Acid base

Acid/base balanceAcid/base balance

Medhat Hashem, MD, FCARCSI, Medhat Hashem, MD, FCARCSI, FRCAFRCA

Professor of Anaesthesia, Cairo Professor of Anaesthesia, Cairo UniversityUniversity

Director of Surgical ICUDirector of Surgical ICU

ObjectivesObjectives

Basic physiologyBasic physiology DefinitionsDefinitions How to read ABGHow to read ABG ExamplesExamples

Normal concentration in Normal concentration in ECF ECF

(m mol/L)(m mol/L)NaNa HH++

140140 0.000040.00004

Normal HNormal H++ concentration: 40 n mol/L concentration: 40 n mol/L Viable limits (20-160 n mol/L)Viable limits (20-160 n mol/L) Na ion conc is 1000 times higher.Na ion conc is 1000 times higher. ECF Electroneutrality essential for cellular ECF Electroneutrality essential for cellular

activity.activity.

pHpH

[H[H++] concentration 40 n mol/L (20-] concentration 40 n mol/L (20-160)160)

100 = (10)100 = (10)22

LogLog1010 100 = 2 100 = 2 LogLog1010 1000 = 3 1000 = 3 pH; negative logarithm of reciprocal pH; negative logarithm of reciprocal

[H[H++] concentration. ] concentration. pH = - LogpH = - Log1010 1/[H 1/[H++]]

pHpH [H[H++] (nmol/L)] (nmol/L)7.67.6 2525

7.57.5 3232

7.47.4 4040

7.37.3 5050

7.27.2 6363

7.17.1 8080

7.07.0 100100

6.96.9 125125

6.86.8 160160

pH of body fluidspH of body fluids

PlasmaPlasma 7.47.4

Gastric HClGastric HCl 0.80.8

UrineUrine 4.54.5

Pancreatic juicePancreatic juice 88

DefinitionsDefinitions

Acid: A substance that dissociates in Acid: A substance that dissociates in water to Hwater to H++..

Acidosis: A process that causes acids Acidosis: A process that causes acids to accumulate.to accumulate.

Acidaemia; if pH<7.36Acidaemia; if pH<7.36 Alkali: a substance that accepts HAlkali: a substance that accepts H++

Alkalosis; A process that causes Alkalosis; A process that causes bases to accumulate.bases to accumulate.

Alkalaemia: if pH > 7.44Alkalaemia: if pH > 7.44

Buffers and Buffers and compensationcompensation

Buffers: Hb, proteins, HBuffers: Hb, proteins, H22POPO4 4 , H, H22COCO33

HH22O + COO + CO22 → H → H22COCO3 3 → HCO→ HCO33-- + +

HH++

Respiratory compensation:Respiratory compensation: ImmediateImmediate Hypo/hyperventilation to ↑or↓ COHypo/hyperventilation to ↑or↓ CO22

Renal compensation:Renal compensation: After 24-48 hoursAfter 24-48 hours Excretion or retention of filtered HCOExcretion or retention of filtered HCO33

Henderson Hasselbach Henderson Hasselbach EquationEquation

pH pH ≈≈ HCO HCO33/CO/CO22 pHpH

>7.44 → Alkalosis (alkalaemia)>7.44 → Alkalosis (alkalaemia) <7.36 → Acidosis (acidaemia)<7.36 → Acidosis (acidaemia)

PaCOPaCO22 and HCO and HCO33 (and Base deficit) (and Base deficit) PaCOPaCO22 → Respiratory → Respiratory

HCOHCO33 → Metabolic → Metabolic

Blood gas analysisBlood gas analysis

Blood gas machine measuresBlood gas machine measures pH (7.36 - 7.44)pH (7.36 - 7.44) PaOPaO22 (75 -100 mmHg), room air (75 -100 mmHg), room air

PaCOPaCO22 (35 – 45 mmHg) (35 – 45 mmHg)

HCOHCO33 calculated (22 – 26 mmol/L) calculated (22 – 26 mmol/L)

