PURPOSE: Previous attempts at measuring respiratory muscle blood flow (RMBF) in humans have required technically demanding and invasive methods on anesthetised subjects. The purpose of this study was to determine if a less invasive method could be used to measure RMBF in conscious humans using near infrared spectroscopy (NIRS) and indocyanine green (ICG) dye.
METHODS: Six healthy humans were instrumented with venous and esophagus balloon catheters. Surface electromyography (EMG) electrodes and NIRS optodes were placed on the 7th intercostal space and on the sternocleidomastoid muscle. Subjects performed 4 minutes of isocapnic hypernpea at 25, 40, 55 and 70% of maximal voluntary ventilation in randomized order. A bolus of ICG (5mg) was injected at rest and during the final minute of each hyperpnea bout. Blood flow index (BFI) was calculated as the change in maximal ICG concentration divided by the rise time of the NIRS derived ICG curve. BFI of the intercostal and sternocleidomastoid muscles were correlated with simultaneously measured work of breathing (WOB) and EMG.
RESULTS: BFI was closely related to the WOB and EMG for both respiratory muscles. The coefficients of determination (R2) on pooled data for BFI vs. the WOB for the intercostal and sternocleidomastoid muscles were 0.992 (P < 0.001) and 0.995 (P < 0.01), respectively, whereas the R2 for BFI vs. EMG for the intercostal and sternocleidomastoid muscles were 0.952 (P < 0.01) and 0.995 (P < 0.01), respectively.
CONCLUSION: BFI closely reflects indices of respiratory muscle work suggesting that BFI accurately reflects changes in RMBF in humans across a wide range of ventilatory loads.
CLINICAL IMPLICATIONS: The regulation of blood flow to primary and accessory respiratory muscles remains unknown because a technique has not been available to measure RMBF in conscious humans. Thus, this new method has considerable potential to provide insight into the pathophysiology of several cardio-respiratory conditions at rest and during exercise and may be a valuable tool to assess RMBF in mechanically ventilated patients.
DISCLOSURE: Jordan Guenette, No Financial Disclosure Information; No Product/Research Disclosure Information