CHAPTER 1
Introduction to Acid-Base and Electrolytes

Angela Randels-Thorp, CVT, VTS (ECC, SAIM) and David Liss, RVT, VTS (ECC, SAIM)

Understanding of acid-base and electrolyte chemistry and physiology is both an important and valuable knowledge base for all veterinary technicians, and especially those in emergency and critical care or specialty practice. These very topics, however, are frequently thought of as boring at best or utterly confusing at worst. In actuality neither is true. These topics are often approached in a piecemeal or qualitative way, which lends itself to confusion. It is the goal of this text to provide a useful, easy-to-learn, and practical approach to the concepts regarding acid-base and electrolytes.

Introduction to acid-base

Assessment of acid-base status provides insight into three physiologic processes: alveolar ventilation, acid-base status, and oxygenation. Evaluating acid-base status has become an integral part of the emergent/critical care patient workup and should be performed as a baseline on all emergent patients. Deviation from normal acid-base balances is indicative of clinical disease processes and can aid the clinician in identifying underlying causes of illness in the patient. Venous samples can provide most of the information needed regarding acid-base status and even alveolar ventilation. Arterial samples are required, however, in order to provide oxygenation status (Sorrell-Raschi 2009). It is ever more important for the emergency and critical care (ECC) technician to be familiar in his/her understanding of acid-base values and what they mean.

What is acidity?

In the simplest of terms, the acidity or alkalinity of a solution is based on how many hydrogen (H+) ions, or molecules of carbon dioxide (CO2), are present. Hydrogen ions are produced daily as a normal part of metabolism of protein and phospholipids, and are considered a fixed, non-volatile acid. Carbon dioxide is a byproduct of the metabolism of fat and carbohydrates in the body, and is considered a volatile acid (volatile?=?readily vaporized). Gaseous CO2 is soluble in water. CO2 is considered an acid because it readily combines with H2O in the presence of carbonic anhydrase (enzyme/catalyst) to form carbonic acid (H2CO3). Without the catalyst, this change occurs very slowly. CO2 is continually removed by ventilation and thereby kept at a stable partial pressure (pCO2) in the body. The change in dissolved CO2 in body fluids is proportional to pCO2 in the gas phase. Elimination of these acids is dependent on the function of the lung, kidney, and liver.

Bronsted and Lowry state an acid is a proton donor (H+) and a base is a proton acceptor (A-) (DiBartola 2006: 229). The H+ concentration ([H+]) of body fluids must be kept at a constant level to prevent detrimental changes in enzyme function and cellular structure. Levels compatible with life are between 16 and 160 nEq/L. Excessive hydrogen ions in the blood result in acidemia. Decreased hydrogen ions in the blood result in alkalemia (Kovacic 2009). Hydrogen ions are not typically measured or tested in clinical practice. Therefore, Sorenson developed pH notation in order to provide simpler notation of the wide range of [H+] (DiBartola 2006: 229). There is an inverse relationship between pH and [H+] (Ex:?[H+]??pH). Normal pH ranges between 7.35 and 7.45, approximately. The processes which lead to changes in production, retention, or excretion of acids or bases, which may or may not result in a change in pH, are called acidosis or alkalosis.

Buffering systems

The body contains several mechanisms in order to maintain the desired "normal" pH level, which is called buffering. A buffer is a compound that can accept or donate protons (H+) and minimize a change in pH. Buffers consist of a weak acid and its conjugate salt (Sorrell-Raschi 2009). If a strong acid is added to a buffer, the protons from the acid dissociate to the salt of the buffer and the change of pH is therefore minimized. With these buffers the body is continually converting CO2, H2O, H+, and to maintain pH within normal ranges. The following equation represents this constant interaction:

There are several compounds that serve as buffers in the body. The primary buffer of extracellular fluid (ECF) is bicarbonate (). Non-bicarbonate buffers consist of proteins and inorganic and organic phosphates, which are primarily intracellular fluid (ICF) buffers. Bone is a prominent source of buffer (calcium carbonate and calcium phosphate). Up to 40% of buffering can be done from resources found in bone. Upon treatment/administration of sodium bicarbonate (), carbonate that has been released to buffer can then be deposited back into the bone. In the blood, proteins, including hemoglobin and plasma, serve as buffers. Hemoglobin constitutes 80% of the buffering capacity of blood, whereas plasma proteins only account for 20% of buffering in the blood.

