Acid base balance

Acid and base balancephysiology and disturbances

Dr. Ayad Abbas Salman


If hydrogen ion of pure H2O at room temperature is equal to 1/10.000.000 mol/L → [H+] = 1/10.000.000 → [H+] = 10-7 → log [H+] = -7 → -log [H+] = 7→ pH = 7

Neutral solution → [H+] = [OH-] = 10-7 → pH =7 Acidic solution → [H+] > [OH-] → pH < 7 Alkaline solution → [H+] < [OH-] → pH > 7Range of blood pH which compatible with life is only (7-7.8) 6 times change in hydrogen ion concentration.


Physiological responses to changes in [H+] are characterized by 3 phases: 1.Immediate chemical buffering. 2.Respiatory compensation. 3.Slower but more effective renal response that may nearly normalize arterial pH.

Chemical buffering

1.Bicarbonate (H2CO3/HCO3-):is most important buffering in the extracellular fluid.H2O + CO2 (ac)↔(al) H2CO3 (ac) ↔(al) H+ +Hco3 ↑ PaCO2 Normally eliminated by lung

2.Haemoglobin (HbH/Hb-):

Although restricted inside RBC, but it is an important buffer in the blood.H+ + KHb (al) ↔(ac) HHb +K+

3.Protein other than Hb:

play a major role in buffering of intracellular fluid.Protein possess both acidic & alkaline groups.Acidic group called carboxyl group. Alkaline group called amino group. NH2 + H+ (al) ↔ (ac) NH3 COOH + OH- (ac) ↔ (al) COO + H2O


4.Phosphate (H2PO4-/ HPO4-2):
it is important urinary buffer. ↑ [H+] →HCl + Na2HPO4 →NaH2­PO4 (kidney) + NaCl di (Na) phosphate mono (Na) phosphate ↑[OH-] →NaOH + NaH2PO4 → NaHPO4 + H2O excreted by kidney

5-Amonia (NH3/NH4+):

it is also important urinary buffer.NH3 + H+ → NH4 (excreted by kidney)

Change in pH sensed by :chemoreceptors in brain stem↑pH → hypoventilation → ↑PaCO2 ↓ pH → hyperventilation → ↓PaCO2

1.Ability of kidney to control the amount of HCO3- reabsorption.2.Ability of kidney to form new HCO3-.3.Ability of kidney to eliminate H+ in the form of titrable acid and ammonium ion.EXAMPLES:► H+ + HPO4-2 → H2PO4 (excreted by kidney) ► NH3 + H+ NH4 (excreted by kidney)

Normal values and analysis

PaCO2 = 40 +- 5 HCO3 = 22 +- 2 SBC* = 24+- 2 BE** = +- 3 -zero.* SBC: it is plasma [HCO3] from fully oxygenated (PaO2 = 100) blood which has been equilibrated to PCO2 40 mmHg at 37○C.** BE: Base Excess: is an amount of strong acid or base required to titrate the pH to 7.4 at PCO2 40 mmHg

Clinical disorders

PaCO2 Change: Respiratory acidosis Respiratory alkalosis Hc03 Change: Metabolic acidosis Metabolic alkalosis

examples

Diag.
BE
Hco3
Paco2
PH
UMac
-20
10.8
41
7.05
URal
5
24
21
7.58
CMac
-15
13.4
23
7.27
CMal
20
42.8
55
7.55



1.pH < 7.20 → direct myocardiac depression and ↓ peripheral vascular resistance → hypotension.2.Tissue hypoxia (although O2 dissociation curve shift to the left)3.Hyperkalaemia 4.CNS depression mainly in respiratory acidosis (CO2 narcosis) → ↑ cerebral blood flow → ↑ intracranial pressure.

Causes of respiratory acidosis:1.Alveolar hyperventilation● CNS depression e.g: 1.cerebral ischemia 2.cerebral trauma 3.drug induced● Neuromuscular disorder: myopathy, neuropathy● Chest wall abnormality e.g kyphoscoliosis● Pleural abnormality e.g pneumothorax, pleural effusion ● Airway obstruction: foreign body, tumor → upper airway asthma, COPD → lower airway● Paranchymal lung disease:1.Pulmonary Oedema 2.Pulmonary Emboli 3.Pneumonia 4.Aspiration 5.Interstitial lung disease

● ↑ in enteral or paraentral nutrition of carbohydrate ● Intense shivering ● Prolong seizure activity● Thyroid storm● Excessive thermal injur

● Treatment of the cause● May → respiratory depression → mechanical ventilation● I.V NaHCO3 is rarely necessary (i.e not used)

anion Gap: defined as the difference between major measured cations and major measured anions. Anion gap = [Na+] -([Cl-] + [HCO3-] ) normally (9-15).

● 1.Incresed anion gap↑ production of non volatile acids e.g renal failure, DKA (diabetic ketoacidosis), lactic acidosis. Ingestion of toxins e.g salicylatye, methanol, sulfer Rhabdomyositis

2.Normal anion gap

↑ GIT loss of HCO3: diarrhoea, ingestion of CaCl2/MgCl2, fistula (pancreatic, biliary, small bowel) ↑ renal loss of HCO3: renal tubular acidosis, hypoaldosteronismDilutional: large amount of HCO3 free fluidTotal paraenteral nutrition↑ intake of chloride containing acid e.g ammonium chloride

ph < 7.2 → Na HCO3 (8.4 %) should be given NaHCO3 = (BE X body weight) /3giving 1/2 the dose and reassess the values● Treatment of the cause

Alkalosis (physiological effect)

1.Hypokalemia2.Difficult to give up O2 to tissue → hypoxia.3.↓ ionized Ca+2 → circulatory depression and neuromuscular irritability.4.Respiratory alkalosis → ↓ cerebral blood flow, ↑ systemic vascular resistance and may → coronary vasospasm.


Resp. Alkalosis
1○ ↓ in PaCO2 by inappropriate ↑ in alveolar ventilation relative to CO2 product.It is compensated by ↓ [HCO3] in plasma.

causes

● Central stimulation: 1.pain 2.anxiety (hystria) 3.stroke 4.ischemia 5.feve6.infection 7.drug induced e.g salicylate● Peripheral stimulation: 1.hypoxia 2.high altitude 3.pulmonary disease (CHF, pulmonary oedema, pulmonary emboli, asthma (severe)● Iatrogenic: ventilator induced

management

Treatment of the cause is enough For severe (pH > 7.6) i.v HCl or NH4Cl4 (ammonium chloride) my be indicated

II.Metabolic alkalosis1○ ↑ in HCO3 Causes 1. Chloride sensitive: those causes associated with ↓ NaCl and ECF depletion● GIT: vomiting, gastric drainage, chloride diarrhoea● renal: diuretic● sweat: cystic fibrosis

2.Chloride resistance: those associated with enhanced mineralocorticoid activity● hyperaldosteronism, cushing syndrome3.Miscellanous● massive blood transfusion ● alkaline treatment with renal insufficiency ● hyper Ca+2 e.g metastasis

management

Treatment of underlying cause For chloride sensitive causes i.v normal saline and K+ replacement When pH > 7.6 i.v : 1.HCl 2.ammonium chloride 3.Vit.C (5-10 g/day) 4.haemodialysis may be considered