Signals Analysis in Abdominal Pain and Sepsis Study (SAAPS)

Undifferentiated abdominal pain remains the diagnosis for approximately 25% of patients discharged from the ED and between 35 – 41% for those admitted to the hospital. Approximately 80% of patients discharged with undifferentiated abdominal pain improve or become pain-free within two weeks of presentation, but discriminating these patients is a significant challenge. Furthermore, older patients (>65) with abdominal pain have a six to eight-fold increase in mortality compared to younger patients, account for 20% of ED visits, of which 3 – 4% are for abdominal pain. About 1/2 to 2/3 of these patients requires hospitalisation, while 1/3 require surgical intervention. Some studies suggest that mortality rates among older adults with abdominal pain increase when their diagnosis is not determined in the ED.

Continuous single lead ECG measurement can provide beat-to-beat fluctuations in heart rate, known as heart rate variability (HRV). The diagnostic and prognostic value of HRV, based on frequency spectrum analysis, has been extensively demonstrated in patients suffering from a range of cardiac disorders, including myocardial infarction and coronary artery diseases. Similar fluctuations are also present in the peripheral blood volume waveform or photoplethysmogram (PPG) measured in extremities such as the fingertip and the earlobe.

Continuous single lead ECG measurement can provide beat-to-beat fluctuations in heart rate, known as heart rate variability (HRV). The diagnostic and prognostic value of HRV, based on frequency spectrum analysis, has been extensively demonstrated in patients suffering from a range of cardiac disorders, including myocardial infarction and coronary artery diseases.

Similar fluctuations are also present in the peripheral blood volume waveform or photoplethysmogram (PPG) measured in extremities such as the fingertip and the earlobe. Red and infrared PPG signals are widely used in pulse oximeters for the measurement of arterial oxygen saturation (SpO2); hence, the PPG waveform is also known as the pulse oximetry waveform POW). The beat-to-beat PPG variability (PPGV) derived from the peak, baseline or amplitude of the pulsatile waveform exhibits sympathetic nervous system-related low-frequency fluctuations, which are augmented during active standing or blood loss and suppressed during anesthesia. The potential use of these slow waves as an indicator of the severity of sepsis has also been proposed, based on the reported lysis of sympathetically-mediated peripheral vascular tone; however, whether these spontaneous low-frequency oscillations may be useful in the diagnosis of abdominal pain remains to be elucidated.

Early studies have reported the presence of excessive sympathetic or vagal activity in patients diagnosed with AMI, for instance, manifested as transient tachycardia and bradycardia, suggesting the presence of autonomic disturbance or imbalance, however, there is minimal evidence of the performance of cardiovascular variability in the diagnosis or management of abdominal pain.