Acid-Base Balance:- Part 4 – Arterial Blood gases (Blood Gases), Acid-Base balance Mechanism
Arterial Blood Gases
Sample
- The better choice is the Radial artery.
- Can draw the blood from the femoral artery or brachial.
- Blood can be drawn from the indwelling arterial line.
- The tests are done immediately because oxygen and carbon dioxide are unstable.
- Place the sample on ice and immediately transfer it to the lab.
- Arterial blood is better than venous blood.
- For venous blood syringe or tube is filled, and apply a tourniquet for a few seconds.
- Arterial blood is risky, and a trained person should do it.
- Never apply a tourniquet.
- Don’t apply the pull to the plunger of the syringe.
Arterial vs. Venous blood
Arterial blood gases (ABG):
- It gives a good mixture of blood from various areas of the body.
- Arterial blood color is bright red.
- Arterial blood measurement gives a better status of lung oxygenation.
- If arterial O2 concentration is normal, indicate lung function is normal.
- If mixed venous O2 concentration is low, indicating the heart and circulation are failing.
- Arterial blood gives information about the lung’s ability to regulate the acid-base balance through the
retention or release of CO2.- Can also be checked the effectiveness of the kidneys in maintaining the appropriate bicarbonate
level.
- Can also be checked the effectiveness of the kidneys in maintaining the appropriate bicarbonate
Venous blood gases (VBG)
- It gives information about the local area from where the blood sample is taken.
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- Venous blood color is dark red.
- Metabolism of the extremity varies from area to area.
- In shock, the extremities are cold, and less blood perfusion.
- During the local exercise of the extremities, such as opening and closing the fist with power.
- In case if there is an infection of the sample area.
- A blood sample from the central venous catheter is not a good mix of blood from various parts of the
body. For well-mixed blood sample should be taken from the right ventricle or the pulmonary artery, which
is not an easy procedure. - A blood sample from the central venous catheter:
- Shows low O2 concentration, which means that:
- Either the lungs have not oxygenated the arterial blood well.
- Or the Heart is not circulating the blood effectively.
- Difference between arterial and venous blood:
Biochemical parameters Arterial blood Venous blood Use For blood gases For all routine lab test Color Bright red Dark red pH 7.35 to 7.45 (7.40) 7.32 to 7.43 (7.37) pCO2 mmHg 35 to 45 (40) 41 to 51 (45) CO2 contents meq/L 22 to 28 (25) 24 to 30 (27) Bicarbonate (HCO3–) - 22 to 28 mmol/L
- 20 to 28 meq/L
- 23 to 29 mmol/L
- 22 to 30 meq/L
pO2 mmHg 80 to 100 37 to 43 (40) O saturation 95% 70 to 75%
- Shows low O2 concentration, which means that:
Precautions for the collection of blood:
- Avoid pain and anxiety in the patient, which will lead to hyperventilation.
- Hyperventilation due to any cause leads to decreased CO2 and increased pH.
- Keep blood cool during transit.
- Don’t clench your finger or fist. This will leads to lower CO2 and increased acid metabolites.
- pCO2 values are lower in the sitting or standing position in comparison with the supine position.
- Don’t delay the performance of the test.
- Avoid air bubbles in the syringe.
- Excess of heparin decreases the pCO2 by maybe 40% less.
- Not proper mixing of the blood before running the test may give a false result.
- A prolonged tourniquet or muscular activity decreases venous pO2 and pH.
- The best way to collect arterial or venous blood is anaerobic.
- Arterial blood precautions:
- Only syringe and needle, no tourniquet, no pull on the plunger.
- Venous blood precautions:
- Needle and syringe of the heparinized evacuated tube filled, drawn a few seconds after the
tourniquet. - Liquid heparin is the only suitable anticoagulant with the proper amount.
- Less amount will lead to clot formation.
- The increased amount will lead to an increase in CO2 and a decrease in pH.
- This will leads to a dilutional error.
- Glass collection devices are better than plastic.
- Needle and syringe of the heparinized evacuated tube filled, drawn a few seconds after the
Purpose of the test (Indications)
- This test is done on mostly hospitalized patients.
- Mostly the patients are on a ventilator or unconscious.
