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Pulse Pressure and Carotid Artery Doppler Velocimetry as Indicators of Maternal Volume Status: A Prospective Cohort Study

Lappen, Justin R. MD*; Myers, Stephen A. DO; Bolden, Norman MD; Shaman, Ziad MD§; Angirekula, Venkata MBBS§; Chien, Edward K. MD, MBA

doi: 10.1213/ANE.0000000000003304
Obstetric Anesthesiology: Original Clinical Research Report
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BACKGROUND: Narrow pulse pressure has been demonstrated to indicate low central volume status. In critically ill patients, volume status can be qualitatively evaluated using Doppler velocimetry to assess hemodynamic changes in the carotid artery in response to autotransfusion with passive leg raise (PLR). Neither parameter has been prospectively evaluated in an obstetric population. The objective of this study was to determine if pulse pressure could predict the response to autotransfusion using carotid artery Doppler in healthy intrapartum women. We hypothesized that the carotid artery Doppler response to PLR would be greater in women with a narrow pulse pressure, indicating relative hypovolemia.

METHODS: Intrapartum women with singleton gestations ≥35 weeks without acute or chronic medical conditions were recruited to this prospective cohort study. Participants were grouped by admission pulse pressure as <45 mm Hg (narrow) or ≥50 mm Hg (normal). Maternal carotid artery Doppler assessment was then performed in all patients before and after PLR using a standard technique where carotid blood flow (mL/min) = π × (carotid artery diameter/2)2 × (velocity time integral) × (60 seconds). The velocity time integral was calculated from the Doppler waveform. The primary outcome was the change in the carotid Doppler parameters (carotid artery diameter, velocity time integral, and carotid blood flow) after PLR. Outcomes were compared between study groups with univariable and multivariable analyses with adjustment for potential confounding factors.

RESULTS: Thirty-three women consented to participation, including 18 in the narrow and 15 in the normal pulse pressure groups (mean and standard deviation initial pulse pressure, 38.3 ± 4.4 vs 57.3 ± 4.1 mm Hg). The 2 groups demonstrated similar characteristics except for initial pulse pressure, systolic and diastolic blood pressure, and race. In response to PLR, the narrow pulse pressure group had a significantly greater increase in carotid artery diameter (0.08 vs 0.02 cm; standardized difference, 2.0; 95% confidence interval [CI], 1.16–2.84), carotid blood flow (79.4 vs 16.0 mL/min; standardized difference, 2.23; 95% CI, 1.36–3.10), and percent change in carotid blood flow (47.5% vs 8.7%; standardized difference, 2.52; 95% CI, 1.60–3.43) compared with the normal pulse pressure group. In multivariable analysis with adjustment for potential confounding factors, women with narrow admission pulse pressure had a significantly larger carotid diameter (0.66 vs 0.62 cm; P < .0001) and greater carotid flow (246.7 vs 219.3 cm/s; P = .001) after PLR compared to women with a normal pulse pressure. Initial pulse pressure was strongly correlated with the change in carotid flow after PLR (r2 = 0.60; P < .0001).

CONCLUSIONS: The hemodynamic response of the carotid artery to autotransfusion after PLR is significantly greater in women with narrow pulse pressure. Pulse pressure correlates with the physiological response to autotransfusion and provides a qualitative indication of intravascular volume in term and near-term pregnant women.

From the *Division of Maternal Fetal Medicine, University Hospitals Cleveland Medical Center, Case Western Reserve University School of Medicine, Cleveland, Ohio

Division of Maternal Fetal Medicine, MetroHealth Medical Center, Case Western Reserve University School of Medicine, Cleveland, Ohio

Departments of Anesthesiology

§Pulmonary and Critical Care Medicine, MetroHealth Medical Center, Case Western Reserve University School of Medicine, Cleveland, Ohio.

Published ahead of print March 1, 2018.

Accepted for publication December 19, 2017.

Funding: None.

The authors declare no conflicts of interest.

Institutional review board: MetroHealth Medical Center IRB Office, Rammelkamp Room 103, 2500 MetroHealth Dr, Cleveland, OH 44109. Address e-mail to cgorden@metrohealth.org.

This study was presented as a poster at the Society for Obstetric Anesthesia and Perinatology 49th Annual Meeting, Bellevue, WA, May 12, 2017 (Abstract #269).

