? 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 viceversa.
? 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 anelevator
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 bicarbonatelevels 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 planImprovement 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 analysisArterial 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 >femoralartery.
? 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 beinserted 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 hourswithout 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