- It monitors critically ill nonventilator patients.
- For patients in pulmonary distress.
- To assess the respiratory (ventilation), metabolic (renal) acid/base, and electrolyte imbalance.
- Its primary use is to monitor arterial blood gases and the pH of the blood.
- Also used to monitor oxygenation.
- Used to qualify a patient for the use of oxygen at home.
- This is used as preoperative baseline parameters.
Precautions
- Avoid in patients with coagulopathy.
- Avoid in a patient with AV fistula.
Definition of the arterial blood gases
- Arterial blood gases are a common test that provides useful and potentially life-saving treatment.
- The following parameters are measured, and the rest are calculated:
- pH will give us information about the acid-base balance.
- pO2 will tell us oxygenation (ventilation).
- pCO2 will also tell us about the acid-base balance.
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- pH = 7.35 to 7.45.
- pO2 = >80 mm Hg.
- pCO2 = 35 to 45 mm Hg.
- The above three parameters are really needed to help the patients with their management.
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Acid-base system:
The H+ ions concentration is commonly expressed as the pH.
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- The buffering system becomes active in response to a change in the acid-base status.
- Buffers can absorb excessive H+ (acid) or OH‾ (base) without any
change in the pH.
- The body normally maintains the arterial blood pH within a definite range of 7.35 to 7.45.
- The H+ ions concentration must be regulated within the narrow range for the body to maintain the body’s
normal functionality.- The slight change in the H+ concentration will change the body pH.
- H+ is needed for the :
- For the maintenance of cell integrity.
- It helps in the speed of the enzymatic reaction.
pH maintained by the interaction of three buffer systems in our body are:
The buffer system
- It works through the retention or excretion of the H+.
- Other minor buffer systems are phosphate and proteins.
The respiratory buffer system:
- It works through Carbonic acid-bicarbonate (H2CO3 – HCO3¯).
This will cover 80% of the pH control, and the renal system does the rest. - There is a ratio of one part of Carbonic acid (H2CO3) and twenty parts of bicarbonate.
(HCO3−).Buffer system, carbonic acid, and bicarbonate
- The carbonic acid level can be measured indirectly by measuring the pCO2 level.
- The lung controls the pCO2.
- More CO2 retained and more H2CO3 will lead to acidosis.
- Less CO2 and there will be fewer H2CO3 will lead to alkalosis.
Bicarbonate (20 parts) / carbonic acid (one part):
- = (HCO3 / H2CO3) is the most important buffering system.
- This buffer system works in the lungs and as well as in the kidneys.
- The greater the CO2 partial pressure pCO2, the more carbonic acid (H2CO3) forms.
Can express this relationship as :
H2CO3 = 0.03 x pCO2 (mm Hg)
0.03 represents the solubility coefficient for the CO2 in the water.
The pCO2 of the arterial blood is around 44 mm Hg. Therefore the amount of H2CO3 is
equal to 1.2 mmol/L = 0.03 x 40 = 1.2 mmol/L
Acids or chemical substances can donate H+ ions.
- Bases or substances that can accept H+ ions.
- Strong acids readily give up H+, whereas strong base readily accepts H+.
- Respiratory and metabolic disorder depends on the correct measurement of:
- O2
- CO2
- Acid/base assessed by:
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- Total CO2
- Plasma pH
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- pCO2
- The lungs can decrease the amount of H2CO3 by blowing off the CO2 and leaving H2O.
H2CO3 → CO2 + H2O - The kidneys can reabsorb HCO3¯or regenerate new HCO3¯ from the CO2 and water.
H2O + CO2 → HCO3¯ + H+
Renal regulation is slow, / Pulmonary regulation is fast.
- The lungs can decrease the amount of H2CO3 by blowing off the CO2 and leaving H2O.
The renal system:
- It works through excretion or retention of H+, HCO3–, Na+, K+, and Cl¯.
- The distle tubule of the kidney regulates the acid-base balance.
- It secretes H+ ions into the urine and reabsorbs the HCO3¯.
- H2PO4¯ and NH4+ are also secreted into the urine.
The body acids exist in two forms:
- Volatile can be eliminated as CO2.