Reprints will not be available from the authors.

Address correspondence to Justin R. Lappen, MD, Division of Maternal Fetal Medicine, University Hospitals Cleveland Medical Center, 11100 Euclid Ave, MAC 5034, Cleveland, OH 44106. Address e-mail to Justin.Lappen@UHhospitals.org.

The assessment and management of maternal volume status are integral elements of intrapartum obstetric care for both normal and complicated pregnancies. Numerous physiological cardiovascular changes occur in pregnancy that may influence the assessment of maternal volume status including plasma volume expansion; increased heart rate, stroke volume, and cardiac output; and decreased colloid oncotic pressure.1–7 Neuraxial analgesia, used by the majority of parturients in the United States,8 significantly alters the maternal hemodynamic profile and is associated with risks of hypotension and new-onset fetal heart rate (FHR) abnormalities.9 Pregnancies complicated by maternal hypertensive, renal, or cardiac disorders pose additional challenges to the assessment of volume status and risks related to intravenous (IV) fluid administration such as volume overload. As such, noninvasive techniques to assess intravascular volume could assist in thoughtful intrapartum fluid management in the obstetric population.

Observational and experimental studies in the literature have demonstrated that pulse pressure is an indicator of central volume status with narrow pulse pressure correlating with hypovolemia.10,11 However, these clinical studies have not included pregnant women and have been conducted primarily in the setting of traumatic hemorrhage. With the availability of compact portable ultrasound units, additional static and dynamic measurements of volume status and fluid responsiveness have become available as clinical tools.12,13 A novel, noninvasive technique for the assessment of volume status is the use of Doppler velocimetry to assess hemodynamic changes in the carotid artery in response to autotransfusion with passive leg raise (PLR).14 PLR increases cardiac preload by inducing gravitational transfer of blood from the lower extremities to the central vasculature, which produces a hemodynamic response similar to that of a 200–300 mL fluid bolus.15,16 Published data have demonstrated that an increase in carotid flow of ≥20% accurately predicts volume responsiveness in critically ill patients.14

Neither pulse pressure nor the carotid artery Doppler response to PLR has been prospectively evaluated in an obstetric population. Additionally, prior studies have not combined these noninvasive parameters in the assessment of volume status. The objective of this study was to test the hypothesis that the carotid artery Doppler response to PLR would be greater in women with a narrow pulse pressure, indicating relative hypovolemia.

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METHODS

Study Oversight

We performed a prospective cohort study of intrapartum women at an urban, tertiary-care maternity hospital. Institutional Review Board approval was obtained at MetroHealth Medical Center before initiation of this study (Institutional Review Board 15-00366; March 23, 2016), and written informed consent was obtained from all study participants. All authors take responsibility for the completeness of the reporting and fidelity of the report to the study protocol.

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Definition of Narrow and Normal Pulse Pressure in Pregnancy

A prior study by Miller et al17 that assessed the relationship between admission pulse pressure and postepidural FHR abnormalities in intrapartum women defined narrow and normal pulse pressure in pregnancy as <45 and ≥45 mm Hg, respectively. These definitions were extrapolated from observations in healthy term pregnant women that stroke volume increases to ≈85 mL18 and from observations in nonpregnant patients that stroke volume is ≈1.7 times the pulse pressure.10,11 Therefore, an estimated normal pulse pressure for a term pregnant woman is 85/1.7 or ≈50 mm Hg. A longitudinal study of blood pressure measurements in healthy pregnant women across gestation reported mean pulse pressure at 36-week gestation of 51 ± 3 mm Hg.19 As such, we chose to define a “narrow” pulse pressure as <45 mm Hg and a “normal” pulse pressure as ≥50 mm Hg for our study to remain consistent with published literature and physiological data.

