Disorders of Acid-Base Flashcards
Why do we care about acid-base homeostasis?
- Critical to life
- Must be tightly controlled for proteins
- Metabolic processes produce acid= cellular respiration (CO2), other processes produce non-volatile acids (ketones and lactate)
How do our bodies maintain acid-base homeostasis?
-Buffers =Bind H+ ions to reduce acidity =Proteins- haemoglobin =Bicarbonate -Lungs =Remove CO2 from cellular respiration and non-volatile acid by shifting equilibrium (limited by bicarbonate reserves) -Kidneys =excrete H+ into urine, recover and regenerate bicarbonate =24-48hrs to maximum efficiency
How can pathology lead to respiratory acid-base disorders?
- Acidosis= lung disease (COPD), airway obstruction, neurological problems resulting in reduced respiratory rate (head injury, opiate overdose)
- Alkalosis= panic attack, reaction to pain, acute asthma exacerbation (increased RR)
How can pathology lead to metabolic acidosis?
-Overproduction of acid
=lactic acidosis from inadequate oxygen supply to tissues (hypovolaemia, severe anaemia, damage to major arteries or cardio-respiratory arrest_
=Ketoacidosis secondary to inadequate cellular glucose supply (Type 1 diabetes, starvation, alcohol intoxication)
-Impaired excretion of H+ by kidneys
=Global loss of renal function (severe acute kidney injury/ end-stage chronic kidney disease)
=Specific impairment of renal tubule’s ability to excrete acid (renal tubular acidosis)
-Losing large amounts of bicarbonate
=Severe diarrhoea, surgical formation of fistula in small bowel, kidney via renal tubular acidosis
How can pathology lead to metabolic alkalosis?
-Unusual loss of H+ ions
=protracted vomiting
=sever hypokalaemia and hyperaldosteronism
=reabsorption of sodium in exchange for H+ ions (given low availability of K+ ions) causes H+ ions to be excreted
-Ingesting unusual quantity of bicarbonate (overdose of sodium bicarbonate)
What indicates acidaemia or alkalaemia?
-H+ ion concentration (usually measured from a blood sample)
=Outside the reference range
-pH in terms of blood concentrations of CO2 or bicarbonate
=caused by underlying pathology
=body compensation for acid-base disturbance
Describe metabolic correction of respiratory disturbances
- Acidosis= kidneys regenerate bicarbonate
- Alkalosis= tends to be too acute for kidneys to react
Describe correction for metabolic disturbances
- Acidosis= rapid and deep breathing (Kussmaul) and regenerate bicarbonate
- Alkalosis= marginal, hypoventilation limited by need for oxygen, few severe metabolic alkaloses
How do we assess an acid-base disorder?
- History = respiratory, fluid balance, intoxication, drug therapy, inadequate blood supply to tissues
- Physical examination= GCS, pupil reactivity, RR, oxygen saturation, heart rate, bp, temperature (sepsis)
- Blood gas (arterial/ venous)
- Venous bicarbonate (total CO2)
How does a blood gas analyser work?
- Uses electrochemistry to measure concentrations of oxygen, CO2 and H+ ions
- Bicarbonate result calculated
- Henderson-Hasselbalch equation (water concentration constant so K)
How is total CO2 measured?
-Measured by enzymatic reaction
How is blood for blood gas analysis collected?
-Radial artery
=Compared to venous collection, painful and risk of damage to artery thus compromising supply to hand
=In an arrest- femoral artery
-Collected into syringe with anti-coagulant (heparin)
=presence of bubbles reduces stability as air exchanges with oxygen and CO2 in blood
=Pod systems to send blood pressurised and affects pO2 and pCO2 values
What are the 4 parameters of blood gas analysis?
-pH
-pO2
-pCO2
-Bicarbonate
(Calculate standardised bicarbonate, base excess, anion gap)
What questions are asked to interpret the gas?
-• Is your patient adequately oxygenated? Does your pO2 fall within the reference range and given their “fraction of inspired” oxygen is that pO2 what you’d expect. Lastly what’s their [Hb]? As even if their pO2 is encouraging, if their [Hb} is very low they may not be delivering enough oxygen to their body.
• What is the pH? Reference range, low or high?
• Next – how did the pH get to where it is? Is it reflected in a derangement of CO2? Or bicarbonate? Or perhaps both? And which one of the changes in pCO2 or bicarbonate is caused by pathology, and which one is the body’s attempt at restoring acid-base homeostasis?
What is base excess?
Base excess is defined as the concentration of H+ required to return pH to reference range assuming a specific pCO2 of 5.3. It should therefore only be deranged in a metabolic disorder, and is negative in metabolic acidosis and positive in metabolic alkalosis.