- Nonvolatile. are sulphuric acid, phosphoric acid, and other organic acids. These are produced by the metabolism of protein, carbohydrates, and fats.
- The volatile acid is carbonic acid (H2CO3) which will form by the hydration of the CO2.
In short, lungs and kidneys, with the help of buffer systems, are the prime regulator of acid-base
balance.
- The respiration process supplies oxygen to the tissues and removes the carbon dioxide produced by cellular
metabolic activity. - External respiration:
- Where oxygen in the air is exchanged at the alveolar level with carbon dioxide in the blood.
- Internal respiration:
- Take place at the tissue level, where oxygen in the blood is delivered to the cells, and
carbon dioxide is transferred from the cells to the blood for disposal.
- Take place at the tissue level, where oxygen in the blood is delivered to the cells, and
- The medullary center, the brain stem, controlled respiration by increasing CO2 and decreasing O2.
Hydrogen (H+) ions and pH:
- The H+ ions concentration is commonly expressed as the pH, The negative logarithm of H+ ions in the
solution. - The logarithm value means that if the pH changes from 7 to 6, then the H+ ions change tenfold.
- As the H+ ions increase, the pH decreases; likewise, if the H+ions decrease, then pH increases.
- The more H+ ions =, the more acidic solution, and the lower the pH.
- The lower the H+ ions =, the more basic solution and higher pH.
- pH <7.4 = acidic.
- pH >7.4 = basic.
Different body fluids pH value Reason for the pH Arterial blood 7.38 to 7.42 There is less H2CO3 Venous blood 7.37 There is more H2CO3 Cerebrospinal fluid 7.32 Decrease HCO3¯ and increased CO2 Urine 5.0 to 6.0 Increased H+ ions excretion Pancreatic fluid 7.8 to 8.0 There are HCO3¯ Gastric contents 1.0 to 3.0 HCL production - Henderson- Hasselbalch equation give the idea about the blood gas measurement.
- Henderson-Hasselbalch equation = pH = pKa + log (base/acid)
- pH calculation formula:
- Where pH = 7.4
- pKa = 6.1
- Base (bicarbonate) = 24
- Acid = dissolved PaCO2 = 0.03 x PaCO2 = 0.03 x 40 = 1.2
- pH = 7.4 = 6.1 + log(24/1.2) = 6.1 + 1.3 = 7.4
- The body makes every effort to maintain the validity of the above equation.
- Our body will maintain a pH of 7.4, and pK is constant to alter the bicarbonate and paCO2
(H2CO3). - The pH is dependent upon the total concentration of:
- CO2.
- HCO–3.
- Dissolved CO2.
- H+ ions.
- All these are interrelated.
- The body normally maintains the arterial pH between 7.35 to 7.45. This takes place through the buffer system of bicarbonate.
Buffer pairs | Buffer system | pK value | Chemical reaction |
HCO3¯ / H2CO3 | Bicarbonate/carbonic acid | 6.1 | H+ + HCO3 >< H2O + CO 2 |
HPO4- /H2PO4- | Phosphate | 6.8 | H2PO4 ↔ H+ + HPO4- |
Hb / HHb | Hemoglobin | 7.3 | HHb ↔H+ + Hb |
Pr- /HPr | Protein in blood | 6.7 | HPr ↔ H+ + Pr- |
Common acid/base disorders examples are:
- Lactic acidosis and diabetic ketoacidosis.
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- Intermediate organic acids are lactic acid and β-hydroxybutyric acid.
- The above acids are metabolized to CO2 and water.
- These may accumulate and cause acidemia.
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- Body acids are formed as end products of cellular metabolism. The average person generates acid 50 to 100 meq/day from the metabolism of protein, carbohydrates, and fats and stool loss.
- To neutralize this acid formation, the body needs to maintain the pH in the range; in that case, an equal amount of acid needs to be neutralized or excreted.
Now our various buffer systems come into play like:
- Retention or excretion of H+ ions.
- Respiratory system.
- Renal system.
- These systems are interrelated and work together.
Blood arterial gases
pH
- The pH of a solution is the negative logarithm of the hydrogen ion activity.
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- pH = -logaH+.