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Screening and Recruitment

Eligible participants were healthy, normotensive women with a nonanomalous singleton gestation ≥35 weeks who presented for delivery (either spontaneous labor or induction of labor). Potential participants were considered ineligible if any of the following medical complications were present: hypertensive disorders (chronic hypertension, gestational hypertension, preeclampsia, or eclampsia),20 gestational or pregestational diabetes mellitus, cardiomyopathy, maternal congenital heart disease, pulmonary edema, renal insufficiency (serum creatinine, ≥1.0 mg/dL), or substance abuse. Additional exclusion criteria were age <18 years, inability to consent in English, admission systolic blood pressure <90 mm Hg, multiple gestation, suspected intrauterine growth restriction, congenital or chromosomal fetal anomalies, intrauterine fetal demise, and a category 2 or 3 FHR pattern.21

Potentially eligible women underwent blood pressure evaluation before the initiation of intrapartum maintenance IV fluid to determine admission pulse pressure. Blood pressure measurements were obtained using automated oscillometric blood pressure monitors with the patient in a semirecumbent position with left lateral displacement of ≥30°. Measurements were obtained between contractions and at least 1 minute from completion of a contraction as assessed using a tocodynamometer. A pulse pressure value <45 mm Hg (narrow) or ≥50 mm Hg (normal) on both of the first 2 consecutive blood pressure measurements was required for study enrollment. At our institution, women are not routinely instructed to be nil per os (NPO) before admission for labor management (either spontaneous or induction); therefore, time of last oral intake was not assessed for study participants.

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Carotid Doppler Evaluation

After informed consent was obtained, maternal carotid artery Doppler assessment was performed in all patients before and after PLR using a standard technique described by Marik et al14 adapted for pregnancy. Carotid Doppler assessments were completed in all study patients on admission to labor and delivery before the initiation of maintenance IV fluid or any obstetric or anesthetic interventions. A single carotid Doppler measurement was obtained before and after PLR for all study participants. All ultrasonographic images were obtained by a single member of the research team (J.R.L.) who had been trained in this technique. Images were obtained with a commercially available portable ultrasound system (SonoSite MicroMaxx; SonoSite Inc, Bothell, WA) using a 6–13 MHz variable frequency linear array transducer.

Carotid artery Doppler assessment was performed 1 cm proximal to the carotid bulb using the following technique. The course of the common carotid artery was obtained in the short-axis view. The transducer was rotated 90° to obtain a long-axis, longitudinal view of the common carotid artery to visualize the carotid bulb (Figure 1). When a satisfactory view was obtained, the position of the transducer was marked to ensure the same location and acoustic window for all images. At 1 cm proximal to the carotid bulb, the diameter of the common carotid artery was measured from intimal edge to intimal edge perpendicular to the vessel wall. The velocity time integral was measured in the midportion of the vessel using spectral Doppler and an angle correction of 60° with a fixed gate size of 1 mm. Auto-trace was used to determine area under the curve for the velocity time integral using a single-pulse wave.

Figure 1.

Figure 1.

Figure 2.

Figure 2.

Baseline carotid Doppler evaluation was performed with the patient in a 45° semirecumbent position with left lateral displacement of ≥30° to avoid aortocaval compression from the gravid uterus. A standard PLR was then performed in each patient by flattening the head of the bed and elevating the legs to 45° for 30 seconds (Figure 2). Left lateral tilt was maintained during the PLR maneuver to minimize aortocaval compression. After PLR with the patient supine, ultrasound assessment of the common carotid artery was performed again using the aforementioned technique. Repeat assessment was obtained within 1 minute of the PLR maneuver as the maximal change in cardiac output occurs within this time frame.16 Blood flow per minute was calculated automatically by the ultrasound software using the following equation: carotid blood flow (mL/min) = π × (carotid artery diameter/2)2 × (velocity time integral) × (60 seconds) with a single-pulse wave used to calculate the velocity time integral (Figure 1).

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Primary Outcome and Data Collection

The primary outcome of this study was the change in carotid Doppler parameters, including the carotid diameter, velocity time integral, and carotid flow in response to PLR. One member of the research team (J.R.L.) abstracted pertinent clinical information from patient charts. Data were entered in REDCap, a Health Insurance Portability and Accountability Act (HIPPA)-certified, secure web-based data storage platform for research studies.22

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Statistical Analysis and Sample Size

