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Acid Base Balance and ABG Interpretation. Dr Kim Khaw Anaesthesia and Intensive Care. Objectives. How to perform ABG sampling How to Interpret ABG How to Make Use of ABG Clinical Diagnosis Clinical Implications of Disturbance Management Monitoring. Where is Arterial Blood Sampled from?.

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Acid base balance and abg interpretation

Acid Base Balance and ABG Interpretation

Dr Kim Khaw

Anaesthesia and Intensive Care


Objectives
Objectives

  • How to perform ABG sampling

  • How to Interpret ABG

  • How to Make Use of ABG

    • Clinical Diagnosis

    • Clinical Implications of Disturbance

    • Management

    • Monitoring


Where is arterial blood sampled from
Where is Arterial Blood Sampled from?

  • Any artery in the body……

    ………..EXCEPT Pulmonary Artery

  • Commonly from:

    • Radial artery- Non dominant arm

    • Femoral artery

    • Brachial artery


Radial artery
Radial Artery

  • Easily accessible

  • Collateral circulation

  • Confirm prior to procedure

    • Allen’s Test




Arterial sampling
Arterial Sampling

  • Single arterial stab

  • Insertion of Arterial Catheter





Arterial sampling1
Arterial Sampling

  • Single arterial stab

  • Insertion of Arterial Catheter


Arterial catheter
Arterial Catheter

Monitoring:

  • Arterial Blood Pressure

  • Arterial Blood Gas


Handling arterial blood
Handling Arterial Blood

  • Kept in ice at 4 ºC

  • Analyzed ASAP

    • Red and white cells continue to metabolize oxygen and produce CO2


Arterial blood gas analysis
Arterial Blood Gas Analysis

  • Blood gas analyser

PICTURE



Complications of arterial sampling
Complications ofArterial Sampling

  • Haematoma

  • Thrombosis

  • Ischaemia

  • Infection


Basic medical science data interpretation
Basic Medical Science – Data Interpretation

  • Foundation for Data Interpretation

  • Basic Diagnosis of Acid base Status:

    • pH

    • PaCO2

    • HCO3

  •  Abnormality


Abg parameters
ABG Parameters

  • pH

  • PCO2

  • PaO2

  • HCO3 –a –- actual bicarbonate

  • HCO3 –s –- standardised bicarbonate

  • BE - Base Excess


pH

  • A way of expressing ACIDITY

  • Better application for chemistry than Physiology

  • Negative logarithm of hydrogen ions

    pH = - log10 [H+]


pH

Biological range is: 20 - 160 nmoles/l

Normal range is 35 - 45 nmoles/l (pH 7.35 - 7.45)

pH 6.8 7.1 7.4 7.7

[H+] 160 80 40 20

(nmol/l)

DOUBLING of [H+] changes pH by 0.3


pH

  • Physiologically is influenced by MANY substances

  • BUT we interpret the ABG by looking at only a FEW measurable parameters :-

    => PCO2

    => HCO3/BE (Buffers)


Paco 2
PaCO2

  • Body production is constant 200-250ml/min

  • Kept in narrow range by respiratory centre:-


Paco 2 ph
PaCO2 & pH

Biological range: 2.7 - 10.6 kPa

Normal range: 4.6 - 6.0 kPa

pH 7.2 7.3 7.4 7.5 7.6

PaCO2 (kPa) 10.6 8 5.3 4 2.7

pH change by 0.1 for every

 PaCO2 2.6kPa  pH by 0.1 (Resp. Acidosis)

 PaCO2 1.3kPa  pH by 0.1 (Resp. Alkalosis)


Actual bicarbonate hco 3 a
Actual Bicarbonate - HCO3-a

  • HCO3-a (Actual value measured)

  • Influenced by:

    Metabolic acids (or base)

    Acid + HCO3-  Salt + Water

    Carbon dioxide

    CO2 + H2O  H2CO3  H+ + HCO3-


New parameters
New Parameters

  • Standardised Bicarbonate

  • Base Excess

  • PaO2


Standardised bicarbonate
Standardised Bicarbonate

  • Derived from the actual bicarbonate

  • Imaginary value assuming a standard PaCO2 of 5.3 kPa

  • Thus the influence from CO2 is standardised or eliminated

  • The value reflects purely metabolic influence


Base excess
Base Excess

Definition:

  • Amount of titratable base in the blood to neutralize to a pH of 7.4 (at PaCO2 of 5.3kPa and temp of 37º C)

    Implications:

  • If it is +ve → Alkalosis

  • If it is –ve → Acidosis

  • Normal range  2

  • Clinically significant if  7 mmol/l


Base excess standardised hco 3
Base Excess & Standardised HCO3-

  • Standardised HCO3- & Base Excess reflects specifically:

    Metabolic Acidosis or Alkalosis


Pao 2
PaO2

  • Partial Pressure of Oxygen in blood

  • Relative to the inspired Oxygen %:

    • “Index of oxygenation of the blood”

    • PaO2 = PiO2 - PaCO2/RQ + k

      (Alveolar Gas Equation)


Alveolar gas equation pao 2
Alveolar Gas Equation - PaO2

  • For Practical Purposes:

    PaO2 (kPa) = O2 (%) – 10


Alveolar gas equation
Alveolar Gas Equation

PaO2 = PiO2 - PaCO2/RQ + k

  • PaO2 = FiO2 - 10 kPa

    = % Inspired O2 - 10

    Breathing Air:

