It is the job of the RT on call in the emergency department to assess and treat incoming patients, working shoulder to shoulder with emergency-department physicians. In reaction to injury or illness, blood-gas levels change, and it is through interpreting the changes in these gas levels, along with other physical findings, that a clinician can make appropriate treatment decisions.

Analyses of arterial blood-gas (ABG) and venous blood-gas (VBG) samples are used to evaluate the patient’s oxygenation, ventilation, and acid-base balance. Arterial samples are used for the primary analysis of ventilation and oxygenation because they reflect gas exchange across the alveolar-capillary membrane without the variable of cellular metabolism. Acid-base abnormalities can be assessed using either arterial or venous samples.1

Clinician using machine

Acid-base balance (blood pH) is precisely controlled; blood is normally slightly basic, with a pH range of 7.35 to 7.45 and a homeostatic level of 7.4. Approximately 5% of the blood’s carbon dioxide is dissolved in the plasma and is measured as PaCO2, 30% is attached to hemoglobin, and 65% functions as an acid-base buffer.

Normally, Paco2 ranges from 35 mm Hg to 45 mm Hg. A PaCO2 value below 35 mm Hg reflects hyperventilation or increased carbon dioxide elimination; a value above 45 mm Hg indicates hypoventilation or decreased carbon dioxide elimination. Tachypnea may or may not result in a lower PaCO2 value. People with asthma, for example, may have a respiratory rate of 30 but half the usual tidal volume, so the PaCO2 will stay within the normal range.2 By adjusting the speed and depth of respiration, the body can regulate the blood pH almost immediately. Because the kidneys make these adjustments more slowly than the respiratory system, compensation may take several days. It takes at least 24 hours for the kidneys to adjust bicarbonate excretion in response to an acid-base disturbance, so this change indicates that the respiratory condition has been present for at least 24 hours. Patients with chronic obstructive pulmonary disease commonly exhibit this effect.

Drawing a sample

The body’s buffer system also protects against sudden shifts in acidity and alkalinity by using carbonic acids and bicarbonate ions. Acidosis and alkalosis are symptoms of a wide array of disorders categorized as either metabolic or respiratory. Normally, there is a 20:1 relationship between bicarbonate- and carbonic acid, and optimal cellular function depends upon maintaining this balance. Bicarbonate is measured in a blood-gas sample, whereas carbonic acid can be calculated by multiplying PaCO2 by 0.03.3

Medication Administration

Alterations in blood pH can affect both cellular function and the potency of many pharmaceutical agents. While acidotic tissue can reduce the efficacy of local anesthetics, relative alkalinity can enhance uptake of them. It can also increase the potency of drugs such as meperidine and morphine by passively allowing them to cross the blood-brain barrier. It is advantageous to know the blood pH in case medication dosage must be adjusted to produce the expected results.

Traditionally, blood-gas samples have been drawn arterially. Studies4 have shown, however, that VBG analysis can provide enough information to make a correct diagnosis. One advantage of VBG sampling is a decrease in pain for the patient. In addition, VBG samples can be drawn from the same puncture used for drawing other blood samples.5 VBG and ABG values are correlated, except for PO2.6

Cardiac and Metabolic Emergencies

A 1989 study7 of patients in cardiac arrest or severe hemodynamic failure unrelated to cardiac arrest evaluated both ABG and VBG analysis. The authors concluded that, in cardiac arrest, knowledge of either the arterial or venous pH or PCO2 did not alter treatment decisions. The study also found that as cardiac output declines, the difference between ABG and VBG values increases, with VBG levels (specifically in cardiac arrest) providing a truer picture of the patient’s cellular environment.6 A 2003 study8 found that there were insignificant differences between ABG and VBG results in evaluating patients in diabetic ketoacidosis; VBG analysis can be used in treating patients with severe metabolic conditions.9

Case Examples

A 34-year-old female was transported to the emergency department comatose, with a suggested diagnosis of overdose. While the laboratory ran toxicology tests and performed urinalysis, the RT drew the ABG specimens. The results of ABG analysis indicated a pH of 7.15, a bicarbonate level of 28 mmol/L, a PCO2 of 80 mm Hg, and a PO2 of 42 mm Hg. These findings were interpreted as indicating acute respiratory acidosis. In this case, the RT quickly assessed the ABG values, assisted the emergency department physician, and intubated the patient to secure the airway.

Machine

In a second case example, a 48-kg, 17-year-old female diabetic entered the emergency department with Kussmaul breathing and an irregular pulse. Analysis of an ABG sample obtained while the patient breathed room air found a pH of 7.05, a bicarbonate level of 5 mmol/L, a PCO2 of 12 mm Hg, and a PO2 of 108 mm Hg. These indicated severe, partly compensated metabolic acidosis without hypoxemia. Glucose and insulin, delivered intravenously, were immediately started. It was felt, however, that the severe acidosis was not being tolerated well by the cardiovascular system; the physician elected to target the pH to 7.2. The sodium bicarbonate push was calculated at 180 mEq by the RT and administered intravenously by the registered nurse over 15 minutes. Analysis of an ABG sample obtained 10 minutes later indicated a pH of 7.27, a bicarbonate level of 14 mmol/L, a PCO2 of 25 mm Hg, and a PO2 of 92 mm Hg.