Demographic and obstetric characteristics of the study population were evaluated, and standardized differences were calculated. Continuous variables were compared using the t test or Wilcoxon rank sum test where appropriate, and categorical variables were compared using the χ2 or Fisher exact test where appropriate. Standardized differences with 95% CIs were calculated to compare the change in carotid Doppler parameters (carotid diameter, velocity time integral, and carotid flow) in response to PLR between study groups. Analysis of covariance was performed to assess the post-PLR carotid Doppler parameters between study groups with adjustment for baseline differences and potentially confounding factors. Variables with P < .10 in univariable analysis and baseline carotid Doppler parameters were retained in the model. Pearson correlation was used to assess the relationship between admission pulse pressure and the change in carotid artery flow after PLR. With 3 individual parameters comprising the primary study outcome, a 2-tailed α <.017 was used to define statistical significance. Otherwise, all hypothesis testing was 2-tailed with α of .05 used to define statistical significance. Statistical analysis was performed using Stata 13.1 (StatCorp, College Station, TX). Given that there are no published studies of this technique in an obstetric population to guide a power calculation, a target sample size of 30 was chosen based on the primary study assessing carotid Doppler in a nonpregnant population that demonstrated significant intergroup differences with a cohort of 34 patients.14

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RESULTS

We recruited 33 participants, including 18 with a narrow and 15 with a normal pulse pressure. Patient characteristics and admission data are presented in Table 1. The mean pulse pressure before PLR was significantly different between groups (38.3 ± 4.4 vs 57.3 ± 4.1 mm Hg, narrow versus normal pulse pressure; P < .0001). Except for admission pulse pressure, systolic and diastolic blood pressure, and maternal race, the study groups were similar with regard to other obstetric and demographic characteristics, including admission heart rate, mean arterial pressure, hemoglobin, and body mass index (Table 1).

Table 1.

Table 1.

Carotid artery Doppler data for the study cohort are presented in Table 2. At baseline before PLR, carotid artery diameter, velocity time integral, and carotid flow were similar between the pulse pressure groups (P > .017 for all comparisons). We then compared the changes in carotid Doppler parameters after PLR between the narrow and normal pulse pressure groups. In response to PLR, women with narrow admission pulse pressure had a significantly greater increase in carotid artery diameter (0.08 vs 0.02 cm; standardized difference, 2.0; 95% confidence interval [CI], 1.16–2.84), increase in carotid blood flow (79.4 vs 16.0 mL/min; standardized difference, 2.23; 95% CI, 1.36–3.10), and increase in percent change in carotid blood flow (47.5% vs 8.7%; standardized difference, 2.52; 95% CI, 1.60–3.43) compared to those with normal pulse pressure. The velocity time integral after PLR was not significantly different between study groups (standardized difference, 1.28; 95% CI, 0.53–2.03). Using analysis of covariance with adjustment for baseline carotid Doppler parameters as well as race, labor type (spontaneous versus induction), and gestational age, women with narrow admission pulse pressure had a significantly larger carotid diameter (0.66 vs 0.62 cm; P < .0001) and greater carotid flow (246.7 vs 219.3 cm/s; P = .001) after PLR compared to women with a normal pulse pressure.

Table 2.

Table 2.

Figure 3.

Figure 3.

A scatterplot with best fit line was generated to assess the relationship between initial pulse pressure and percent change in carotid blood flow in response to PLR (Figure 3). The data suggest an inverse linear relationship, with increasing carotid Doppler response to PLR with decreasing pulse pressure. A strong correlation was found between initial pulse pressure and carotid flow after PLR (r2 = 0.60; P < .0001).

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DISCUSSION

In this prospective study, we demonstrated a significant correlation between admission pulse pressure and the hemodynamic response of the carotid artery to PLR. Women with a narrow pulse pressure had a greater increase in carotid artery diameter and carotid blood flow after PLR compared to women with a normal pulse pressure. Given that autotransfusion from PLR increases preload, our findings provide physiologic evidence of the relationship between pulse pressure and central volume status and suggest that women with a narrow pulse pressure have relative hypovolemia, with cardiac function on a steeper part of the Frank–Starling curve. If corroborated, these noninvasive, qualitative indicators of intravascular volume may be applied in intrapartum care to direct fluid management.

