Acid-base balance Flashcards
By measuring ABG’s (the partial pressure of gases dissolved in arterial blood) what information does it provide us with ?
Both ventilation status and acid base balance.
What patients need an ABG?
- Resp disease - Asthma/COPD
- CVS derangement/ shock/ bleeding
- DKA
- Renal failure
- Collapsed/ unconcious
- Cardiac arrest
- ICU patients
List some of the instances where ABG measurements can be useful
- Exacerbations of COPD
- Severe asthma,
- DKA
- severe sepsis
- Acute LVF and pulmonary oedema
- Post-resuscitation
- Tri-cyclic antidepressant overdose etc
What should be done prior to ABG sampling ?
Allens test
Describe what allens test is
A test use to confirm the patency of the Ulnar Artery.
- Occlude both radial and ulna arteries with direct digital pressure.
- The hand will become pale.
- Release pressure over the ulnar artery.
- If colour returns to the hand, this confirms ulnar artery patency
- Therefore sampling from the radial artery may proceed
State the key equation you need to know to understand acid-base stuff ?
What is bicarbonate and what is its levels controlled by ?
What is important to remember about the time taken for this control (metabolic compensation) to be exerted?
It is a base (alkali) that is generated or resorbed by the kidneys
These systems have a large capacity to but unlike respiratory compensation, the response of the kidneys is slow and takes several days.
Appreciate this
- CO2 is a waste product of metabolism of all cells in the body
- Its retention/excretion is controlled by the lungs
What will cause PaCO2 levels to rise and what does this result in?
Hypoventilation (or even worse resp arrest).
Makes the patient more acidaemic due to the acid-base equation where CO2 combines with water to give H+ ions and bicarbonate:
CO2 + H2O = H+ + HCO3-
What will cause PaCO2 levels to fall and what does this result in?
- Hyperventilation.
- This results in making the patient alkalaemic. Due to CO2 falling and considering the acid-base equation, H+ and HCO3- will then combine, to restore equilibrium of the reaction, resulting in a fall in H+ conc, making the patient alkalaemic.
What is normal atmospheric pressure and how does this allow you to calculate the atmospheric pressure of oxygen?
- Normal atmospheric pressure is 100kPa.
- At atmospheric pressure the partial pressure of a gas in a mixture is numerically the same as the percentage of the gas by volume.
- Atmosphere is 21% oxygen ==> partial pressure of O2 in atmosphere is 21kPa.
Why/how is it helpful to know the atmospheric pressure of oxygen ?
It is important because increasing the partial pressure of oxygen delivered to someone can help a damaged/impaired lung take more of the gas/oxygen. Through increasing the pressure gradient between the gas in the lung and the blood.
Therefore knowing 21% oxygen is 21kPa allows you to understand giving a patient e.g. 40% oxygen is like 40kPa. This is almost twice what they are breathing in air.
What is meant by the abbreviation PaO2 and what is its normal range ?
- PaO2 sometimes called PO2 is the partial pressure of oxygen in arterial blood i.e. the pressure exerted by the dissolved oxygen in plasma. This reflects how much oxygen there is.
- Normal range = 10-13kPa
Why is the partial pressure of oxygen in blood (PaO2) lower than the partial pressure in atmosphere/air? i.e. what is inhaled.
Because air is humidified as it is inhaled and then mixes with CO2 in the alveoli, therefore diluting the inhaled O2.
As a rough rule of thumb what should the PaO2 of arterial blood be compared to the % of inhaled oxygen ?
In healthy patients, the PaO2 should be about 10 less than the inspired concentration (%)
Therefore a normal PaO2 in someone on supplemental oxygen may actually represent very abnormal/impaired gas exchange.
What is the normal range of PaCO2 & what is it a measure of ?
- Normal range = 4.5-6 kPa
- It is the partial pressure of carbon dioxide gas dissolved in arterial blood plasma & is a measure of aveolar minute volume
- pCO2 falls if minute volume increases pCO2 rises if minute volume falls
What is alveolar minute volume and what does it reflect?
- Minute volume = RR x alveolar tidal volume
- The amount of air that moves between the atmosphere and the alveoli per minute
- This reflects ventilatory adequacy
In chemistry what is a base defined as ?