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- The acid-base status of the body is assessed by:
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- pH.
- pCO2.
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- While blood passes through the lung, O2 moves to the blood, and CO2 goes into the lung.
- As the blood hydrogen concentration increases, the pH decreases, and if hydrogen ions decrease, the pH increases.
- The decrease of one unit of pH represents a 10 times increase in H+ activity.
- The average pH of the blood of 7.40 is equal to the H+ ions concentration of 40 nmol/L.
- The lungs and the kidneys regulate the pH of the plasma.
- The acids found in the blood are carbonic acid (H2CO3), dietary acids, keto acid, and lactic acid.
- pH indicates acidity and alkalinity.
- In respiratory or metabolic alkalosis, the pH will be high.
- Respiratory acidosis or metabolic acidosis will decrease the pH value.
- pH alkaline when it is >7.4.
- pH acidic when it is <7.35.
- Acidemia = pH <7.35
- Alkalemia = pH >7.45
pCO2 (Partial pressure of the carbon dioxide CO2)
- pCO2 measures the partial pressure of CO2 gas in the blood (arterial blood, plasma, or serum) measured in mm Hg.
- This is proportional to the amount of dissolved CO2 or the concentration of the CO2.
- pCO2 is a measurement of ventilation capability.
- The units are the same as pO2.
- pCO2 in the blood is 10% in the plasma and 90% carried by the red blood cells.
- With respiration, CO2 is breathed out, and pCO2 level drops will depend upon the breathing rate.
- The faster and more deeply one breaths, the more CO2 is blown off, and the pCO2 level drop.
- pCO2 is referred to as the respiratory component in acid-base determination because the lungs control this value.
- As the CO2 level increases, the pH level will decrease.
- The pCO2 level in blood and CSF is a major stimulant to the breathing center in the brain.
- As the pCO2 level rises, breathing is stimulated.
- When the brain can not cope with increased pCO2 and cannot blow off excess CO2, the brain is depressed, ventilation decreases, and the patient goes into a coma.
- In metabolic acidosis, the lungs try to compensate by more blowing of CO2 to raise pH.
- In metabolic alkalosis, the lungs try to compensate by retaining the CO2 to lower pH.
HCO3 or CO2 content
- Method to measure CO2 contents:
- The total CO2 contents are determined from the heparinized arterial or venous blood drawn anaerobically.
- This can be done in a vacuum tube or a syringe which is quickly capped.
- The blood is centrifuged and plasma is separated.
- Next, plasma is analyzed for CO2 by converting HCO3– and H2CO3 to the gas form.
- Most of the CO2 contents are HCO3¯ in the blood because the dissolved amount of the CO2 and H2CO3 contents are very small.
- The HCO3– ions can be measured directly as HCO3– or indirectly by CO 2 contents.
- Total CO2 = HCO3¯ + Dissolved CO2.
- The most important buffer system of the plasma is HCO3¯ / H2CO3.
- It is also present in the RBC but at a lower concentration.
- The ratio of base: acid = 20: 1 in plasma.
- The kidney regulates HCO3¯ ions, and it is the measure of the metabolic (Renal) component of the acid-base balance.
- CO2 contents should not be confused with pCO2.
- CO2 contents are indirectly measured by HCO3¯.
- HCO3¯ : Dissolved CO2 = 25 : 1
- Any change in the above equation leads to a change in the pH.
- As the HCO3¯ level increase and the pH also increases.
- The HCO3¯ level:
- In metabolic alkalosis, the HCO3– level is elevated.
- In metabolic acidosis, the HCO3– level is decreased.
- In respiratory acidosis, kidneys attempt to compensate for increased reabsorption of HCO3¯.
- In respiratory alkalosis, kidneys excrete an increased amount of HCO3¯ to lower the pH.
- In metabolic alkalosis, the HCO3– level is elevated.
Clinical conditions | pH | Bicarbonate (HCO3) level |
Metabolic acidosis | decreased | decreased |
Metabolic alkalosis | increased | increased |
Respiratory alkalosis | increased | decreased |
Respiratory acidosis | decreased | increased |
pO2
- Oxygen in the blood is carried in two forms:
- Dissolved in plasma = <2%.
