ECC MEMBERS

Define the pH equation → and explain how to evaluate them and why

H+ + HCO3- = H2CO3 = H2- + CO2, Assess the respiratory portion first as it can respond to pH shifts within minutes. After that evaluate the metabolic portion.

What are the immediate buffer systems for pH changes?

Intracellular - protein, haemoglobin, phosphate

Extracellular - plasma protein, bicarbonate, plasma phosphate

Transcellular - K+/H+ exchange

Respiratory

What is the delayed buffer system for pH changes, and how long does that take?

The Renal system - excretes H+/HCO3- (and conserves it). Starts within 12-24 hours and can take 2-5 days

What are the signs of acidaemia?

Tachycardia

Hypotension

Arrythmias

Decreased Cardiac Contractility

Insulin Antagonism

Blunted response to Catecholamines

Compensatory hyperkalaemia

What are the signs of Alkalaemia?

Muscle spasms

Stuporous mentation

Arrythmias

Reduces O2 unloading in tissues → hypoxemia

Hypocalcemia

Hypokalaemia

Altered enzyme activity + alterations to cell structures that are dependent on H+ ions

What are the 4 primary acid-base disturbances?

Respiratory Acidosis → Increased PCO2

Respiratory Alkalosis → Decreased PCO2

Metabolic Acidosis → Decreased HCO3/BE-

Metabolic Alkalosis → Increased HCO3/BE-

Why do we use BE (Base excess), and explain what it means

More accurate measure of metabolic changes, is a measure of the amount of acid/base required to bring 1L of blood to pH 7.4.

+ve BE = metabolic alkalosis, -ve BE = metabolic acidosis

Normally will change in same direction as HCO3-, however HCO3- is linked with respiration, therefore BE- is more accurate

What are the causes of Respiratory Acidosis?

Overarching - Hypoventilation

• Neuronal, neuromuscular, muscular disease - think ticks, snakes, botulism, myasthenia gravis etc.,

• rebreathing/increase in dead space

• upper airway obstruction

• anterior displacement of diaphragm

• restrictive extrapulmonary disease

• pleural space disease

• severe pulmonary parenchymal disease

• bicarbonate therapy in respiratory compromised patients

• certain medications - e.g., sedation

• malignant hyperthermia

• compensation for metabolic alkalosis

What are the causes for Respiratory Alkalosis?

Overarching - Hyperventilation

• FAS

• hyperthermnia/fever

• pulmonary thromboembolism

• early pulmonary parenchymal disease

• respiratory centre disease - e.g., neoplasia or head trauma

• hypoxemia

• hyperadrenocorticism (cushings)

• excessive mechanical ventilation

• compensatory response to metabolic acidosis

Causes of Metabolic Acidosis?

Toxic - ethylene glycol, methanol

Intestinal - loss of HCO3- through diarrhoea

DKA

Dilutional acidosis - dilution of HCO3- (e.g., fluid therapy with non-balanced fluids

Renal - reduced excretion of H+ and reduced update of HCO3-

Lactic Acidosis - marker of shock and poor perfusion

What is the Anion Gap Formula and when is it useful?

[Na+] + [K+] - [Cl-] + [HCO3-]. Can be useful when trying to interpret a metabolic acidosi. Can only interpret it when there are no obvious significant abnormalities to the plasma protein (albumin) levels, as plasma protein makes up a significant portion of the unaccounted for factors in the equation.

Differentials for a normal Anion Gap (metabolic acidosis)

Dilutional Acidosis

Diarrhoea

Kidney Disease

Differentials for a increased anion gap (metabolic acidosis)?

M - methanol, methaldehyde

U - uraemic acid

D - DKA

P - Paracetamol, propylene glycol, paraldehyde

I - Iron, Isoniazid

L - Lactic Acidosis

E - Ethylene Glycol Toxicity

S - Salicylic Acid (Aspirin)

What are the causes of Metabolic Alkalosis?

Gastric or intestinal obstructive processes: lose H+, K+ and Cl- → then reduced resorption of H+ and CL- in the proximal SI

Frusemide administration

Hyperaldosteronism

Hypovolemia/dehydration + hypochloridaemia → Na+ + H20 resorbed by kidneys without CL- to maintain electroneutrality

Compensation for respiratory acidosis

Severe hypokalaemia → Potassium is exchanged with H+ across cell membranes - causes an intracellular acidosis in all the cells including the kidneys → leading to a increase in H+ excretion by the kidneys + an increase in HCO3- resorption

What is Contraction Alkalosis?

When a large amount of Sodium rich fluid is lost from the body → leads to an increase in net concentration of bicarbonate

When should you be concerned about 2 processes occuring at the same time when reading pH?

When you notice that the pH, metabolic markers and respiratory markers all seem to trend in the same direction.

What are the two main mechanisms by which metabolic acidosis occurs?

1. A fall in bicarbonate concentration with a rise in chloride concentration (hyperchloremic metabolic acidosis)

2. Gain of acid in the system leading to a fall in bicarbonate concentration.

What causes renal loss of bicarbonate?

Appropriate response to a persistent respiratory acidosis (metabolic compensation)

Or renal tubular acidosis as a primary disease process

Two types of renal tubular acidosis (RTA) and causes?

1. Proximal tubular dysfunction - congenital abnormalities e.g., Fanconi syndrome, toxins, drugs and various diseases

2. Distal RTA - inadequate hydrogen ion secretion in the distal tubule that prevents maximum acidification of the urine e.g., pyelonephritis and IMHA

When is bicarbonate therapy indicated?

When the acidosis is severe or when the compensatory respiratory alkalosis is considered detrimental to the patient.

How can hypokalemia play a role in generation and maintenance of s metabolic alkalosis?

Promotes more loss of hydrogen ions from the kidneys

K+ moved from cells to intravascular space in exchange with H+

What are alternative alkalanising therapies and what is the major difference with sodium bicarbonate?

Tromethamine - no CO2 production

Carbicarb - less CO2 production

What are the main contraindications and risks of sodium bicarbonate administration to the patient with metabolic acidosis?

1. Patients that are hypoventilating - bicarbonate becomes carbonic acid which becomes water and CO2, if the patient isnt ventilating adequately can cause a decrease in pH

2. Paradoxical intracellular acidosis - CO2 crreated by bicarbonate can freely enter cells → lowers pH

3. Increases in blood lactate in cases of DKA, lactic acidosis and haemorrhagic shock.