Metabolic acidosisMetabolic acidosis ↓↓pH (↑HpH (↑H++)) ↓↓HCOHCO33 (22-26 mmol/l) (↑Base deficit) (22-26 mmol/l) (↑Base deficit) PaCOPaCO2 2 → normal→ normal Compensation: Hyperventilation to wash COCompensation: Hyperventilation to wash CO22

pH pH ≈≈ HCO HCO33/CO/CO22

pHpH HCOHCO33-- PaCoPaCo22 CompensationCompensation

↓↓ ↓↓↓↓ ↓↓ HyperventilationHyperventilation

↓↓PaCOPaCO22

Metabolic acidosisMetabolic acidosis

Overproduction of organic acids Overproduction of organic acids Lactic acid: shock, infection, hypoxiaLactic acid: shock, infection, hypoxia Urate: renal failureUrate: renal failure Ketones: DM, alcoholKetones: DM, alcohol Drugs/toxins: (salicylate, biguanide, Drugs/toxins: (salicylate, biguanide,

methanol)methanol) Excessive loss of bicarbonateExcessive loss of bicarbonate

Metabolic acidosisMetabolic acidosis

Overproduction of organic acids Overproduction of organic acids Excessive loss of bicarbonate:Excessive loss of bicarbonate:

DiarrhoeaDiarrhoea Pancreatic or small intestinal fistulaPancreatic or small intestinal fistula Renal tubular acidosisRenal tubular acidosis Uretero-sigmodostomyUretero-sigmodostomy Addison’s diseaseAddison’s disease

Metabolic acidosis (CP)Metabolic acidosis (CP)

of the causeof the cause Tachypnea (↑rate and depth)Tachypnea (↑rate and depth) Severe acidosis → cardiovascular Severe acidosis → cardiovascular

collapse → cardiac failure (↓BP)collapse → cardiac failure (↓BP) DeathDeath

Rx of metabolic acidosisRx of metabolic acidosis

Rx of the cause (e.g., insulin if DKA)Rx of the cause (e.g., insulin if DKA) Mild to moderate: Mild to moderate:

beneficial (shift Obeneficial (shift O22 dissociation curve to dissociation curve to Rt → ↑ORt → ↑O22 delivery at tissue level). delivery at tissue level).

Severe (↓pH < 7.2) → iv NaHCOSevere (↓pH < 7.2) → iv NaHCO33

Amount of HCOAmount of HCO33= Bwt x base deficit x 0.3= Bwt x base deficit x 0.3 Start with half correctionStart with half correction Repeat assessmentRepeat assessment

35 yrs, M, RTA, bilateral 35 yrs, M, RTA, bilateral ## femur & pelvis, femur & pelvis,

pH pH ≈≈ HCO HCO33/CO/CO22 SaOSaO22 98% 98% pH 7.25pH 7.25 PaCOPaCO22 37 mmHg 37 mmHg

HCO HCO33 17 mmol/L 17 mmol/LBE -7BE -7

PaOPaO22 130 mmHg (O 130 mmHg (O22 by face mask) by face mask) Hb 4.1 gm/dlHb 4.1 gm/dl Rx: Blood Tx, ( open book Rx: Blood Tx, ( open book ##pelvis)pelvis)

17 yrs, M, unconscious, syringe 17 yrs, M, unconscious, syringe next to himnext to him pH pH ≈≈ HCO HCO33/CO/CO22

SaOSaO22 81% 81% pH 7.2pH 7.2 PaCOPaCO22 80 mmHg 80 mmHg

HCO HCO33 22 mmol/L 22 mmol/LBE 0BE 0

PaOPaO22 45 mmHg (O 45 mmHg (O22 40%) 40%) Hb 14.5 gm/dlHb 14.5 gm/dl

Respiratory acidosis Respiratory acidosis (uncompensated)(uncompensated)

Respiratory acidosisRespiratory acidosis

↓↓pH (↑HpH (↑H++)) ↑↑PaCOPaCO2 2 (35-45 mmHg)(35-45 mmHg)

