Are daily chest radiographs and arterial blood gas tests required in ICU patients on mechanical ventilation?
ARTERIAL BLOOD GAS TESTING: DAILY VS CLINICALLY INDICATED
Arterial blood gas testing has value for managing patients undergoing mechanical ventilation, and it is one of the most commonly performed diagnostic tests in the ICU. It provides reliable information about the patient’s oxygenation and acid-base status. It is commonly requested when changing ventilator settings.
Downsides. Arterial blood gas measurements account for 10% to 20% of the cost incurred during ICU stay.15 In addition, they require an arterial puncture—an invasive procedure associated with potentially serious complications such as occlusion of the artery, digital embolization leading to digital ischemia, local infection, pseudoaneurysm, hematoma, bleeding, and skin necrosis.
Is daily testing needed?
Guidelines say no. The 2013 American Association for Respiratory Care16 guidelines suggest that arterial blood gas testing should be based on the clinical assessment of the patient. They recommend blood gas analysis to evaluate the patient’s ventilatory status (reflected by the partial pressure of arterial carbon dioxide [PaCO2], acid-base status (reflected by pH), arterial oxygenation (partial pressure of arterial oxygen [PaO2] and oxyhemoglobin saturation), oxygen-carrying capacity, and whether the patient likely has an intrapulmonary shunt. They state that testing is useful to quantify the response to therapeutic or diagnostic interventions such as cardiopulmonary exercise testing, to monitor severity and progression of documented disease, and to assess the adequacy of circulatory response.
Studies agree
The ACR recommendation to test “as clinically indicated” is supported by studies showing that patient outcomes are not inferior for arterial blood gas testing when clinically indicated instead of daily, and that this practice is associated with fewer complications, less resource use, and reduced overall patient care costs.
A 2015 study compared the efficacy and safety of obtaining arterial blood gases based on clinical assessment vs daily in 300 critically ill patients.17 Overall, fewer samples were obtained per patient in the clinical assessment group than in the daily group (all patients 3.7 vs 5.5; ventilated patients 2.03 vs 6.12; P < .001 for both). In ventilated patients, there was a 60% decrease in arterial blood gas orders without affecting patient outcomes and safety, including a lower risk of complications and overall cost of care.
In another study, Martinez-Balzano et al18 evaluated the effect of guidelines they developed to optimize the use of arterial blood gas testing in their ICUs. These guidelines encouraged testing of arterial blood gases after an acute respiratory event or for a rational clinical concern, and discouraged testing for routine surveillance, after planned changes of positive end-expiratory pressure or inspired oxygen fraction on mechanical ventilation, for spontaneous breathing trials, or when a disorder was not suspected.
Compared with data collected before implementation, these guidelines reduced the number of arterial blood gas tests by 821.5 per month (41.5%), or approximately 1 test per patient per mechanical-ventilation day for each month (43.1%; P < .001). Appropriately indicated testing rose to 83.4% from a baseline of 67.5% (P = .002). Additionally, this approach was associated with saving 49 liters of blood, reducing ICU costs by $39,432, and freeing up 1,643 staff work hours for other tasks. There were no significant differences in days on mechanical ventilation, severity of illness, or mortality between the 2 periods.18
Extubation effects. Routine arterial blood gas testing has not been shown to affect extubation decisions in patients on mechanical ventilation. In a study of 83 patients who completed a spontaneous breathing trial (total of 100 trials), Salam et al19 found arterial blood gas values obtained during the trial did not change the extubation decision in 93% of the cases.
In a study of 54 extubations in 52 patients,20 65% of the extubations were performed without obtaining an arterial blood gas test after the patient completed a trial of spontaneous breathing. The extubation success rate was 94% for the entire group, and it was the same regardless of whether testing was done (94.7% vs 94.3%, respectively).
Alternatives to arterial blood gases
There are less-invasive means to obtain the information that comes from an arterial blood gas test.
Pulse oximetry is a rapid noninvasive tool that provides continuous assessment of peripheral arterial oxygen saturation as a surrogate marker for tissue arterial oxygenation. However, it cannot measure PaO2 or PaCO2.21
Transcutaneous carbon dioxide (PTCO2) monitoring is another continuous noninvasive alternative. The newer PTCO2 devices are useful in patients with acute respiratory failure and in critically ill patients on vasopressors or vasodilators. Studies have shown good correlation between PTCO2 and PaCO2.22,23
End-tidal carbon dioxide (PetCO2) is another alternative to estimate PaCO2. It can also be used to confirm endotracheal tube placement, during transportation, during procedures in which the patient is under conscious sedation, and to monitor the effectiveness of cardiopulmonary resuscitation and return of circulation after cardiac arrest. PetCO2 measurements are not as accurate as arterial blood gas testing owing to a difference of approximately 2 to 5 mm Hg between PaCO2 and PetCO2 in normal lungs due to alveolar dead space. This difference may be much higher depending on the clinical condition and the degree of alveolar dead space.21,24,25
Venous blood gases, which can be obtained from a peripheral or central venous catheter, are adequate to assess pH and partial pressure of carbon dioxide (PCO2) in hemodynamically stable patients. Walkey et al26 found that the accuracy of venous blood gas measurement to predict arterial blood gases was 90%. They recommended adjusting the venous pH up by 0.05 and the PCO2 down by 5 mm Hg to account for the positive bias of venous blood gases. A limitation of this method is that the values are not reliable in patients who are in shock.
These alternatives can be used as a substitute for daily arterial blood gases. However, in certain clinical scenarios, arterial blood gas measurement remains a necessary and useful clinical tool.
TAKE-HOME MESSAGE
Most scientific evidence suggests that chest radiographs and arterial blood gas measurement in patients undergoing mechanical ventilation—and critically ill, in general—are best done when clinically indicated rather than routinely on a daily basis. This will reduce cost and harm to patients that may result from these unnecessary tests and not adversely affect outcomes.