The body's buffering system is considered an open buffering system, with both bicarbonate and carbonic acid systems. In a closed system the exchanges would have to occur in a reciprocal manner. Since the body eliminates the majority of CO2 through ventilation, keeping pCO2 constant, a reciprocal reaction does not have to occur, which allows the body's buffering systems to be considered open. Both hydrogen ion excretion and bicarbonate regeneration are regulated by the kidneys.

Physiologic response system

The balance of acid-base in the body is regulated by metabolic, respiratory, and renal pathways. In terms of acid-base discussion, generally either a metabolic or a respiratory derangement occurs with the renal or respiratory system compensating for either/both.

When an excess of H+ ions occurs, this causes a decrease in pH. Within minutes of this imbalance, the hydrogen ions begin to titrate with bicarbonate ions in ECF and then titrate with ICF buffers in order to minimize changes in pH. Next, alveolar ventilation is stimulated in order to decrease CO2 until levels are below normal, thereby raising the pH back up to near normal. Within hours (2-3 days peak effect), the renal system begins to regenerate . As is increased, the body's pH is increased. Alveolar ventilation no longer needs to be increased, so returns to normal rates, restoring pCO2 levels to normal.

CO2 concentrations are a balance of mitochondrial production and alveolar removal by ventilation. An excess of CO2 (in excess of ventilatory regulation) cannot be buffered directly by . CO2 is converted to carbonic acid by the mechanisms described above, which then allows H+ from carbonic acid to titrate with intracellular buffers (proteins/phosphates). The renal system also adapts by increasing reabsorption (2-5 days peak effect).

There is a variety of terms that can be used to describe acid-base imbalances, including: acidosis, alkalosis, acidemia, and alkalemia. While it may seem overly technical, knowing the differences between the terminologies can be important. The terms acidosis/alkalosis refer to the pathophysiologic processes that cause the net accumulation of acid or alkali in the body. The terms acidemia/alkalemia refer to the actual change in pH of ECF. In cases of acidemia, the pH is lower than normal, or?<?7.35 ([H+]). With alkalemia, the pH is higher than normal, or?>?7.45 ([H+]). For example: a patient with chronic respiratory acidosis may have normal pH due to renal compensation. The patient has acidosis, but not acidemia. Mixed acid-base disorders may also have an overall normal pH, due to one counter-balancing the other (Murtaugh 2002). These concepts will be covered in more detail in subsequent chapters.

Primary acid-base disturbances

There are four primary acid-base disturbances that may occur in the body: metabolic acidosis, metabolic alkalosis, respiratory acidosis, and respiratory alkalosis. When evaluating a patient's acid-base status, the following parameters are primarily needed: pH, (or TCO2), and pCO2 (Figure 1.1). If blood gases, or oxygenation, are being evaluated on an arterial blood sample, then PaO2 values are also provided. Simple acid-base analysis may be done on either venous or arterial blood samples. Arterial samples are mandatory if one is attempting to assess the oxygenation status of a patient.

Figure 1.1 Arterial blood gases chart.

These imbalances will be discussed in greater detail throughout the chapters of this text, as well as practical approaches and applications for day-to-day practice.

Introduction to electrolytes

Just as fluid imbalances can affect the patient's electrolyte balance, electrolyte imbalances can, in turn, result in fluid imbalances, as well as a host of other problems. Imbalances involving sodium, potassium, chloride, calcium, phosphorus, and magnesium can all result in potentially life-threatening problems for animals. It is imperative for technicians to understand the role of electrolytes in the body and recognize the signs of imbalances in these electrolytes in order to aid in the quick recognition, diagnosis, and treatment of these problems. In this section we will introduce electrolyte physiology and regulation in the body, and specific and greater detail on each is covered in its respective...