HCOHCO33 (22-26 mmol/l) normal (22-26 mmol/l) normal Renal compensation (after 24 Renal compensation (after 24

hours): hours): Reabsorption of filtered HCOReabsorption of filtered HCO33

pH pH ≈≈ HCO HCO33/CO/CO22pHpH HCOHCO33-- PaCoPaCo22 CompensationCompensation

↓↓ ↑↑ ↑ ↑↑ ↑ Renal retention of Renal retention of HCOHCO33

--

Respiratory AcidosisRespiratory Acidosis

Always associated with hypoxiaAlways associated with hypoxia ↑↑PaCOPaCO22 → ↑respiratory depression → ↑respiratory depression

(CO(CO22 narcosis) → more hypoxia narcosis) → more hypoxia

Respiratory Acidosis Respiratory Acidosis (causes)(causes)

↓↓Respiratory centerRespiratory center Drugs (opioids-anaesthesia)Drugs (opioids-anaesthesia) ICT brain tumours-head injuryICT brain tumours-head injury

Cervical spinal cord lesions: trans-section Cervical spinal cord lesions: trans-section of SCof SC

AHC: e.g. poliomyelitisAHC: e.g. poliomyelitis Chest wall: Chest wall: ##ribs (flail), obesity ribs (flail), obesity Intercostal ms: Myasthenia GravisIntercostal ms: Myasthenia Gravis Lung disease: severe COPD (blue bloater)Lung disease: severe COPD (blue bloater)

Respiratory Acidosis (CP)Respiratory Acidosis (CP)

Acute COAcute CO22 retention: retention: restlessness restlessness flapping tremorsflapping tremors

Further COFurther CO22 rise: rise: VD (warm limbs, papilloedema) VD (warm limbs, papilloedema) Drowsiness, confusion, comaDrowsiness, confusion, coma

Chronic COChronic CO22 retention: retention: poor sleeping →day somnolencepoor sleeping →day somnolence

CyanosisCyanosis

Respiratory Acidosis Respiratory Acidosis (treatment)(treatment)

Depends on the cause and timescaleDepends on the cause and timescale Patent airwayPatent airway Oxygen therapy Oxygen therapy Rx of the cause e.g., Rx of the cause e.g.,

Naloxone (opioid overdose) Naloxone (opioid overdose) Drainage of hydrocephalus Drainage of hydrocephalus fixation of flail ribsfixation of flail ribs

Mechanical ventilationMechanical ventilation

69 yrs, M, smoker, COPD, SOB69 yrs, M, smoker, COPD, SOB pH pH ≈≈ HCO HCO33/CO/CO22


HCOHCO33 32 mmol/L 32 mmol/LBE 7BE 7


Compensated respiratory Compensated respiratory acidosisacidosis

Nurse story, post colecystectomy, Nurse story, post colecystectomy, 40 yrs, fit40 yrs, fit





Metabolic alkalosis Metabolic alkalosis (uncompensated)(uncompensated)

Metabolic alkalosisMetabolic alkalosis ↑↑pH (↓HpH (↓H++)) PaCOPaCO2 2 (35-45 mmHg) normal(35-45 mmHg) normal

↑ ↑ HCOHCO33 (22-26 mmol/l) (22-26 mmol/l) Compensation: Respiratory inhibition Compensation: Respiratory inhibition

but hypoxia limit rise of PaCObut hypoxia limit rise of PaCO22 50 mmHg 50 mmHg


pHpH HCOHCO33-- PaCoPaCo22 CompensationCompensation

↑↑ ↑ ↑↑ ↑ ↑↑ HypoventilationHypoventilation

↑↑ PaCOPaCO22

Metabolic alkalosisMetabolic alkalosis

Loss of gastric HClLoss of gastric HCl Repeated vomiting (pyloric stenosis)Repeated vomiting (pyloric stenosis) Gastric suction Gastric suction