- Combined with hemoglobin = 98%.
- This partial pressure of the oxygen gas determines the force it exerts in attempting to diffuse through the pulmonary membrane.
- The pO2 reflects the amount of oxygen passing from the pulmonary alveoli to the blood.
- pO2 is the measure of the pressure of O2 present in the plasma.
- pO2 is the indirect measure of O2 contents of arterial blood.
- The pO2 level is decreased in:
- Pneumonia.
- Shock lung.
- Congestive heart failure.
- Congenital heart diseases.
- Patient under-ventilated.
O2 saturation
- O2 saturation indicates % of hemoglobin saturated with oxygen. OR:
- This measurement is the ratio between the actual O2 content of the hemoglobin and the potential maximum carrying capacity of the hemoglobin.
- O2% saturation is the percentage indicating the relationship between O2 and hemoglobin.
- This is not the O2 content.
- The combined measurement of:
- O2 saturation.
- pO2.
- Hemoglobin.
- This indicates the amount of O2 available to the tissues for oxygenation.
- When hemoglobin 92 to 100% carries O2, then perfusion or oxygen supply to the tissue is normal.
- With the decrease of the pO2 level, the saturation of hemoglobin also decreases.
- When the O2 saturation is 70% or low, the tissues cannot get adequate oxygen.
- Precaution:
- Please avoid smoking or exposure to close second-hand smoke or CO (carbon monoxide). In such cases, the COHb level is increased.
- Avoid the use of paint or varnish.
O2 content
- The actual amount of O2 in the blood is termed the O2 content.
- Normally all O2 is bound to hemoglobin.
- About 98% of all O2 delivered to the tissue is transported in combination with the hemoglobin.
- The following formula calculates O2 contents:
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- O2 content = O2 saturation x Hb x 1.34 + pO2 × 0.003
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Normal electrolytes and blood gases
Source 1
pH
- Adult / child = 7.35 to 7.45
- Newborn = 7.32 to 7.49
- 2 months to 2 years = 7.34 to 7.46
- pH venous blood = 7.31 to 7.41
Body fluids pH Arterial blood 7.38 to 7.42 Venous blood 7.37 Cerebrospinal fluid 7.32 Pancreatic fluid 7.8 to 8.0 gastric juice 1.0 to 3.0 Urine 5 .0 to 6.0
pCO2
- Adult /child = 35 to 45 mm Hg
- Child <2 years = 26 to 41 mm Hg
- Venous blood = 40 to 50 mm Hg
HCO3
- Adult / child = 21 to 28 mEq/L
- Newborn / infants =16 to 24 mEq/L
pO2
- Adult / child = 80 to 100 mm Hg
- Newborn = 60 to 70 mm Hg
- Venous blood = 40 to 50 mm Hg
O2 saturation
- Adult / child = 95 to 100%
- Old people = 95%
- Newborn = 40 to 90%
O2 content
- Arterial blood = 15 to 22 vol%
- Venous blood = 11 to 16 vol%
Source 2
Chemicals | Arterial | Venous | |
pH | 7.35 to 7.45 | 7.31 to 7.41 | |
pCO2 | 35 to 45 mm Hg | 40 to 50 mm Hg | |
pO2 | 75 to 100 mm Hg | 40 to 50 mm Hg | |
O content | 15 to 22 % | 11 to 16 % | |
HCO3 | 21 to 28 meq/L | ||
Anion Gap | 3 to 10 |
Source 3
Normal Values of Analytes
Blood | Venous | Arterial | |
pH | 7.32 to 7.43 | 7.35 to 7.45 | |
Bicarbonate (HCO3) | 22 to 26 mmol/L | ||
Albumin | 3.5 to 5.0 G/dL | ||
pCO2 | Male = 35 to 48 mm HgFemale = 32 to 45 mm Hg | ||
Anion Gap | 3 to 10 | ||
Oxygen saturation O2 | 94 to 98% (decrease with age) | ||
O2 content | 11 to 16% | 15 to 22% | |
pO2 | 80 to 108 mm Hg (depends on altitude) |
Source 4
Arterial blood gases ordered in routine:
Biochemical parameter | Adult | Pediatric group |
pH | 7.35 to 7.45 | 7.32 to 7.42 |
pCO2 | 35 to 45 mm Hg | 30 to 40 mm Hg |
pO2 | >80 mm Hg | 80 to 100 mm Hg |
O2 saturation | >94% | |
CO2 content | 45 to 51 vol% (19.3 to 22.4 mmol/L) | |
O2 content | 15 to 22 vol% (6.6 to 9.7 mmol/L) | |
Base Excess | >2 meq/L (>2 mmol/L) | |
Base deficit | < – 2 meq/L (< – 2 mmol/L) | |
HCO3 | 22 to 26 meq/L (22 to 26 mmol/L) |
Interpretations and role of arterial gases in the acid-base balance:
Normal picture = pH normal, PCO2 normal, HCO3 normal.
