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Download MBBS General Surgery PPT 1 Arterial Blood Gas Lecture Notes

Download MBBS (Bachelor of Medicine, Bachelor of Surgery) General Surgery PPT 1 Arterial Blood Gas Lecture Notes

This post was last modified on 07 April 2022


ABG

(Arterial Blood Gas)

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The basic physiology of acid ?base balance

? Our body functions in a relatively narrow alkaline environment

pH: 7.35-7.45

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? Normal physiologic function = the maintenance of pH within this range.

? Two main mechanisms ? Respiratory and Metabolic.

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? If pH <7.35, the blood is said to be acidic.

? If pH >7.45, the blood is said to be alkalotic.
The respiratory buffer response

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? Carbon dioxide (CO2) is a normal by-product of cel ular metabolism.

? Partial pressure of CO2 in arterial blood (paCO2) is determined by alveolar ventilation.

? The excess CO2 combines with water to form carbonic acid.

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? The blood pH changes according to

Amount of carbonic acid in the body i.e. the depth and rate of ventilation.

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? As blood pH decreases (acidosis), CO2 is exhaled (alkalosis as compensation).

? As blood pH increases (alkalosis), CO2 is retained (acidosis as compensation).

? The respiratory response is fast and activated within minutes.

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The renal buffer response

? The kidneys secrete Hydrogen ion (H+) and reabsorbs bicarbonate.

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? In response to metabolic acid formation.

? Bicarbonate is a metabolic component and considered a base.

? As blood pH decreases (acidosis), the body retains bicarbonate (a base).

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? As blood pH rises (alkalosis), the body excretes bicarbonate (a base) in urine.

? This compensation is slow and takes hours to days to get activated.
The acid-base control

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? The pH is dependent on the paCO2 /HCO3- (bicarbonate) ratio.

? A change in CO2

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compensated by a change in HCO3- and vice

versa.

? The initial change is cal ed the primary disorder.

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? The secondary response is cal ed the compensatory disorder.

Basic facts to remember

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? CO2 is a respiratory component and considered a respiratory acid.
? Moves opposite to the direction of pH and is visualized as a see-saw
Basic facts to remember.......

? Bicarbonate - A metabolic component and considered a base.

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? It moves in the same direction as pH and is visualized as an

elevator

Basic facts to remember.......

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? If CO2 and HCO3- move in the same direction, it is considered a primary

disorder.

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? For example, if there is respiratory acidosis in body (CO2 retention), the bicarbonate

levels increase as a compensation (metabolic alkalosis). The direction of both CO2 and
HCO3- are the same in this case.

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? If CO2 and HCO3- move in opposite directions, it is considered a mixed disorder.

? For example, mixed disorder in the case of salicylate poisoning: Primary respiratory

alkalosis due to salicylate-induced hyperventilation and a primary metabolic acidosis due

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to salicylate toxicity.
Conditions causing acid-base imbalance

? Respiratory acidosis

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? Respiratory alkalosis

? Any condition causing the

? Due to decrease in CO2 .

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accumulation of CO2 in the body.

Hyperventilation occurs and CO2 is

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? Central nervous system (CNS)

washed out causing alkalosis.

depression due to head injury

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? Sedation, coma

? Psychological: Anxiety, fear

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? Chest wal injury, flail chest

? Pain

? Respiratory obstruction/foreign body

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? Fever, sepsis, pregnancy, severe

anemia.

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Conditions causing acid-base imbalance

? Metabolic acidosis -due to excess of acids

? Metabolic alkalosis - caused by excess

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or deficit of base.

base or deficit of acids.

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? Increased acids

? Acid Deficit:

? Lactic acidosis (shock, haemorrhage, sepsis)

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? Prolonged vomiting, nasogastric suction,

? Diabetic ketoacidosis

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diuretics

? Renal failure

? Excess base:

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? Deficit of base

? Severe diarrhoea

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? Excess consumption of diuretics and antacids

? Intestinal fistulas.

? massive blood transfusion (citrate metabolized to

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bicarbonate).
Arterial blood gas analysis

? Important routine investigation to monitor

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the acid-base balance of patients
effectiveness of gas exchange

? A vital role in monitoring of

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Postoperative patients,
Patients receiving oxygen therapy,

Those on intensive support,

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Patients with significant blood loss, sepsis, and comorbid conditions like diabetes,

kidney disorders,

Cardiovascular system (CVS) conditions

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Why do we order a blood gas analysis?

Aids in establishing diagnosis

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Guides treatment plan

Improvement in the management of acid/base; allows for optimal function of medications

Acid/base status may alter levels of electrolytes critical to the status of a patient.

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? Limitations of blood gas analysis

Can not yield a specific diagnosis. (e.g. A patient with asthma may have similar values to

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another patient with pneumonia).

Does not reflect the degree to which an abnormality actually affects a patient.

Cannot be used as a screening test for early pulmonary disease.

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Arterial vs Venous blood gas analysis

Arterial vs Venous blood gas analysis

? If the venous sample is obtained

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Values compared and interpreted keeping in consideration.

Significance in hemodynamically unstable patients and should not be

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discarded.
Obtaining an arterial sample

? Order of preference:

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Radial > brachial >femoral

artery.

? Radial artery is preferred:

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ease of palpation and

access

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good collateral supply.