A base is a substance that can neutralize the acid by reacting with hydrogen ions.
What is base excess and its normal range
- It is a measure of how much acid (H+ ions) has to be removed or added to the system to correct the pH as a result of metabolic derrangement (assuming a normal CO2)
- It is therefore a measure of metabolic acidosis or alkalosis
- The normal range is -2 to +2
Think back to the definition of a base, the base excess is telling you if you have -ve base i.e. need to remove acid or +ve base i.e. too much base and need to add acid.
What does a base excess of < -2 indicate ?
- < -2 = sick patient
- < -5 = very sick patient
- < -10 = exstremly sick patient, consider ITU
What does a negative or a positive base excess indicate ?
- A negative BE indicates a metabolic acidosis (-ve i.e. need to remove acid)
- A positive BE indicates a metabolic alkalosis (+ve i.e. need to add acid)
Why is urine profoundly acidic ?
Because Hydrogen ion concentration ([H+], pH) is primarily excreted through the urine
What is the normal range of H+ ion concentration in the body ?
- [H+] is 35-45 nanomol/L
- or pH 7.35-45
Small changes in pH reflect what in terms of H+ concentration?
Very large changes in H+ conc.
e.g. a change in pH from 7.4 to 7.1 reflects a change in H+ concentration from 40nmol/L to 80nmol/L i.e. H+ concentration has doubled.
Why does pH/H+ concentration have to be so tightly controlled?
Because many of the complex reactions within cells are controlled by enzymes that function only within a very narrow pH.
What is the normal range of HCO3-?
22-26mmol/l
What is bicarbonate (HCO3-) in terms of acid-base balance ?
This is the main buffering agent (alkali or base) used to compensate for excess carbonic, lactic or any other acid dissolved in plasma
What is the only limit to the following equation ?
The initial concentration of HCO3-
Define what is meant by Acidosis and Alkalosis
- Acidosis = increased [H+], or a process tending to cause increased [H+]
- Alkalosis = decreased [H+], or a process tending to cause decreased [H+]
In terms of the acid-base equation, what is considered the respiratory and conversley the metabolic component ?
- pCO2 is the respiratory component
- HCO3- is the metabolic component - excretion/reabsorption is renal
Define what respiratory acidosis is
Increased [H+] due to increased pCO2
Define what respiratory alkalosis is
Decreased [H+] due to decreased pCO2
Define what metabolic acidosis is
Increased [H+] due to decreased HCO3-
Define what metabolic alkalosis is
Decreased [H+] due to increased HCO3-
How does the body try to compensate for increased H+ ion concentration (acidosis)?
The lungs blow off CO2 (hyperventilation)
When you’ve got too much CO2, how does the body compensate for this?
The kidneys get rid of H+ (and in the process reclaim HCO3-)
How does the body compensate for metabolic acidosis ?
PRIMARY problem is too much H+ ==> COMPENSATE by blowing off CO2
How does the body compensate for respiratory acidosis ?
PRIMARY problem is too much CO2 ==> COMPENSATE by excreting H+ (and simultaneously regenerating HCO3-)
Go over the steps for evaluating blood gases
List some of the clinical causes of respiratory acidosis
- Acute COPD
- Decompensation in other respiratory conditions e.g. life-threatening asthma / pulmonary oedema
- CNS (depression) - opiate overdose, stroke, neuropathy, myopathy
The underlying cause is alveolar hypoventilation
What type of acid-base problem is displayed by these biochemistry results ?
Uncompensated respiratory acidosis
What type of acid-base problem is displayed by these biochemistry results ?
Compensated respiratory acidosis
List some of the causes of respiratory alkalosis
- Early acute asthma with over ventilation
- Pulmonary embolus, pneumonia, pulmonary oedema
- Anxiety attack
- Severe pain
- Mechanically ventilated patients with too high a RR/ or tidal volume
- CNS disorders (raised intra-cranial pressure): stroke, subarachnoid haemorrhage, encephalitis
- Altitude
- Pregnancy
The underlying cause is alveolar hyperventilation
What type of acid-base problem is displayed by these biochemistry results ?
Respiratory alkalosis uncompensated
What type of acid-base problem is displayed by these biochemistry results ?
