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When it comes to mastering anesthesia, a solid understanding of acid-base balance and blood gas interpretation is non-negotiable. Whether you’re a nursing student, an ICU nurse preparing for CRNA school or already deep into your Nurse Anesthesia Resident training, this topic is essential for clinical decision-making.

In today’s episode, we are honored to welcome Dr. Carly Mitchell, DNP, APRN, CRNA, the Program Director of Bellarmine University’s Juneja Nurse Anesthesia Program. With extensive experience in obstetric anesthesia and clinical education, Dr. Mitchell brings a wealth of knowledge to break down the complexities of acid-base physiology and arterial blood gas (ABG) analysis. Get ready to boost your critical thinking as Dr. Mitchell breaks down key concepts to help you understand and interpret ABG results with confidence.

From identifying respiratory versus metabolic imbalances to understanding compensation mechanisms, Dr. Mitchell delivers a comprehensive yet practical approach to acid-base management in anesthesia practice. She simplifies the science behind pH regulation, bicarbonate buffering, and the intricate role of the respiratory and renal systems in maintaining homeostasis.

If you’ve ever second-guessed an ABG interpretation or struggled to pinpoint whether an acid-base disturbance is compensated, this episode is for you. So, let’s dive in and elevate your expertise—because as future CRNAs, precision in acid-base management isn’t just important, it’s critical. Be sure to grab your free ABG Fast Facts at the end of the episode!

Get access to application & interview preparation resources plus ICU Educational Workshops that have helped thousands of nurses accelerate their CRNA success. Become a member of CRNA School Prep Academy: https://cspaedu.com/join

Get CRNA School insights sent straight to your inbox! Sign up for the CSPA email newsletter: https://www.cspaedu.com/podcast-email

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Watch the episode here

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Understanding Acid-Base Balance: A Guide to Arterial Blood Gas Interpretation

Hello, future CRNA. Welcome back to CSPA podcast. Today we’re going to talk about ABG interpretation and management. This topic is going to be presented to you by our guest host, Dr. Carly Mitchell, who is the Program Director of Bellarmine. She brings extensive experience in obstetric anesthesia, having served as a Chief Certified Registered Nurse Anesthetist for obstetrics at Norton Hospital and Norton Women’s Children’s Hospital.

She has also played a key role in clinical education as a Clinical Coordinator for student nurse anesthetists. She is currently the Program Director and she is dedicated to collaborating with clinical partner students, faculty and leadership to advance nurse anesthesia education and training. So without further ado, welcome Dr. Mitchell to the show. We’re so excited to have you and let’s go ahead and get into today’s episode.

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I am Dr. Carly Mitchell, Program Administrator for Bellarmine University‘s Juneja Nurse Anesthesia Program, and I want to welcome you to our discussion on the fundamental concepts of acid-base balance in the blood as evaluated through arterial blood gas analysis.

Key Terms in Acid-Base Balance

To begin, I want to start by introducing several key terms that are essential for understanding the physiological and pathological aspects of acid-base balance. So first, we’ll define an acid and a base. An acid is a substance that donates protons or hydrogen ions in a given solution. Commonly, acids are associated with lowering the pH of a solution due to this donation. On the other hand, a base acts as a proton acceptor, usually raising the pH of a solution by reducing the concentration of hydrogen ions.

Moving on to acidemia and alkalemia; these terms describe the pH state of the blood. Acidemia occurs when the blood pH is less than 7.35, and this low pH can be due to an excess of acid in the body or a loss of base, both of which increase the concentration of hydrogen ions in the blood, making it more acidic. The term acidosis refers to the process or condition leading to this state. So it’s important to note that acidemia describes the measurable state of the blood, while acidosis refers to the process that causes the state.

Similarly, alkalemia occurs when the blood pH is greater than 7.45, indicating a blood environment that is too alkaline. Now this can happen through a loss of hydrogen ions or an excess of base. The process leading to an increase in blood pH is referred to as alkalosis. So like acidemia, alkalemia describes the state of the blood and alkalosis refers to the underlying process.

Lastly, we have base excess. This is a calculated value representing the amount of strong acid or strong base required to return one liter of blood to a normal pH of 7.4, assuming a normal carbon dioxide partial pressure, or pCO2 of 40 millimeters of mercury. So this measurement helps us understand the metabolic component of acid-base balance, indicating whether there is an excess or deficit of base in the blood relative to the normal buffer system.

The Bicarbonate Buffering System and pH Regulation

The body must be able to tightly regulate the concentration of hydrogen ions within a narrow range in order to function optimally. So this balance affects everything from the activation of enzymes to the integrity of cellular membranes. Now, we quantify hydrogen ions using the pH scale, which represents the negative logarithm of the hydrogen ion concentration.

