Pulse oximetry in the adult ED patient with sickle cell

Bernard L. Lopez MD, MS*, Jacob F. Cogen, Leemu Kerkula, Theodore Corbin MD, Pamela Flenders MSN

Department of Emergency Medicine, Jefferson Medical College, Philadelphia, PA 19107, USA

Received 14 August 2004; revised 1 September 2004; accepted 13 September 2004

Abstract We performed a retrospective chart review of adult patients with sickle cell presenting with vasoocclusive crisis to determine the association between pulse oximetry and emergency department (ED) disposition. Subjects were divided into a NORMAL (pulse oximetry z95%) and a LOW (pulse oximetry b95%) group. Two hundred ten consecutive charts showed no significant difference between NORMAL (n = 163) and LOW (n = 47) groups regarding admission or discharge from the ED ( P N .05). A higher percentage in the LOW group received chest radiographs (40.9% vs 29%, P b .05), suggesting that pulse oximetry may have influenced ordering of this test. No significant differences in historical and physical exam characteristics were found. The pulse oximetry level does not appear to be associated with a particular ED disposition in adult Sickle cell anemia.

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Introduction

Sickle cell anemia (SCA) is the most common inherited blood disorder in the United States, affecting about 1 in 600 African Americans [1]. Uncomplicated vasoocclusive crisis (VOC) represents the most common emergency department (ED) presentation and is the result of microvascular sludging and thrombosis causing tissue hypoxia and ischemia. The emergency physician has 2 main duties regarding a patient with SCA: to provide analgesic therapy and to evaluate for the presence of coexisting and potentially life-threatening illness. The patient with SCA is at risk for a number of Disease states. They are at high risk for infection with encapsulated organisms and are thus more susceptible to pneumonia [2]. More ominous is acute chest

Presented at the ACEP Research Forum, October 2003.

T Corresponding author. Tel.: +1 215 955 6844; fax: +1 215 923 6225.

E-mail address: [email protected] (B.L. Lopez).

syndrome, a condition of new Pulmonary infiltrates, fever, chest pain, and hypoxia. Acute chest syndrome is the leading cause of death and second leading cause of hospitalization in the patient with SCA [3]. For this reason, the ability to accurately evaluate the pulmonary status of patients with SCA is important.

Pulse oximetry is a noninvasive and commonly used test in the ED for detecting hypoxemia. Although it has been found useful in many conditions, its usefulness in SCA, especially in the ED, has not been fully determined. The few studies available, typically comparing pulse oximetry to arterial blood gas and/or measured oxygen saturation, show conflicting results regarding the accuracy of pulse oximetry in determining oxygenation in the sickle cell population [4-6]. Further, the vast majority of the studies have been done in children and in either an inpatient or an outpatient setting; the ED literature is deficient. In the only ED study done to date, we found that pulse oximetry underestimated Arterial oxygenation in 13 adult ED subjects

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presenting with uncomplicated VOC [7]. No studies have examined its actual use in the ED setting. The purpose of this study was to examine the association between pulse oximetry and ED disposition in VOC through a retrospec- tive chart review. We hypothesize that a categorical range of pulse oximetry reading is not associated with differences in the disposition of the adult patient with VOC presenting to the ED.

Methods

This study was approved by the Institutional Review Board. We conducted a retrospective chart review of all subjects who presented to an urban university ED from March through October 2003 with either an initial complaint of or a final ED diagnosis of VOC. Included were those 18 years or older with at least one chief complaint of VOC and who had a pulse oximetry reading obtained in the ED. Subjects with sickle cell heterozygous SS, homozygous SC, and b thalassemia disease as docu- mented by prior hemoglobin electrophoresis in our SCA center were eligible. Excluded were those younger than 18 years.