Respiratory alkalosis compensated
List some of the potential causes of metabolic acidosis
- Lactate: shock (septic, cardiogenic, hypovolaemic), hypoxia
- ketones: diabetic ketoacidosis, alcohol
- Urate: renal failure
- Acid poisoning: salicylates, methanol
- gastrointestinal bicarbonate loss: diarrhoea, ureterosigmoidostomy, fistula
- Renal tubular acidosis
- Drugs: e.g. acetazolamide
- Ammonium chloride injection
- Addison’s disease
Underlying cause is excess production of H+ ions or sometimes loss of HCO3- ions
What type of acid-base problem is displayed by these biochemistry results ?
Uncompensated metabolic acidosis
What type of acid-base problem is displayed by these biochemistry results ?
Metabolic acidosis with respiratory compensation
Describe what respiratory compensation is
The respiratory centre within the brain stem is very sensitive to changes in blood H+ concentration. If H+ conc rises within minutes it will sense this and increase RR. Resulting in a fall in PaCO2 and therefore in turn H+ concentration - this is called respiratory compensation.
List some of the potential causes of metabolic alkalosis
- Loss of gastric fluid - Vomiting / aspiration (e.g. peptic ulcer leading to pyloric stenos, nasogastric suction)
- Diuretics causing K+ loss & excess renal HCO3- reabsorption
- Post hypercapnic mechanical ventilation
- Liquorice, carbenoxolone
- Hypokalaemia
- Primary hyperaldosteronism
- Cushing’s syndrome
- Bartter’s syndrome
- Congenital adrenal hyperplasia
The underlying cause is loss of hydrogen ions or a gain of bicarbonate. It is due mainly to problems of the kidney or gastrointestinal tract
What type of acid-base problem is displayed by these biochemistry results ?
Uncompensated metabolic alkalosis
What type of acid-base problem is displayed by these biochemistry results ?
Compensated metbaolic alkalosis
What are the blood gases which suggest type 1 and type 2 respiratory failure ?
Define what is meant by a hypoxic respiratory drive
The hypoxic drive is a form of respiratory drive in which the body uses oxygen chemoreceptors instead of carbon dioxide receptors to regulate the respiratory cycle. … And that in this case, when there is an increase in oxygen levels the body will decrease the rate of respiration.
What is the normal respiratory response to hypercapnia or hypoxia ?
Mammals respond to hypercapnia and hypoxia by increasing minute ventilation (Ve)
In acute asthma what is the respiratory response, expected blood gases and the oxygen therapy you should give
- Expected blood gases - low PaO2 & low PaCO2
- Respiratory response - Alveolar hyperventilation due to increased respiratory drive
- Oxygen therapy = give high flow, high concentration of O2 (60%)
In late asthma what is the respiratory response, expected blood gases and the oxygen therapy you should give
- Expected blood gases - low PaO2 & high PaCO2 (due to the hypoventilation)
- Respiratory response - Alveolar hypoventilation due to decreased respiratory drive
- Oxygen therapy = give high flow, high concentration of O2 (60%)
What may happen in COPD which means when there is an acute exacerbation you may need to give different O2 therapy?
- Many patients who are know to have COPD function with an abnormally low PaO2 in the long term, and with an abnormally raised PaCO2. These patients lose their normal increased respiratory drive in the presence of hypercarbia, so hypoxia is the only stimulus that keeps them breathing. This is why the target O2 saturation levels by pulse oximetry in patients with known COPD are 88 - 92% (as opposed to 94 - 98% in patients without COPD).
- Giving supplemental oxygen to a patient with COPD risks removing their hypoxic drive. This results in reduced ventilation, with increasing PaCO2, drowsiness and acute respiratory failure
- ==> need to Initially use low concentration of O2 (24-28%) to avoid suppressing possible hypoxic drive in acute exacerbations
Note - This initial management of an acutely unwell patient is high flow oxygen at 15l/min using a mask with a reservoir bag. (NO MATTER WHAT)
Not all COPD patients retain CO2 i.e. are in type II resp failure, how do you determine a COPD patient is a chronic CO2 retainer & therefore is at risk of resp despression from high flow oxygen, compared to simply showing acute CO2 retention on ABG?
