Episode 131. Acid Base Balance with Associate Professor Adrian Regli (Part 1)

Acid-Base theory is often considered a difficult subject. As long ago as 1962, Creese et al wrote in the Lancet … “There is a bewildering variety of pseudoscientific jargon in medical writing on this subject “My suspicion is that some degree of confusion and thus avoidance of the subject continues to this day. Hopefully, this podcast conversation will resonate with some of our listeners and smooth out any misunderstandings should they exist.

As a background, Bronsted and Lowrys definitions of acids and bases are as follows: A base is a substance that accepts a proton (a hydrogen ion) an acid is a compound that dissociates in water to release a proton. A strong acid is one that readily dissociates in water to release a proton (eg HCL), and a weak acid does not readily dissociate in water (uric acid). pH is the negative logarithm of the hydrogen ion concentration to the base 10. Thus, the negative logarithm of 0.0000001 which may be expressed as 10 to the power of -7 is 7.

The reason blood and cellular pH are so important is that their stability is essential to the integrity of enzymes, metabolic processes, and cell membrane potential. Homeostasis holds our blood pH tightly between 7.35 and 7.45 with an intracellular pH of 6.8.

Where does the acid come from?

Acid production results from the production of CO2 by metabolism of glucose, fatty acids, and amino acids. CO2 combines with water and is converted to carbonic acid -H2CO3 by carbonic anhydrase and then dissociates to H+ and HCO3-. That enzyme carbonic anhydrase pops up everywhere.

Acid production also results from anaerobic glucose metabolism whereby glucose is converted to H+ and lactate in ketogenesis as well as from the catabolism of the amino acids: methionine and cysteine.

Which organs play a major role in the maintenance of pH?

Both the lungs and kidneys play critical roles in acid-base balance. We exhale CO2 from the lungs effectively blowing off acid but may also retain CO 2 by underventilation.

The kidneys have the potential to excrete or absorb bicarbonate and to excrete or reabsorb protons (hydrogen ions) influencing and compensating for pH disturbance through an intricate juggling of these two. The excretion of protons is by combination with ammonia from the metabolism of muscle glutamine or in combination with monohydrogen phosphate. These ingenious biological systems may be influenced by multiple disease processes and respiratory forms of acidosis and alkalosis as well as metabolic processes leading to acidosis and alkalosis are well recognised.

Whilst arterial blood gas assessment is used in critical care units to determine the degree of oxygenation, adequacy of ventilation, and the presence and severity of acid-base disturbances in the body, arterial puncture may result in complications, and the difficulty in acquiring arterial blood may delay care. Venous blood gas (VBG) is a more accessible alternative to ABG sampling and correlates well with arterial sampling in pH measurement (slightly lower in venous sample) and HCO3 - (1.41 mmol/l higher in venous) with pCO2 approximately 5.6 mmHg higher in venous blood. These differences may be exaggerated however in circulatory failure.

In this podcast with ICU physician Associate Professor Adrian Regli, we will explore the subject further, delve into some of the typical metabolic and respiratory disturbances we are likely to encounter as clinicians and also review some handy rules of thumb to draw upon in practical acid-base interpretation. Currently, Adrian works as an ICU consultant at Fiona Stanley Hospital Perth. Please welcome Adrian to the Podcast.

References

Assoc Professor Adrian Regli - via Google

Oh’s Intensive Care Manual, Bersten et al 6 th ED, Butterworth

Medical Biochemistry at a Glance, Salway,3rd ED, Wiley-Blackwell

Acid-Base Disorders in the Critically Ill Patient, Achanti et al CJASN, Sept 2022