - although gas exchange tends to occur within the first 0.25s
An arterial blood gas (ABG) s is a point of care test.
What is meant by the term point of care test?
Analyser sits very close to the patient - i.e. blood drawn from the patient and placed into machine by the analyser where the patient is situated
Not a blood sample collected from the patient and sent off to labs
Why is a fresh sample of blood for the ABG useful?
As the machine detects temperature, so if the analyser collects the blood and places it into the machine immediately, it doesnβt allow much time for the blood to cool
Gives a more accurate result
How do you convert a proton concentration to pH?
ππ»= - ππg 10 [π»^+]
As proton concentration increases, pH decreases
What else does the ABG show?
[H+] - mmol/L PCO2 - kPa PO2 - kPa HCO3- - mmol/L BE (base excess) - mmol/L
What does the co-oximetry in an ABG show?
Haematocrit percentage - i.e. how much of the whole blood sample is FBCs total Hb - g/L O2 sats - % OxyHb - % CarboxyHb - % metHb - %
What else does an ABG show about the kind of air the person is breathing?
What kind of air the person is breathing in e.g. on Air it would show F1O2 to be 0.21 (i.e. 21% oxygen in the air)
And it would show the kPa of the atmosphere e.g. 101kPa is room pressure
The body produces a signifcant amount, but how much of the acid is respiratory Vs metabollic?
99% respiratory (99% of the acid produced is produced and cleared by the lungs)
1% metabolic (e.g. pyruvic acid, lactic acid etc.)
How is CO2 output / production measured in tissues?
What is a CO2 flux?
What is the average CO2 ml produced by an adult male?
Measure CO2 in the blood before and after the tissues
e.g. for every decilitre of blood that passes through, 4ml of CO2 is produced
200ml per minute of CO2 produced in an average adult male
300L of acid and cleared by the lung in a day
What is the buffering capacity of the blood?
Blood has an enormous buffering capacity that can react almost immediately to imbalances
What is corrective compensation?
Changes in ventilation (hyper or hypo ventilation) can help adjust the amount of CO2 that is being cleared - helps alter and restore optimal pH rapidly
What is difference between corrective compensation in the lungs VS the kidneys?
Changes in HCO3- and H+ retention/secretion in the kidneys can stimulate a SLOW compensatory response to increase/decrease pH
RAPID in the lungs - via changes in speed and depth of ventilation
What is the difference between alkalemia and alkalosis VS acidemia and acidosis?
-osis = describing circumstances that lead to a change in the direction it is describing
e.g. acidosis = conditions within the body that is driving the pH to be lower
alkalosis = conditions within the body that is driving the pH to be higher
Whilst -osis is the change in pH, -emia is the position of the pH
e.g. alkalemia = alkaline pH
acidemia = acidic pH
How can acidosis and alkalosis be corrected?
An acidosis will need an alkalosis to correct
An alkalosis will need an acidosis to correct
What are the 4 steps in the procedure to interpret ABG?
Type of imbalance?
Acidosis (or acidaemia) / Alkalosis (or alkalaemia) / Normal
Aetiology of imbalance?
Respiratory (change in breathing results in change in pH) / Metabolic (any other change in the body resulting in change in pH) / Mixed (respiratory and metabolic) / Normal
Any homeostatic compensation?
Uncompensated (fresh, body hasnβt had time) / Partially compensated (on its way to compensation) / Fully compensated (issue driving the continuous compensation)
Oxygenation?
Hypoxaemia / Normoxaemia / Hyperoxaemia
What are the steps in reporting an ABG?
- Assess pH - disturbance
- Assess the PCO2 - aetiology (respiratory, metabolic, mixed, normal)
- Assess the BE - compensation / metabolic status
- Assess the PO2 - oxygenation
- Evaluate the acid-base status (i,e, compensation aetiology distubrance e.g. partially compensated metabolic acidosis)
- Evaluate the oxygenation (hypoxaemia, normoxaemia, hyperoxaemia)
Why is it important to maintain a stable pH?
Changes in proton concentration affects the hydrogen bonding in proteins leading to changes in protein shape
This can affect enzyme binding, hormone binding, working membrane transporters
A patient is hypoventilating.
Their ABG shows: low pH, high PCO2, normal BE
What is this called?
How can the body reduce proton concentration?
Uncompensated respiratory acidosis
H2CO3 H+ + HCO3-
- Acute phase: Equilibrium shifts to the left, bicarbonate ions bind to H+ to form H2CO3
- Chronic phase: gut absorbs more HCO3- to aid acute phase
Whilst the excess protons are being cleared, PCO2 remains high as ventilation has not changed.
The updated ABG shows: low pH, high PCO2, high BE
What is this called?
Partially compensated respiratory acidosis
Eventually the compensation will be enough to return the pH to the normal range. Ventilation remains unchanged.
The ABG now shows: normal pH, high PCO2, high BE
What is this called?
Full compensated respiratory acidosis
A patient is hyperventilating.
Their ABG shows: high pH, low PCO2, normal BE
What is this called?
How can the body increase proton concentration?
Uncompensated respiratory alkalosis
H2CO3
Whilst the body is trying to raise the pH, PCO2 remains unchanged as the patientβs ventilation has not changed.
The ABG now shows: high pH, low PCO2, low BE
What is this called?
Partially compensated respiratory alkalosis
Eventually the compensation will be enough to return the pH to the normal range. Ventilation remains unchanged.
The ABG now shows: normal pH, low PCO2, low BE
What is this called?
Fully compensated respiratory alkalosis
A patient has diarrhoea.
What would their ABG show, and what would this be called?
Diarrhoea = HCO3- that should have been absorbed in the gut is instead passed out
H2CO3 H+ + HCO3-
Equilibrium shifts right. H2CO3 dissociates to compensate for HCO3-, but this also increases H+
ABG shows: low pH, normal PCO2, low BE
Uncompensated metabolic acidosis
The patient then hyperventilates to reduce H+ concentration to normalise pH.
What does their ABG show now and what is this called?
CO2 + H2O H2CO3 H+ + HCO3-
Increased ventilation reduces PCO2, hence reducing H2CO3 formed
Causes equilibrium to shift to the left to reduce proton concentration
ABG shows: low pH, low PCO2, low BE
Partially compensated metabolic acidosis
Eventually the pH normalises.
What does their ABG show now and what is this called?
ABG shows: normal pH, low PCO2, low BE
Fully compensated metabolic acidosis
A patient is losing protons due to HCl being lost from the stomach due to excessive vomiting
What does the ABG show and what is this called?
H2CO3 H+ + HCO3-
Equilibrium shifts to the right to compensate for H+ loss, but this also increased HCO3-
ABG shows: high pH, normal PCO2, high BE
Uncompensated metabolic alkalosis
The patient then hypoventilates to increase their H+ concentration to normalise pH.
What does their ABG show now and what is this called?
Increased PCO2 due to hypoventilation
CO2 + H2O H2CO3 H+ + HCO3-
Equilibrium shifts to the right
ABG shows: high pH, high PCO2, high BE
Partially compensated metabolic alkalosis
Eventually the pH returns to normal.
What does the ABG show now and what is this called?
ABG shows: normal pH, high PCO2, high BE
Fully compensated metabolic alkalosis
