Acid-base disturbances are physiological responses to a wide variety of underlying conditions and critical illnesses. Homeostasis of acid-base physiology is complex and interdependent with the function of the lungs, kidneys, and endogenous buffer systems. Traditionally, these disturbances have been classified in terms of being caused by either a primary respiratory or a metabolic insult and by chronicity and compensation. While existing literature consists largely of physiology reviews, several well-designed studies and clinical practice guidelines provide relevant new perspectives on interpreting and managing acid-base disturbances. This review outlines several approaches to characterizing disturbances, with a case-based format and algorithms to aid in diagnostic testing and interpretation of arterial blood gases.
This issue of Emergency Medicine Practice utilizes a workbook approach in order to highlight the strengths and weaknesses of respective approaches to acid-base disturbances. Clinical vignettes are provided with accompanying steps for interpretation. To test your skills, try to interpret the cases before reading the “Methods” and “Interpretation” sections.
It is Friday night and you have just received sign-out from your partner, who was finishing the swing shift, leaving you to function as the single provider in a critical access ED. Immediately after he leaves, local EMS radios in to give report about a patient en route:
“This is rescue 59 coming to your facility with a 56-year-old white woman with fever, chills, and lethargy. She is a patient being treated at University Hospital for ovarian cancer and last had chemotherapy 1 week ago. Her family says she has had fever up to 103°F since last night and recently had a CT scan showing a mass impinging on her ureter. Initial vital signs are: heart rate, 148 beats/min; blood pressure, 88/52 mm Hg; respiratory rate, 30 breaths/min; temperature 39° C; and oxygen saturation, 95% on 2L NC. We have not been able to obtain IV access, and we will be at your back door in 4 minutes.”
As you begin to prepare a resuscitation room, you appreciate that this febrile patient may be in septic shock, which is an inherently acidemic state. You wonder how best to determine whether an acidosis is purely due to sepsis or is confounded by additional acid-base disturbances. You also wonder what kind of IV fluids are best for resuscitation and whether there is a role for bicarbonate…
After your initial evaluation, the charge nurse informs you that there have been several new patient arrivals, and you review the track board, along with the results of their laboratory tests. (See Tables 1 and 2.)
Acid-base disturbances are observed frequently in patients with complex or critical illness and may be associated with a wide range of underlying conditions. Recognition of how homeostasis is maintained and the ability to identify and characterize disturbances is fundamental to the practice of emergency medicine. Acidemia is defined as the state in which the extracellular pH is abnormally low, typically < 7.35. Alkalemia is the state in which the pH is abnormally high, typically > 7.45. By contrast, the terms acidosis and alkalosis refer to the underlying physiological processes that result in an abnormal pH. A patient can exist in a state of acidemia or alkalemia, never in both. Nonetheless, several independent disturbances may coexist such that a patient experiences concomitant acidosis and alkalosis, known as mixed disturbances, the cumulative sum of which determines overall pH.
This issue of Emergency Medicine Practice provides a review of the basic science and physiological principles from which clinical paradigms are based, complimentary approaches to the characterization and diagnosis of acid-base disturbances, a series of real-life case presentations that highlight the approaches, principles for initial evaluation and management, and consideration for special populations and circumstances.
A literature search was undertaken utilizing multiple available databases. A MEDLINE search with the MeSH heading of acid base imbalance restricted to humans from 1965 through 2018 yielded over 26,000 results. Additional databases and search strategies were queried, including Google Scholar and the Cochrane Database of Systematic Reviews. Reference lists from the identified articles/abstracts as well as major textbooks in internal medicine, critical care, and nephrology were screened for additional citations. Clinical practice guidelines and position statements from major relevant professional organizations were also reviewed. (See Table 3.)
Boolean operators and MeSH headings were applied to structure the literature search and included: acid base imbalance, acidosis, alkalosis, acidemia, alkalemia, ketoacidosis, anion gap, with or without additional terms including poisoning, toxicology, emergency department, arterial blood gas, COPD, metformin, lactates, sepsis, crystalloid, colloid, sodium bicarbonate, practice guidelines, and emergency medical services. Both authors screened these results independently for articles considered to be landmark publications, highly impactful, or from the highest-quality journals. Fifty-one citations were subsequently included in this review.
1. “The patient was hypoxic, so I applied supplemental oxygen by nonrebreathing mask until the SpO2 was 100%.”
Oxygen-induced hypercapnia and hyperoxia are proven consequences of excessive supplemental oxygen therapy in patients with COPD. Mechanisms are complex and not due simply to decreased hypoxic respiratory drive, but a titrated oxygen strategy targeting maximum SpO2 of 88% to 92% should be employed.5
3. “The septic patient was severely acidotic, so I gave her sodium bicarbonate boluses.”
Existing evidence does not support the routine administration of sodium bicarbonate for most causes of acidosis. In some cases, this may be harmful and should be considered only if there is immediately life-threatening acidemia (eg, pH < 6.9), bicarbonate-losing conditions, or at the direction of a specialist.
6. “I had to intubate the COPD patient – his pCO2 was 70!”
Many patients with obstructive airway disease chronically retain carbon dioxide that is accompanied by a compensatory renal response of increased bicarbonate production and resorption, over time, to raise the pH back closer to normal. The decision to initiate mechanical ventilation should be based on mental status, historical ABG comparisons, and consideration for compensated or uncompensated processes.
Evidence-based medicine requires a critical appraisal of the literature based upon study methodology and number of subjects. Not all references are equally robust. The findings of a large, prospective, randomized, and blinded trial should carry more weight than a case report.
To help the reader judge the strength of each reference, pertinent information about the study, such as the type of study and the number of patients in the study is included in bold type following the references, where available. In addition, the most informative references cited in this paper, as determined by the author, are highlighted.
Michael Boniface, MD; Ivan Porter, MD
Daniel J. Egan, MD; Gabriel Wardi, MD, MPH
June 1, 2020
July 1, 2023
4 AMA PRA Category 1 Credits™, 4 ACEP Category I Credits, 4 AAFP Prescribed Credits, 4 AOA Category 2-A or 2-B Credits
Date of Original Release: June 1, 2020. Date of most recent review: May 10, 2020. Termination date: June 1, 2023.
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