To assure capture of consecutive patients with VOC, subjects were identified retrospectively on a daily basis. The research nurse or assistant reviewed both the daily ED computer log as well as the ED charts for the preceding 24 hours for Sickle cell disease and VOC. In addition, the names of commonly treated patients with sickle cell (identified from a list of patients with SCA treated at our institution-we are a National Institutes of Health desig- nated regional sickle cell center with a regular cohort of

150 patients) were searched. Charts were reviewed to determine if the subject presented with typical (defined by the subject as pain of typical severity and location based upon prior episodes) or atypical (different severity and/or location or presence of a coexistent infectious illness) VOC. Because of our designation as an NIH Regional Sickle Cell Center, we routinely document if a patient with sickle cell has typical (defined by the patient based upon their past experiences on severity and location) or atypical pain (defined as pain of different severity, location, or duration). Because of our documentation practices with regards to patients with sickle cell, if an item was not recorded, it was considered normal.

Table 1 Disposition by pulse oximetry category

NORMAL (z95%)

Admitted (n = 88)

Discharged (n = 120)

Left during treatment (n = 2)

P N .05, v².

67

94

2

LOW

(b95%)

21

26

0

B.L. Lopez et al.

Table 2 Chest radiographs

NORMAL (n = 47)

Chest pain (n = 44) Cough (n = 2)

Shortness of breath (n = 1)

LOW (n = 19)

Chest pain (n = 15) Cough (n = 2)

Shortness of breath (n = 2)

P b .05, v²

For the purposes of comparison, subjects were divided into 2 groups-a Normal group (pulse oximetry z95%) and a LOW group (pulse oximetry b95%). Ninety-five percent was chosen as the cutoff based upon the fact that it represents a Pao₂ of 75 mm Hg, considered by many to be the lower end of normal. The groups were compared for differences in historical/physical examination data, labora- tory testing, radiographs, and final ED disposition. Cate- gorical data were analyzed using v² testing; continuous data were analyzed with the t test. Continuous data were presented as mean F SD.

Results

Two hundred ten consecutive charts representing all patients with sickle cell who presented to the ED met Inclusion/exclusion criteria and were reviewed. All subjects enrolled were patients of our Sickle Cell Center. The mean age of the group was 34.9 F 1.1 years; 114 were men and 96 were women.

In the NORMAL group (n = 163), the mean pulse oximetry was 97.87% F 1.7%; the LOW group (n = 47) was 91.55% F 2.44%. There was no significant difference between NORMAL and LOW groups with regards to patient disposition (Table 1, P N .05). There was no significant difference in historical and physical exam characteristics between groups.

A higher percentage of subjects in the LOW group had a chest radiograph done when compared to the NORMAL group (19 [47%] vs 47 [29%], P b .05) (Table 2). All 47 in the NORMAL group and all but one in the LOW group were read as bno new infiltrate.Q The one subject with the new infiltrate presented with atypical VOC accompanied by chest pain and shortness of breath had a temperature of 100.58F, a heart rate of 123, a pulse oximetry reading of 86% (the lowest reading of our cohort), and was admitted with a final ED diagnosis of sickle cell crisis with possible acute chest syndrome. The most common blood tests ordered in the evaluation of the patient with sickle cell in the ED are the complete blood count and reticulocyte count. In our study, 87 (53.3%) in the NORMAL and 26 (55.3%) in the LOW group had a complete blood count/reticulocyte count drawn ( P N .05), illustrating that no significant association with pulse oximetry level and blood testing was noted. There was a nonsignificant trend toward lower Hemoglobin levels in the LOW group (8.75 F 2 g/dL NORMAL vs 7.92 F 2.2 g/dL LOW, P = .07; t test).

Pulse oximetry in the adult ED patient with sickle cell 431

Most of the patients (196/210) presented to the ED with typical VOC. The other 14 presented with atypical VOC alone or with a coexistent illness. Interestingly, 11 of these were categorized in the NORMAL pulse oximetry group.