The assessment of maternal volume status, while nuanced due to the numerous physiological changes associated with pregnancy and labor,1–7 remains a critical element of intrapartum management. As such, the usage of noninvasive indicators of volume status in the general obstetric population may facilitate the identification of patients at risk for complications related to hypovolemia and the development of tailored prophylactic strategies. For example, the results of this cohort study support those of a recent randomized controlled trial, which highlighted admission pulse pressure as a predictor of postepidural FHR abnormalities, maternal hypotension, and resuscitative interventions and demonstrated a decrease in the frequency of these outcomes with the administration of a larger volume of IV crystalloid coload at epidural placement.23 Additionally, as predictive tools, pulse pressure and the carotid Doppler response to PLR could serve as bedside screening tools for the early identification and treatment of clinically significant obstetric hemorrhage. While our study excluded women with medical comorbidities, pulse pressure and the carotid Doppler response to PLR may also be applicable in women with preeclampsia, cardiac or renal disease, or other clinical conditions where assessment of volume status is more complex. Therefore, future studies of noninvasive indicators of volume status should be assessed in these specific obstetric populations and settings.

Recent systematic reviews and meta-analyses have confirmed the ability of PLR to predict volume responsiveness.13 However, these data were generated in critically ill patients with refractory hypotension, the majority of whom were mechanically ventilated and whose cardiac output was determined using transpulmonary thermodilution techniques or echocardiography. As such, these published data may not be generalizable to the general obstetric population. To our knowledge, our study is the first prospective evaluation of admission pulse pressure (a static parameter) and the carotid Doppler response to PLR (a dynamic parameter) as indicators of volume status in normotensive intrapartum pregnant women.

Strengths of this study include the prospective cohort design, which allowed for the recruitment of balanced study groups equivalent in all clinically relevant characteristics other than the primary exposure (pulse pressure) and race. While maternal race was significantly different between study groups, race was not a significant covariate in multivariable models of pulse pressure and the change in carotid Doppler parameters after PLR. As such, race should not markedly influence our study findings. However, given that the small sample size of our study limits the power to address covariates in a multivariable model, unmeasured confounding cannot be excluded. Additional strengths of this study include that complete data collection was obtained on all patients, and no patients were lost to follow-up. We used an objective, reproducible outcome measure obtained by a single member of the research team with specific training in the technique of carotid Doppler ultrasonography. These characteristics should minimize selection, information, and ascertainment bias and eliminate interobserver variability. Standard techniques for PLR and carotid Doppler assessment were employed14–16 with the modification of uterine displacement to avoid the impact of aortocaval compression. Carotid Doppler assessments were performed in ≤1 minute of the PLR maneuver to capture the maximum hemodynamic effect on cardiac preload.16 We chose to use Doppler changes in the common carotid artery as it is a large, superficial, and readily accessible vessel. While other abdominal central vessels are often used in the assessment of volume status in critical care medicine, such as the inferior vena cava, the use of the common carotid artery avoids potential confounding due to compression by the gravid uterus that may vary between individuals based on fetal size and other anatomic factors.

Limitations of our study also deserve mention. Reviews on the PLR maneuver emphasize the importance of assessing the effect of PLR with direct measurements of cardiac output.12,13 While we were unable to perform echocardiography and obtain contemporaneous, direct measurements of stroke volume and cardiac output, published data have suggested that PLR-induced changes in other vessels such as the carotid or femoral arteries are reliable surrogates.12 Additionally, due to a concomitant randomized trial assessing IV fluid bolus volumes in women with a narrow pulse pressure, we were unable to administer IV fluid boluses to directly assess whether the change in carotid flow predicts volume responsiveness. As such, we cannot characterize or quantify the percentage increase in carotid flow that predicts volume responsiveness in our patient population. However, the difference in the percentage change in carotid flow after PLR between pulse pressure groups in our cohort is consistent with the magnitude of change for responders and nonresponders in the critical care literature, which has used increases of 20% in cardiac output (or surrogate vessels such as the common carotid) as thresholds for predicting volume responsiveness.12–14 Given these limitations, larger validation studies with direct measures of cardiac output, volume responsiveness, and obstetric and anesthetic outcome data are desirable.

While our data suggest an inverse linear relationship between admission pulse pressure and the percent change in carotid flow after PLR, the recruitment of women by pulse pressure groups limits our ability to confirm a linear relationship between these parameters. Similarly, we are unable to comment on women with an admission pulse pressure between 45 and 49 mm Hg. However, the dichotomization of narrow and normal pulse pressure allowed for the appropriate separation of physiologically distinct groups while preserving some ability to demonstrate a continuous effect across a range of pulse pressure values.

