START & STOPP in GWI

Gulf War Illness (GWI) veterans were divided into 2 pathophysiological groups based on their orthostatic tachycardia responses after submaximal exercise. Two thirds had normal increases of 10 to 15 beats per minute between recumbent and standing both before and after exercise. These were termed the Stress Test Originated Phantom Perception (STOPP) phenotype. In contrast, one third had increases in heart rate of more than 30 beats per minute indicating that exercise induced postural tachycardia; there were the Stress Test Activated Reversible Tachycardia (START) group. This study aimed to confirm the original findings of Rayhan (2013).

Gulf War Illness (GWI) veterans had heart rate and blood pressure measured lying down, and then for 5 minutes standing up. Before exercise, all GWI had normal changes of 10 to 15 beats per minute upon standing up. Then they had submaximal bicycle exercise stress tests. Rayhan et al. (2013) discovered that two thirds of the subjects had the same, unchanged response of about 10 to 15 beats per minute upon standing both before and after exercise. These were termed the Stress Test Original Phantom Perception (STOPP) phenotype. In contrast, one third of GWI veterans were found to have normal postural changes before exercise, but all the stress tests they had larger changes in heart rate of over 30 beats per minute. They were termed the Stress Test Activated Reversible Tachycardia (START) phenotype. The importance of the START phenotype was indicated by finding that they had brain stem atrophy by MRI voxel based morphometry, reduced brain blood flow and activation during a cognitive task performed in the fMRI scanner, and differences in biomarkers compared to STOPP and sedentary control subjects. This study was designed to use the identical exercise protocol to verify or refute the presence of START and STOPP phenotypes in GWI. The incremental change in heart rate between recumbent and standing (Delta HR) was determined by having subjects lie quietly at rest. Heart rate and blood pressure was measured at 1 minute intervals. The average recumbent heart rate was determined. Then subjects stood up without assistance. Beginning 1 minute after standing up, heart rate was measured at 1 minute intervals for 5 minutes. Delta HR was found by subtracting each of the 5 standing measurements minus the average recumbent heart rate. If a subject had 2 or more Delta HR measurements of 30 beats per minute or greater while standing, they were called Stress Test Activated Reversible Tachycardia (START). The threshold of 30 beats per minute for Delta HR was based on the criteria for Postural Orthostatic Tachycardia Syndrome (POTS). However, START subjects had normal Delta HR of 10 to 15 before exercise, and so did not have POTS. This was a key finding of the original study that we plan to verify in this study. Study Design: Pre-exercise recumbent and standing heart rate measurements and m Magnetic resonance imaging (MRI). Exercise: Submaximal bicycle exercise stress test. Subjects were monitored while sitting on the bike for 5 minutes. Cycling started with a gradual increase in resistance to increase heart rate to 70% of maximum predicted heart rate (pHR = 220-Age). Cycling continued at 70%pHR for 25 minutes or until the subject wanted to stop. After 25 minutes, the exercise level was increased gradually to reach 85%pHR equivalent to a cardiac stress test. After stopping, heart rate was measured for 5 more minutes while sitting. Post-Exercise: Recumbent and standing heart rate measurements were performed approximately 3, 8, 24 and 36 hours after exercise. Specific times could not be scheduled because of the timing of MRI scans and other procedures. Outcome measure: DeltaHR was the difference between standing heart rates minus average recumbent heart rate. Changes in DeltaHR were measured for up to 48 hr after exercise. START definition: DeltaHR of 30 more greater at 2 or more time points in the 48 hr after submaximal exercise.