? Collateral supply to the

hand:

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Confirmed by the modified

Allen's test

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Modified Allen's test
? Ask the patient to make a tight fist.
? Apply pressure to the wrist:

Using the middle and index fingers of both hands

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Compress the radial and ulnar arteries at the same time

? While maintaining pressure:

ask the patient to open the hand slowly.

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Lower the hand and release pressure on the ulnar artery only.

? Positive test:

The hand flushes pink or returns to normal color within 15 seconds

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? Negative test:

The hand does not flush pink or return to normal color within 15 seconds
indicating a disruption of blood flow from the ulnar artery to the hand

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radial artery should not be used.
Sampling
? Arm of the patient

? palm up on a flat surface

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? wrist dorsiflexed at 45?.

? Puncture site :

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? cleaned with alcohol or iodine

? al ow the alcohol to dry before puncture, as the alcohol can cause

arteriospasm

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? local anesthetic (such as 2% lignocaine)

? Radial artery should be palpated for a pulse

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? A preheparinised syringe with a 23/25 gauge needle should be

inserted at an angle just distal to the palpated pulse.

? After the puncture, sterile gauze should be placed firmly over the

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site and direct pressure applied for several minutes to obtain
hemostasis.

Errors

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? Allow a steady state after initiation or change in oxygen therapy before obtaining a sample

? a steady state is reached between 3 and 10 minutes.
? in patients with chronic airway obstruction, it takes about 20-30 minutes.

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? Always note the percentage of inspired air (FiO2 ) and condition of the patient
? Do not use excess heparin as

? it causes sample dilution
? Excess of heparin may affect the pH.

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? Avoid air bubbles in syringe.
? Avoid delay in sample processing.

? As blood is a living tissue, O2 is being consumed and CO2 is produced in the blood sample.

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? In case of delay, the sample should be placed in ice and such iced samples can be processed for up to two hours

without affecting the blood gas values.

? Accidental venous sampling. The venous sample report should not be discarded and can provide

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sufficient information.
Steps of interpretation

? Step 1: Anticipate the disorder

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? keeping in mind the clinical settings and the condition of the patient

? e.g., the patient may present with a history of insulin-dependent diabetes mellitus (IDDM),

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which may contribute to a metabolic acidosis

? Step 2: Check the pH.

? pH < 7.35: Acidosis

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? pH > 7.45: Alkalosis

? pH = 7.40: Normal/mixed disorder/fully compensated disorder

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? (Note: If mixed disorder, pH indicates stronger component)

Steps of interpretation..............

? Step 3: Check SaO2 /paO2

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SaO2 is a more reliable indicator as it depicts the saturation of hemoglobin in arterial

blood.

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Note: Always compare the SaO2 with FiO2

. the SaO2 could be within normal range but stil much less than FiO2 if the patient is on supplemental

oxygen (difference should be less than 10)

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Steps of interpretation...........

? Step 4: Check CO2 and HCO3 - (bicarbonate) levels-

Identify the culprit

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Is it a respiratory/metabolic/mixed disorder?

Steps of interpretation...........

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? Step 5: Check base excess (BE).

? Defined as amount of base required to return the pH to a normal range.
? If it is positive, the metabolic picture is of alkalosis.
? If it is negative, the metabolic picture is of acidosis.

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? Either of bicarbonate ions/base excess can be used to interpret

metabolic acidosis/alkalosis.
Interpretation of arterial blood gas report on the basis of

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using BE as a metabolic index

Steps of interpretation...........

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? Step 6: Check for compensation.
? Is there a compensatory response with respect to the primary

change?

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? If yes: Compensated
? if no: Uncompensated.

? In case of compensation, does it bring the pH to a normal range?

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? If yes: Fully compensated
? if no: Partially compensated.
Example: 1
? If pH is 7.21, HCO3- is 14, and CO2 is 40.

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? CO2 is normal

? HCO3- is decreased

Example: 1

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? If pH is 7.21, HCO3- is 14, and CO2 is 40.

? CO2 is normal
? HCO3- is decreased

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? A case of metabolic acidosis.
? Expected compensation would be a decrease in CO2 causing respiratory

alkalosis.

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? Now consider this table ---
Example: 2
? pH: 7.55, paCO2: 49.0, HCO3 : 48.2

? pH: 7.55

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alkalosis

? paCO2: 49.0

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increased

? HCO3: 48.2

increased

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? paCO2 is increased - retention of CO2 causes acidosis
? HCO3 is increased - increased base causes alkalosis
? So, the primary disorder is metabolic alkalosis.

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? CO2 is being retained to compensate for the same-

? the pH has stil not returned to a normal range.

? So, the interpretation - Partially Compensated Metabolic Alkalosis

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Example 3

? pH: 7.34, paCO2 40.3, HCO3 : 20.4.

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? The pH is acidic
? paCO2 is normal
? Bicarbonate is decreased.

? Primary disorder is metabolic acidosis

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? but no compensatory response as the paCO2 is normal.

? Interpretation - Uncompensated Metabolic Acidosis
Example 4

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? pH: 7.52, paCO2 : 31.0, HCO3 : 29.4

? pH is alkalotic
? paCO2 is decreased (alkalosis)

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? Bicarbonate is increased (alkalosis).

? As the directions of paCO2 and bicarbonate are opposite and both are

causing alkalosis.

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? The picture is suggestive of a mixed disorder.
? Interpretation - Combined Alkalosis