Discussion

Our study suggests that categorical ranges of pulse oximetry do not have a significant association with the disposition of adult patients with SCA in the ED. It would seem intuitive that a low pulse oximetry reading would be suggestive of and associated with either more severe disease or the presence of a coexistent illness (and hence, a higher amount of admissions to the hospital). Conversely, it would seem likewise intuitive that a high pulse oximetry reading might suggest either a less severe presentation or an absence of a coexistent illness (and thus, a higher likelihood of being discharged from the ED). We found neither to be the case. In our study, a low pulse oximetry reading was not associated with higher admission rate, nor was a high pulse associated with a higher likelihood of being discharged from the ED. In addition, an atypical presentation (with or without coexis- tent illness) was not associated with lower pulse oximetry as 11 of the 14 had normal readings. These findings help to confirm our observation over the years that the pulse oximetry reading in adult VOC may not be a reliable indicator of ED disposition.

Interestingly, there was a higher percentage of subjects in the LOW group that received chest radiographs. Given the potential for a pulmonary process as a cause for hypoxia, this is not surprising and might suggest that the pulse oximetry reading may have influenced diagnostic testing. However, when viewed in another manner, there was a significant number in the LOW group that did not have a chest radiograph done, suggesting that pulse oximetry was not always a determinant. Only one patient in our study had an abnormal chest radiograph-this patient had the lowest pulse oximetry reading of our cohort and had Presenting complaints beyond her typical sickle cell VOC. Laboratory testing did not appear to be affected by pulse oximetry reading as there were equal percentages of patients in the NORMAL and LOW groups having a complete blood count/reticulocyte count drawn.

Pulse oximeters measure arterial hemoglobin saturation and pulse rate by positioning a pulsatile arterial vascular bed between red and infrared light sources and a detector, and detects the differential absorption of light by oxy- and deoxyhemoglobin. A microprocessor then converts this detected light into an arterial blood saturation reading [8,9]. Although it should also work well in SCA, certain characteristics of the disease (such as peripheral arteriolar damage from multiple acute episodes of vascular occlusion over the years, the increased presence of met- and carboxyhemoglobin) [10,11] may prevent it from being a reliable predictor of hypoxemia. In addition, the rightward

shift of the oxyhemoglobin dissociation curve [12] com- monly found in sickle cell disease may also alter the estimation of oxygenation.

The literature on the use of pulse oximetry in SCA is not definitive. Fitzgerald and Johnson [4] measured initial pulse oximetry readings in pediatric ED and intensive care unit patients in a children’s hospital and found correlation with cooximeter-measured saturation in patients with sickle cell. They found that pulse oximetry underestimated arterial O₂ saturation but stated that the bias found was clinically insignificant and concluded that pulse oximetry could be used as a reliable estimate. Blaidsell et al [6] simultaneously measured pulse oximetry and cooximetry in outpatient sickle cell children. They found that pulse oximetry in this setting had poor specificity in detecting hypoxemia and commented that bmany more patients would have been considered hypoxic based on pulse oximetry measurements alone than were hypoxic enough to require intervention.Q The usefulness of pulse oximetry in the adult ED patient with sickle cell is unclear. In our previous study, we found, similar to the Blaidsell study, that pulse oximetry underestimated oxygen- ation in acute VOC. In contrast to these other studies, this study aimed not to determine correlation of pulse oximetry to oxygenation, but rather, attempted to examine pulse oximetry readings as a predictor of disposition in the clinical setting.

Our study has a number of limitations due mostly to its retrospective nature. We are unable to determine the true effect of the pulse oximetry reading on test ordering, decision-making, and disposition. Rather, we could only make the associations or lack thereof between oxygen saturation and the various outcomes. Inadequate documen- tation may have affected the conclusions. This study may also reflect a selection bias of a subgroup of patients with sickle cell who frequent the ED, and thus our findings may not be generalizable to all adult ED patients. Despite these, and given the paucity of literature in this setting, our study appears to shed more light on the actual use of pulse oximetry in the adult patient with sickle cell.

In conclusion, we found that the determination of a normal or abnormal pulse oximetry reading in the adult ED patient with SCA was not associated with a particular patient disposition.

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