Decentralized Analysis

Blood-gas analysis can be an effective tool in management of critically ill patients in the emergency department, and turnaround times for blood-gas analysis can have an impact on patient-care decisions.10,11 Some hospitals have brought analyzers out of the laboratory and into the emergency department for expediency. There are risks and benefits inherent in this decentralization. Having an analyzer in the emergency department shortens turnaround time and improves patient treatment decisions.6 Laboratory analysis, however, ensures adherence to standards, as well as technician competence.12

Oxygenation and Ventilation

While closely related, oxygenation is separate from ventilation. Ventilation may be normal, yet the patient may be hypoxemic. The factors that determine the amount of oxygen present in arterial blood (and whether that amount is enough to support metabolic processes) are more complex than those affecting carbon dioxide levels. Approximately 97% of oxygen is bound to hemoglobin; the remainder is dissolved in the plasma and measured as PaO2. Normal PaO2 levels are 80 mm Hg to 100 mm Hg, but lower values can be considered normal for patients over 60 years old.

Blood-gas analyzers can calculate oxygen saturation based on pH, temperature, PaCO2 and PaO2. Most of the time, a calculated saturation will be the same as a value directly measured, but this correlation does not exist when abnormal hemoglobins (such as carboxyhemoglobin and methemoglobin) are present. In these cases, use of a CO-oximeter is required to differentiate hemoglobin saturated with oxygen from abnormal hemoglobin that cannot be isolated when the saturation value is calculated.13 Obtaining an accurate saturation value in a patient who has inhaled smoke or has otherwise been exposed to carbon monoxide will require CO-oximetry.

Jennifer Lord is an EMT, Norwalk Hospital, Norwalk, Conn; Tracy Evans, MS, MPH, is director of nursing practice, The Stamford Hospital, Stamford, Conn.

References

  1. Drage S, Wilkinson D. Acid base balance. Pharmacology [serial online]. Available at: [removed]www.nda.ox.ac.uk/wfsa/html/u13/u1312_01.htm[/removed]. Accessed August 22, 2007.
  2. Fitzgerald JM, Hargreave FE. Acute asthma: emergency department management and prospective evaluation of outcome. CMAJ. 1990;142:591-595.
  3. Grogono AW. Acid-base tutorial. Available at: www.acid-base.com. Accessed August 22, 2007.
  4. Sherman SC, Schindlbeck M. When is venous blood gas analysis enough? Emerg Med. 2006;38:44-48.
  5. Malinoski DJ, Todd SR, Slone DS, Mullins RJ, Schreiber MA. Correlation of central venous and arterial blood gas measurements in mechanically ventilated trauma patients. Arch Surg. 2005;140:1122-1125.
  6. Simon N, Ramaut C, Getti R, Abderrahim N. Can venous blood gas replace arterial blood gas in emergency patients [abstract]? Acad Emerg Med. 2003;10:563.
  7. Androgué HJ, Rashad MN, Gorin AB, Yacoub J, Madias NE. Assessing acid-base status in circulatory failure. Differences between arterial and central venous blood. N Engl J Med. 1989;320:1312-6.
  8. Ma OJ, Rush MD, Godfrey MM, Gaddis G. Arterial blood gas results rarely influence emergency physician management of patients with suspected diabetic ketoacidosis. Acad Emerg Med. 2003;10:836-41.
  9. Journal Watch. Arterial blood gases not helpful in ED management of DKA. Available at: emergency-medicine.jwatch.org/cgi/content/full/2003/827/1. Accessed August 22, 2007.
  10. Kellerman AL, Cofer CA, Joseph S, Hackman BB. Impact of portable pulse oximetry on arterial blood gas test ordering in an urban emergency department. Ann Emerg Med. 1991;20:130-134.
  11. Le Bourdelles G, Estagnasie P, Lenoir F, Brun P, Dreyfuss D. Use of a pulse oximeter in an adult emergency department: impact on the number of arterial blood gas analyses ordered. Chest. 1998;113:1042-1047.
  12. Meredith RL. Quality improvement: emergency department arterial blood gas turn around. Available at: www.acestar.uthscsa.edu/institute/su06/documents/meredith.pdf. Accessed August 22, 2007.
  13. Anderson IB, Kim SY. Methemoglobinemia. Available at: [removed]www.chestnet.org/education/online/pccu/vol16/lessons21_22/lesson21/print.php[/removed]. Accessed August 22, 2007.