Our calculation of carotid flow did not take into account changes in viscosity associated with variations in red cell mass and its impact on rheologic characteristics. However, the percent change in carotid blood flow in response to PLR reflects changes within individuals at a relatively constant blood viscosity, which minimizes the impact on our study outcomes. Additionally, no differences in mean hemoglobin or hematocrit were identified between study groups. Changes in carotid flow may also be subject to autoregulation, which could impact the correlation between carotid flow and cardiac output. Given that our cohort enrolled healthy intrapartum women without hemorrhage, sepsis, preeclampsia, or other medical comorbidities or obstetric complications, the impact of pathologic blood flow redistribution should be negligible. While pregnancy results in physiologic changes in cerebral blood flow and autoregulation,24,25 these changes would be expected to impact both study groups equivalently.

The precision in the measurement of carotid Doppler parameters represents another potential limitation of our study. The change in carotid diameter after PLR was <1 mm in both pulse pressure groups. While the ultrasound machine provides measurements to the 10th of a millimeter, an impact related to the error in the precision of the measurement cannot be excluded. However, given that a standardized, systematic technique of carotid Doppler evaluation was used and the same ultrasound machine was used for all patients, any error in the precision of the measurement should impact both groups equivalently. Additionally, while a trained member of the research team performed all carotid Doppler assessments, intraobserver variability was not assessed.

The impact of increased intraabdominal pressure on the validity of the PLR test is controversial.12,26 While a paucity of data on the influence of pregnancy on intraabdominal pressure has been published, small series have suggested that mean intraabdominal pressure may be significantly higher in pregnant women than nonpregnant individuals.27 As such, we cannot eliminate any impact of pregnancy on the validity of our measurements. However, the fact that our findings were consistent with the critical care literature on PLR and carotid Doppler minimizes this possibility.

The lack of other objective data, apart from pulse pressure and vital signs, to corroborate admission volume status represents another limitation of our study. Laboratory data including specific gravity, serum or urine electrolytes, or urine osmolality are not routinely collected for intrapartum women, and other invasive measures of volume status (such as central venous pressure) are not possible to obtain as part of routine intrapartum care. Patient NPO status was not available for patients in this study. However, given that pulse pressure characterizes central volume status, variation in NPO status should be reflected in admission pulse pressure. Uterine contractions are known to produce a small autotransfusion to the central vasculature,28 which could produce fluctuations in central volume status and confound our results. Given that there was not a statistically significant difference in the proportion of women presenting in spontaneous labor between study groups and that the contribution of autotransfusion is likely to be small relative to that induced by PLR, this phenomenon should not significantly impact the study findings. Last, given the risk for type II error, the small sample size of our cohort precludes analysis of meaningful clinical outcomes.

In summary, in healthy term and near-term pregnant women, the hemodynamic response of the carotid artery to autotransfusion after PLR is significantly greater in those with a narrow pulse pressure. These physiological data suggest that pulse pressure provides a qualitative indication of intravascular volume and may serve to identify women more likely to respond positively to a fluid bolus, including that which may be administered at the time of initiation of neuraxial labor analgesia. Further studies to evaluate the utility of these static and dynamic noninvasive indicators of volume status may help to individualize and direct intrapartum fluid management.

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DISCLOSURES

Name: Justin R. Lappen, MD.

Contribution: This author helped design and conduct the study, analyze the

data, and write and revise the manuscript.

Name: Stephen A. Myers, DO.

Contribution: This author helped design and conduct the study, analyze the data, and write and revise the manuscript.

Name: Norman Bolden, MD.

Contribution: This author helped design and conduct the study, analyze the data, and write and revise the manuscript.

Name: Ziad Shaman, MD.

Contribution: This author helped design and conduct the study, analyze the data, and write and revise the manuscript.

Name: Venkata Angirekula, MBBS.

Contribution: This author helped design and conduct the study, analyze the data, and write and revise the manuscript.

Name: Edward K. Chien, MD, MBA.

Contribution: This author helped design and conduct the study, analyze the data, and write and revise the manuscript.

This manuscript was handled by: Jill M. Mhyre, MD.

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