So the symbol pH represents the acidity or alkalinity of a solution, and the logarithmic value means that as the pH changes one unit, the hydrogen ion concentration changes tenfold. So it’s important to understand how changes in hydrogen ion concentration affect pH.

An increase in hydrogen ion concentration results in a lower pH, making the solution more acidic. On the other hand, a decrease in hydrogen ion concentration raises the pH, making the solution more basic. In arterial blood and extracellular fluid, the normal hydrogen ion concentration ranges from 35 to 45 nanomoles per liter, and this corresponds to a pH of approximately 7.35 to 7.45. As we mentioned, this range is important for the proper functioning of many enzymatic and biochemical processes.

Intracellularly, the hydrogen ion concentration is around 160 nanomoles per liter, which translates to a more acidic environment with a pH of about 6.8. This difference in pH between intracellular and extracellular environments is important for various cellular functions including metabolism and transport mechanisms.

The normal plasma bicarbonate concentration is approximately 24 milliequivalents per liter. Bicarbonate serves as a major buffer in maintaining our body’s pH within its narrow range, helping to neutralize excess acids generated by metabolic activities.

Our body’s pH balance is primarily regulated by three systems, the buffer systems, the respiratory system, and the renal system. The buffer systems provide an immediate chemical response to changes in pH by neutralizing excess acids or bases. So this is the first line of defense against pH fluctuations.

Respiratory and Metabolic Causes of Acid-Base Disorders

The respiratory system adjusts the pH by changing the rate and depth of breathing, thus altering the carbon dioxide levels in the blood. So carbon dioxide when dissolved in blood forms, carbonic acid influencing the pH. So this system can correct acid-base disturbances within minutes.

The renal system, although slower than the respiratory system, can nearly completely restore the pH balance by adjusting bicarbonate reabsorption or hydrogen ion secretion in the kidneys. This process, however, may take days to fully reestablish the normal pH.

So a little more on buffering systems. Buffering is a biological process that occurs in response to changes in acid-base status. This mechanism helps to absorb excessive hydrogen ions when there is too much acid or hydroxide ions when there is excess base. So the goal is to minimize fluctuations in the pH of body fluids, thus protecting the body from the harmful effects of pH imbalances.

Buffer systems operate on the principle of buffer pairs, which consists of a weak acid and its conjugate base. So these pairs work together to resist changes in pH. For example, when there is an excess of hydrogen ions, the conjugate base in the buffer will bind to these ions, thus reducing the acidity. On the other hand, if there is an excess of hydroxide ions, the weak acid can donate hydrogen ions to neutralize the base.

The primary buffers in plasma include bicarbonate, which is the most significant buffer in the blood. Bicarbonate pairs with carbonic acid to form a buffer system that is regulated by both the respiratory system and the kidneys. Hemoglobin in red blood cells acts as a buffer by binding to hydrogen ions, especially in the venous blood where CO2 is high.

Phosphate is a buffer system that operates both intracellularly and in the kidneys helping to control pH within cells and in urine. Each buffer has a PKA value, which is the pH at which the buffer pair is half dissociated. So at this pH, the buffering capacity is maximized because the concentrations of the weak acid and its conjugate base are equal.

Interpreting Arterial Blood Gases for Acid-Base Balance

The PKA value is important for understanding how effective a buffer will be in a particular environment like within the blood’s typical pH range. In arterial blood gas analysis, we measure several key parameters. So pH tells us about the blood’s overall acid-base status. PaCO2 and PaO2 provide insights into respiratory efficiency, how well carbon dioxide is expelled and oxygen is absorbed.

Bicarbonate levels and base excess help us understand the metabolic contributions to pH balance. And so these measures are important for pinpointing the underlying disorders affecting a patient. A normal pH range is between 7.35 and 7.45; as we’ve said before, values below 7.35 indicate acidosis where there is an excess of hydrogen ions or a loss of bicarbonate, signaling that the blood has become too acidic. This can be due to respiratory problems where CO2 clearance is impaired or metabolic issues where there is either an overproduction of acids or an underproduction of bicarbonate.

On the other hand, a pH above 7.45 suggests alkalosis indicating that the blood is overly alkaline. This condition can arise from excessive loss of CO2 through rapid breathing, known as respiratory alkalosis or through loss of hydrogen ions, which can happen with excessive vomiting or diuretic use, known as metabolic alkalosis. Understanding these disturbances and their underlying causes is important for effective clinical management as the symptoms ranging from confusion and fatigue in acidosis to nausea and numbness in alkalosis guide the urgency and type of treatment needed.