Acute CO2 retention will not have any metabolic compensation from the kidneys, whereas chronic CO2 retention will usually result in a raised HCO3- to compensate
What are the steps for interpreting ABG results ?
- Look at the pO2
- Look at the pH
- Look at the pCO2 - A raised pCO2 associated with acidosis, or a low pCO2 associated with alkalosis indicates a primary respiratory problem. A low pCO2 associated with acidosis indicates that the primary problem is metabolic
- Look at the bicarbonate - If this is raised in association with an alkalosis, or decreased in the presence of an acidosis, this indicates a primary metabolic problem. An increased bicarbonate level associated with an acidosis indicates that the metabolic response to a respiratory acidosis
- Look at the base excess - this should correlate with the bicarbonate level, and gives an indication of the severity of the acid-base imbalance. If the BE is raised in the presence of an alkalosis, this indicates a primary metabolic problem. If there is an abnormally negative base excess (base deficit) associated with an acidosis, this again indicates a primary metabolic problem.
What is the anion gap and what is the use of it ?
- An anion gap (AG) is calculated by using on electrolyte levels, and is of assistance in determining the cause of a metabolic acidosis, including any poisons that may have been ingested.
- Anion gap = (Na + K) – (Cl + HCO3)
- Normal range = 12 +/- 4
What are the causes of a reduced anion gap?
- A reduced anion gap is less common.
- It may be simply the result of laboratory or sampling error, or may be seen associated with myeloma or hypoalbuminaemia.
What are the causes of raised anion gap metabolic acidosis and explain the underlying reason why they cause a raised anion gap?
Think MUDPILES
- Methanol ( formic acid)
- Uraemia – DKA/ alcoholic ketoacidosis
- Paraldehyde
- Isoniazid
- Lactic acidosis
- Ethylene glycol
- Salicyl
They cause a raised anion gap because essentially there is some osmole i.e. MUDPILES causes, which is not accounted for in the anion gap equation resulting in a raised gap.
What are the causes of metabolic acidosis with a normal anion gap ?
Think ABCD:
- Addison’s
- Bicarb loss e.g. diarrhoea, fistula
- Chloride
- Drugs
Ans the following question
Partially compensated metabolic acidosis
Ans the following question
Ans the following question
Partially compensated respiratory acidosis
Ans the following question
Interpret the ABG’s and provide a working diagnosis
- Hypoxia, Respiratory acidosis with no compensation
- Working diagnosis = pnuemonia
He is hypoxic
Metabolic acidosis (the BE is outwidth normal and is very negative indicating you need to remove lots of acid and that its a Metabolic acidosis)
No compensation
What is the most likely diagnosis in this patient ?
DKA
Interpret the ABG’s and state the likely diagnosis
- Non hypoxic, respiratory alkalosis
- No compensation
- Likely diagnosis = anxiety related hyperventilation
Interpret the following ABG’s and state the likely diagnosis
Likely diagnosis = meningococcal septecaemia
Interpret the ABG’s and state the likely diagnosis
Likely diagnosis = exacerbation of COPD
Interpret the ABG’s and state the likely diagnosis
Perforated dudodenal ulcer (diclofenac use is big hint)
Interpret the ABG’s and state the likely diagnosis
Likely diagnosis = hyperemesis
36 y/o admitted after taking her fathers tablets all washed down with a bottle of vodka
Now drowsy and has been vomiting
Salicylate e.g. aspirin
75 y/o had a VF cardiac arrest, paramedics restored spontaneous circulation after 3 shocks, CPR was not initiated prior to this
On arrival:
- Comatose (GCS 3)
- Ventilated with 50% O2 via tracheal tube
- HR 120
- BP 150/95
75 y/o admitted following VF cardiac arrest, spontaneous circulation restored after 2 shocks by paramedics
At present:
- Comtaose
- Apnoeic
- Lungs ventilated on automatic ventilator
What can acidosis do to cardiac contractility ?
It decreases cardiac contractility; the tension generated by isolated myofibrils at a fixed calcium concentration is reduced at low pH.
Appreciate this
Note that technically FiO2 for atmospheric pressure starts at 21% despite it saying 4% increments
Appreciate the limitations of pulse oximetry
Work though case 1
Work through case 2
Work though case 3
Work though case 4
Work though case 5