- Acidemia means arterial blood pH <7.4.
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- Acidosis means a systemic increase in H+ ions.
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- Alkalemia means arterial blood pH >7.4.
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- Alkalosis means a systemic decrease in H+ ions.
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Respiratory acidosis
- pH and HCO3– go in the opposite direction.
- pH lower, pCO2 high, HCO3– high.
- Seen in respiratory depression due to any cause.
- Hypoventilation.
- Excessive retention of CO2.
Metabolic acidosis
- pH and HCO3– go in the same direction.
- pH low, pCO2 low, HCO3– low.
- Seen in diabetes, shock, renal failure, and an intestinal fistula.
Respiratory alkalosis
- pH and HCO3– go in the opposite direction.
- pH high.
- pCO2 low.
- HCO3– is normal or slightly decreased.
- Seen in hyperventilation.
- Excessive loss of CO2.
Metabolic Alkalosis
- pH and HCO3– go in the same direction.
- HCO3 is >30 meq/L.
- pH high, pCO2 high, HCO3– high.
- Urine pH >7.0 (Unless there is severe hypokalemia).
- Serum K is usually low.
- Seen in sodium bicarbonate overdose, prolonged vomiting, and nasogastric drainage.
Urine pH:
- pH = < than 7.4 = acidosis.
- pH = > than 7.4 = alkalosis.
Interpretation of the various parameters:
pCO2
- pCO2 is high = It is respiratory acidosis.
- pCO2 is low = It is metabolic acidosis.
- pCO2 is low = It is respiratory alkalosis.
- pCO2 is high = It is metabolic alkalosis.
HCO3–
- HCO3– high = It is in respiratory acidosis.
- HCO3– low = It is in metabolic acidosis.
- HCO3– low = It is in respiratory alkalosis.
- HCO3– high = It is in metabolic alkalosis.
Calculation of Anion gap = Na (140 ) + K (4) — Cl (110 ) + HCO3(24) = 10 meq/L
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- Normal anion gap = 10 to 12 meq/L = < 12
Table showing the values of pH, HCO3, pCO2, and etiology:
Clinical condition | pH | HCO3 | pCO2 | Etiology |
Metabolic acidosis | <7.4 | low | low | Diabetic ketoacidosis. lactic acidosis. |
Metabolic alkalosis | >7.4 | high | high | Vomiting |
Respiratory acidosis | <7.4 | high | high | COPD, weakness of respiratory muscles |
Respiratory alkalosis | >7.4 | low | low | Anxiety and pain |
Critical values
Biochemical parameter | Less than | More than |
pH | 7.25 | 7.55 |
pCO2 | 20 mm Hg | 60 mm Hg |
HCO3 | 15 meq/L | 40 meq/L |
pO2 | 40 mm Hg | |
O2 saturation | 75% or lower | |
Base Excess | ± 3meq/L |
Signs and symptoms summary of the acid-base system changes:
- Acidosis leads to coma and death due to depression of the CNS.
- Alkalosis leads to irritability, tetany, and possible death due to the stimulation of the CNS.
- The acidosis state is more threatening than alkalosis.
Summary of the parameters needed for the acid-base balance:
Lab test | Importance |
pH | This will tell:
|
pCO2 | This is the partial pressure of CO2, and it will tell:
|
pO2 | This is the partial pressure of the O2 in the arterial blood and tells:
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