Respiratory vs. Metabolic Causes of Acid-Base Disorders

When it comes to diagnosing whether an acid-base imbalance is primarily respiratory or metabolic, it’s important to focus on PaCO2 and bicarbonate levels. And so a primary respiratory disorder is indicated by changes in PaCO2. If PaCO2 is elevated we’re typically dealing with respiratory acidosis likely due to hypoventilation. On the other hand, if PaCO2 is decreased, it suggests respiratory alkalosis often caused by hyperventilation.

Now, on the metabolic side, bicarbonate levels give us the clues that we need. So a decrease in bicarbonate suggests metabolic acidosis, which can result from conditions like renal failure or diabetic crises where acids accumulate or bicarbonate is lost excessively. An increase in bicarbonate suggests metabolic alkalosis often due to excessive loss of acids, such as from prolonged vomiting or diuretic use.

The anion gap is an important diagnostic tool in our arsenal, especially when assessing metabolic acidosis. So by calculating the difference between measured cations and the sum of the most common measured anions, we can get a sense of whether there are excess acids in the blood that aren’t being directly measured. So this calculation helps us determine if the metabolic acidosis is due to an accumulation of acids or a loss of bicarbonate.

For instance, a high anion gap typically points to the presence of additional acids such as lactate in lactic acidosis or keto acids in diabetic ketoacidosis. In these cases, our treatment focuses on managing the underlying cause, whether that’s improving insulin therapy in diabetes or enhancing profusion in shock. On the other hand, a normal anion gap usually indicates that the acidosis results from a significant loss of bicarbonate, which could happen through diarrhea or certain kidney disorders like renal tubular acidosis. Here, treatment may involve replacing bicarbonate and managing the primary disorder affecting the gastrointestinal or renal system.

Compensation Mechanisms in Acid-Base Disorders

Accurate interpretation of arterial blood gases is essential in diagnosing and managing acid-base disorders. So let’s break down the steps involved in determining whether the disturbance is respiratory or metabolic in nature. The first step is to evaluate the pH level. If the pH is high, the condition is alkalosis. If the pH is low, the condition is acidosis. This initial assessment provides us with a broad classification of whether the disturbance is acidotic or alkalotic.

Next, we assess whether the pH change is primarily due to a change in PaCO2 or bicarbonate. If the pH change aligns with an altered PaCO2 level, then the disturbance is respiratory. If the pH change corresponds to an altered bicarbonate level, the disturbance is metabolic. After identifying whether the primary disturbance is respiratory or metabolic, the next step is to assess whether there is appropriate compensation.

Compensation refers to the body’s attempt to restore normal pH by adjusting either bicarbonate levels via the kidneys or carbon dioxide levels via the lungs, depending on the type of disturbance. In respiratory acidosis, the primary problem is elevated PaCO2, often due to inadequate ventilation. Renal compensation occurs by increasing bicarbonate reabsorption in order to buffer the excess hydrogen ions and raise pH towards normal.

In respiratory alkalosis, the primary issue is decreased PaCO2 due to hyperventilation, and so renal compensation occurs by excreting more bicarbonate to lower its concentration and help bring the pH back towards normal. In metabolic acidosis, there’s a primary decrease in bicarbonate due to excess acid or bicarbonate loss. So, respiratory compensation occurs through hyperventilation, which lowers PaCO2 two by blowing off excess carbon dioxide to help increase pH.

In metabolic alkalosis, the primary issue is an increase in bicarbonate levels due to excess base or acid loss. So respiratory compensation involves hypoventilation, which increases PaCO2, helping to reduce the pH back towards normal. Understanding compensation helps clinicians determine whether the body is attempting to correct the disturbance or if multiple disorders may be present. Keep in mind that compensation rarely fully normalizes pH, but mitigates the severity of the imbalance.

Distinguishing Between Uncompensated, Partially Compensated, and Fully Compensated States

Finally, to fully understand acid-base imbalances, it’s important to distinguish between uncompensated, partially compensated and completely compensated states. This table summarizes the patterns of pH, PaCO2 and bicarbonate in different types of acid-base disturbances.

Acid-Base Balance: This table summarizes the patterns of pH, PaCO2 and bicarbonate in different types of acid-base disturbances.

Starting with respiratory acidosis, as we’ve said, this occurs when there’s an excess of carbon dioxide in the blood leading to a decrease in pH. In an uncompensated state, you’ll notice that while the PaCO2 two level is elevated, the bicarbonate remains normal because the kidneys have not yet had time to respond. Now, as compensation begins, the kidneys try to retain more bicarbonate to buffer the excess hydrogen ions stabilizing the pH back toward normal.