  • PaO2 =21 - 10 kPa

  • PaO2 =11 kPa

    Breathing 40% oxygen:

  • PaO2 =40 - 10 kPa

  • PaO2 =30 kPa


Anion gap
Anion Gap

The blood plasma is electrically neutral. Therefore the sums of the positive and negative charges are equal:


Anion gap1
Anion Gap

  • [Na + K] - [HCO + Cl]

    • If >16  Fixed Acidic Material in Body

  • Na - [HCO + Cl]

    • If >12  Fixed Acidic Material in Body


Summary
Summary

Standardised Bicarbonate

  • Metabolic status without respiratory influence

    Base Excess

    Metabolic status:

  • Alkalosis (+ve)

  • Acidosis (-ve)

    Partial Pressure of Oxygen

  • Should be Inspired Oxygen % - 10


Clinical diagnosis
Clinical Diagnosis

Acid Base Status:

  • What is the pH abnormality?

  • Is it Respiratory or Metabolic

  • What is the Magnitude?

  • Is there any compensatory changes?

    • If not, WHY NOT?

  • Are there any tricks?


Clinical diagnosis1
Clinical Diagnosis

Gas Exchange Status:

  • Oxygenation

    • Is there any abnormality?

    • PaO2 = Inspired O2% -10

  • Ventilation

    • Is there any abnormality?

    •  PaCO2= Hypo ventilation

    • PaCO2= Hyper ventilation


Case 1
Case 1

In a class demonstration, medical students have to take arterial blood sample from an unfortunate colleague who ‘volunteered’. He is perfectly healthy otherwise.


Case 2
Case 2

A 74-year-old lady complained of pain one hour after a total hip replacement surgery. She was given an injection of morphine for analgesia. Shortly afterwards she was noted to be drowsy and the house officer took an arterial blood for investigation.


Actual bicarbonate hco 3 a1
Actual Bicarbonate - HCO3-a

Carbon dioxide

CO2 + H2O  H2CO3  H+ + HCO3-

  • Alveolar Gas Equation

  • PaO2 = PiO2 - PaCO2/RQ + k


Case 3
Case 3

A patient who suffered from gastroenteritis had been vomiting for several days.


Case 4
Case 4

A pedestrian who was involved in a road traffic accident was brought to the emergency department bleeding heavily from a compound fracture of the femur. She was hypotensive, cold and clammy on arrival, and was immediately taken to the operating theatre for surgery.


Case 5
Case 5

An elderly patient with chronic obstructive airways disease presented to the emergency department with increasing dypsnoea.


Case 6
Case 6

A 16-year-old girl with insulin-dependent diabetes presents with fever, lethargy and shortness of breath. She has not taken insulin for the past two days. Her arterial blood shows:


Case 7
Case 7

A young man was hit by a truck and brought to the emergency department in a comatosed state. An arterial blood shows:



Stroke pathophysiology
StrokePathophysiology

  • Subarachniod Haemorrhage

  •  Intracranial pressure

  •  Cerebral Blood Flow


Management
Management

Principles:

  • Cerebral Blood flow  Perfusion

    Pressure

  • Perfusion Pressure  MAP - ICP

    Management:

  • Evacuation of blood clots

  • Reduce the ICP – hyperventilation,

    drugs, drain CSF

    Role of ABG:

  • PaCO2 as guide to ventilation

  • Arterial catheter measuring MAP



Renal failure1
Renal Failure

  • Metabolic acidosis -  anion gap

    • Unable to excrete acid by-products (metabolism of amino acids)

  • Role of ABG:

    • Monitor Acidosis or alkalosis post dialysis



Severe burns1
Severe Burns

Lactic acidosis

  • Massive fluid loss – reduced perfusion to tissues

  • Hypermetabolic state

  • Septicaemia

  • Intravascular haemolysis


Severe burns2
Severe Burns

Impaired gas exchange

  • Inhalation injury

  • Airway obstruction

  • Carbon Monoxide or Cyanide toxicity

  • Nosocomial pneumonia

  • ARDS (adult respiratory distress syndrome)


Severe burns3
Severe Burns

Role of ABG analysis

Severity of metabolic acidosis

  • Onset of complications

  • Improvement or deterioration

    Gas exchange problems

  • Onset of ARDS, pneumonia or improvement



Multiple trauma1
Multiple Trauma

  • ABC

    • Airway compromise

    • Breathing difficulty

    • Cardiovascular compromise

  • Role of ABG

    • Monitor gas exchange

      • PaO2 PaCO2

    • Monitor perfusion

      • Base excess



Spinal cord injury1
Spinal Cord Injury

Problems

  • Loss of motor nerve output

  • Paralysis of muscles of respiration

     Respiratory failure

    Role of ABG

  • Monitor PaO2 & PaCO2

  • Alert to ventilatory failure



Heart failure pulmonary odema1

Lung is congested –impairment of gas exchange

Role of ABG

Monitor PaO2

Monitor Metabolic acidosis

Heart failure – pulmonary odema


Summary1
Summary

  • Refinement of ABG interpretation skills

  • Understanding

    • How ABG is taken

    • How ABG is measured

    • Why ABG is important for your patients

    • How to interpret ABG in different clinical settings


Coming soon

Coming Soon

Management of Hypoxia


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