In a critical care setting, the main aim is always oxygen perfusion; perfusion = survival = healing. Acid Base Balance a.k.a. pH balance, is the level of acids and bases in the blood at which the human body functions at its best. A pH between 7.35 and 7.45 is considered to be an optimum pH level since it promotes good oxygen perfusion throughout the body.
A cell without oxygen can compensate with the help of anaerobic respiration. This however produces lactate a.k.a. lactic acid. Thus, anaerobic respiration can only provide compensation for a short period of time.
In normal circumstances, the body aims to maintain a healthy balance between the acid and alkaline within. This process is mostly active thanks to the lungs and the kidneys, both of which play an important role in maintaining the body’s pH balance. This means however, that for individuals with compromised kidneys or lungs, compensating pH imbalance becomes even more difficult.
An acid is a substance which is chemically able to donate a hydrogen ion to another substance. Acids, which have a pH <7, are formed by free H+ ions and carry a positive electrical charge a.k.a. cations.
A base a.k.a. buffer is any substance which is chemically able to accept a hydrogen ion. Most bases are insoluble, however, ones that dissolve in water are also called alkali. Alkalis are formed by OH– ions a.k.a. Hydroxyl ions. They have a pH of >7 and carry a negative electrical charge a.k.a. anions.
pH is the measure of H+ (hydrogen ion) concentration in water.
pH is controlled by the following active organs:
LUNGS: excrete carbon dioxide in the form of carbonic acid (H2CO3), and dissociates into H2O + CO2 for excretion.
KIDNEYS: control bicarbonate excretion; the kidneys can form ammonia which combines with acid products of protein metabolism for excretion.
PLASMA PROTEINS: able to bind both to free H+ and OH– ions, preventing changes in the pH (fine-tuning pH levels that are still within their normal range i.e. between 7.35-7.45).
Bicarbonate and pH Balance
Normal Blood Gases Values
| Arterial | Venous | |
| pH | 7.35-7.45 | 7.33-7.43 |
| PO2 (Partial Pressure of Oxygen) | 80-100mmHg / 11-15KPa | 35-49mmHg / 4.5-6KPa |
| PCO2 (Partial Pressure of Carbon Dioxide) | 35-45mmHg / 4.5-6.1KPa | 41-51mmHg / 5-6.5KPa |
| SO2 (Oxygen Saturation) | 95-100% | 65-80% |
| HCO3 (Bicarbonate) | 22-26mmol/l | 24-28mmol/l |
| Base Excess | -2 to 2 | 0 to 4 |
NOTE: In the UK, PaCO2 and PaO2 are normally measured in kPa (kilopascal) whereas in Malta they are usually measured in mmHg (millimetres of mercury). 1kPa = 7.5mmHg.
- pH – acidity or alkalinity measurement based on the hydrogen ions present
- PaO2 – partial pressure of oxygen which is dissolved in arterial blood
- SO2 – arterial oxygen saturation
- PCO2 – the amount of carbon dioxide dissolved in arterial blood
- HCO3 – the amount of bicarbonate in the blood
- Base Excess – the amount of excess or insufficient level of bicarbonate in the system
Restoring Acid-Base Balance Through Compensation
The human body naturally attempts to keep the pH within normal range by restoring acid-base balance through the opposite unaffected system. For example, if the respiratory system is affected, the metabolic system attempts to compensate so as to restore normal pH.
Respiratory Compensation happens 2-4 HOURS following an established metabolic process.
Metabolic Compensation happens 2-4 DAYS following an established metabolic process.
ABGs Interpretation Algorithm
Acid Base Balance Disorders
ABGs Interpretation
Partially vs Fully Compensated & Uncompensated Arterial Blood Gases
Further information
Arterial Blood Gases Blogpost – http://student-nurse-life.com/arterial-blood-gases-interpreting-abg/
Reference
Featured image retrieved from https://www.medistudents.com/osce-skills/arterial-blood-gases on 18th November 2022
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