Hypokalaemia (intracellular K moves Hypokalaemia (intracellular K moves out exchange with K (maintain electro-out exchange with K (maintain electro-neutrality)neutrality) diureticsdiuretics

Bicarbonate retentionBicarbonate retention NaHCONaHCO33 administration administration Milk alkali syndromeMilk alkali syndrome

Metabolic alkalosis (CP)Metabolic alkalosis (CP)

The cuaseThe cuase Bradypnea (Chyne Stokes Bradypnea (Chyne Stokes

respiration)respiration) Tetany (↓ionized Ca)Tetany (↓ionized Ca)

Metabolic alkalosis Metabolic alkalosis (treatment)(treatment)

Rx of the causeRx of the cause Cl replacement (Normal Saline Cl replacement (Normal Saline

0.9%)0.9%) Correcion of hypokalaemia (irritant, Correcion of hypokalaemia (irritant,

CVC)CVC) Iv ammonium chloride????Iv ammonium chloride???? Rx of tetany: Ca (gluconate-chloride) Rx of tetany: Ca (gluconate-chloride)

10ml SLOWLY10ml SLOWLY

22 yrs, M, soldier, spasticity22 yrs, M, soldier, spasticity pH pH ≈≈ HCO HCO33/CO/CO22



PaOPaO22 189 mmHg (O 189 mmHg (O22 by face mask) by face mask) Hb 13.6 gm/dlHb 13.6 gm/dl

Respiratory alkalosis Respiratory alkalosis (uncompensated)(uncompensated)

Respiratory alkalosisRespiratory alkalosis ↑ ↑ pH (pH (↓↓HH++))

↓↓PaCOPaCO2 2 (35-45 mmHg)(35-45 mmHg)

HCOHCO33 (22-26 mmol/l) normal (22-26 mmol/l) normal Renal compensation (after 24 Renal compensation (after 24

hours): hours): Increased renal excretion of HCOIncreased renal excretion of HCO33

pH pH ≈≈ HCO HCO33/CO/CO22pHpH HCOHCO33-- PaCoPaCo22 CompensationCompensation

↑↑ ↓↓ ↓↓↓↓ ↑↑Renal excretion of Renal excretion of HCOHCO33

--

Respiratory alkalosis Respiratory alkalosis (causes)(causes)

Hyperventilation (wash out of COHyperventilation (wash out of CO22)) Hysteria Hysteria HyperpyrexiaHyperpyrexia

Iatrogenic hyperventilationIatrogenic hyperventilation

Respiratory alkalosis Respiratory alkalosis (CP)(CP)

The causeThe cause Short lived, well toleratedShort lived, well tolerated ↓↓CaCa++ → carpopedal spasm, → carpopedal spasm,

parasthesiaparasthesia Severe respiratory alkalosis → Severe respiratory alkalosis →

respiratory arrestrespiratory arrest

Respiratory alkalosis Respiratory alkalosis (treatment)(treatment)

Breath into paper bag (rebreathing)Breath into paper bag (rebreathing) Readjust ventilator parametersReadjust ventilator parameters Addition of COAddition of CO2 2 ????????

Base ExcessBase Excess

““The amount of acid or base (mmol/l) The amount of acid or base (mmol/l) required to return the pH of 1L required to return the pH of 1L blood to normal at a PaCOblood to normal at a PaCO22 40 40 mmHg.”mmHg.”

It is a measure of the magnitude of It is a measure of the magnitude of the metabolic component to the acid-the metabolic component to the acid-base disorderbase disorder

Standard BicarbonateStandard Bicarbonate

Not the actual bicarbonate in the Not the actual bicarbonate in the samplesample

““Estimate of bicarbonate Estimate of bicarbonate concentration after elimination of concentration after elimination of any abnormal respiratory any abnormal respiratory contribution to HCOcontribution to HCO33 (i.e., an (i.e., an estimate of HCOestimate of HCO33 at PaCO at PaCO22 40 40 mmHg).mmHg).

Thank YouThank You

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Date post:	07-May-2015
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Acid base

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