Moving on to respiratory alkalosis, this happens when there’s too little carbon dioxide in the blood, usually due to hyperventilation, causing the pH to rise. Initially, there’s no change in bicarbonate, but in a partially compensated state, the kidneys may start to excrete more bicarbonate to lower the pH back to normal. In the case of metabolic acidosis, the disturbance is due to a decrease in bicarbonate, either from a metabolic problem or a direct loss of bicarbonate. So the respiratory system may compensate by increasing breathing to blow off CO2, thus raising the pH closer to normal. If compensation is complete, the respiratory effort has normalized the pH even though bicarbonate levels remain low.

Lastly, for metabolic alkalosis, an increase in bicarbonate, usually due to excessive intake or retention, elevates the pH. The respiratory system may respond by reducing breathing, retaining more CO2 to help bring the pH back down. Now, it’s important to note that while the body can often compensate over time, it cannot always restore pH completely to normal, especially in severe or chronic conditions.

These compensatory mechanisms are important for temporary stabilization, buying time for medical intervention, or for the underlying cause of the disturbance to be resolved. Understanding these principles helps in diagnosing the underlying cause of acid-base imbalances and planning appropriate treatment strategies. It shows how our body strives to maintain homeostasis, highlighting the interconnectedness of our respiratory and renal systems in managing pH levels.

Bellarmine University’s Nurse Anesthesia Program

Thank you for taking the time to listen to this presentation. As mentioned, my name is Dr. Carly Mitchell. I am the Program Administrator for Bellarmine University’s Juneja Nurse Anesthesia Program, and as such, I just want to take a few moments to tell you a little bit about our program.

The Bellarmine University Juneja Nurse Anesthesia Program spans 36 months and encompasses 101 credit hours. It is tailored for post-BSN students pursuing doctoral level preparation as nurse anesthetists. Classes begin in August each year with ongoing enrollment of 20 students per cohort. The first two semesters consist of full-time study online in an asynchronous format, followed by seven semesters of full-time study on site.

The curriculum commences with foundational courses introducing advanced practice nursing and fostering doctoral level thinking with courses such as foundations of scholarship, healthcare informatics, and national and global health policy. Anesthesia specific coursework begins in the third semester of the program with courses in applied sciences, anatomy, physiology, and pathophysiology, pharmacology, and basic principles of nurse anesthesia.

Our nurse anesthesia residents also engage in immersive learning experiences facilitated by our advanced simulation center, equipped with high fidelity mannequins, point of care ultrasound simulators, and specialized task trainers. The clinical practicum component begins in semester four and extends throughout the remainder of the program, emphasizing the practical application of theoretical concepts learned in the anesthesia curriculum.

Our residents also gain hands-on experience in administering anesthesia care to individuals across the lifespan at diverse clinical sites within prominent regional hospitals. All clinical rotations, including primary and specialized placements, are conducted locally through our formal collaboration with Norton Healthcare.

Successful completion of the program qualifies graduates to sit for the National Certification Exam administered by the National Board of Certification and Recertification for Nurse Anesthetists marking their readiness to practice as Certified Registered Nurse Anesthetists.

To learn more about our program of study, admission criteria and application process, I invite you to join one of our upcoming information sessions, so please visit our website or follow us on Facebook and Instagram for upcoming dates and times.

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Thank you for tuning in. Be sure to get your Free ABG Fast Facts study guide, head over to our Free Resources page to gather planning and interview prep resources to help you on your CRNA journey. You can also explore our FREE community for aspiring CRNAs, ICU Dreaming of Anesthesia.

Stay strong. We’re rooting for you, future CRNA!

Important Links

Join the Free CSPA Community! Connect with a network of Aspiring CRNAs, Nurse Anesthesia Residents, practicing CRNAs and CRNA Program Faculty Mentors here: https://www.cspaedu.com/community

Get access to application & interview preparation resources plus ICU Educational Workshops that have helped thousands of nurses accelerate their CRNA success. Become a member of CRNA School Prep Academy: https://cspaedu.com/join

Get CRNA School insights sent straight to your inbox! Sign up for the CSPA email newsletter: https://www.cspaedu.com/podcast-email

Book a mock interview, resume or personal statement critique, transcript review and more: www.teachrn.com

Learn more about the Bellarmine University Juneja Nurse Anesthesia Program: https://www.bellarmine.edu/lansing/nursing/graduate/dnp-na/