Impact of intermittent apnea on myocardial tissue oxygenation--a study using oxygenation-sensitive cardiovascular magnetic resonance

Abstract

Background: Carbon dioxide (CO(2)) is a recognized vasodilator of myocardial blood vessels that leads to changes in myocardial oxygenation through the recruitment of the coronary flow reserve. Yet, it is unknown whether changes of carbon dioxide induced by breathing maneuvers can be used to modify coronary blood flow and thus myocardial oxygenation. Oxygenation-sensitive cardiovascular magnetic resonance (CMR) using the blood oxygen level-dependent (BOLD) effect allows for non-invasive monitoring of changes of myocardial tissue oxygenation. We hypothesized that mild hypercapnia induced by long breath-holds leads to changes in myocardial oxygenation that can be detected by oxygenation-sensitive CMR.

Methods and results: In nine anaesthetized and ventilated pigs, 60s breath-holds were induced. Left ventricular myocardial and blood pool oxygenation changes, as monitored by oxygenation-sensitive CMR using a T2*-weighted steady-state-free-precession (SSFP) sequence at 1.5T, were compared to changes of blood gas levels obtained immediately prior to and after the breath-hold. Long breath-holds resulted in an increase of paCO(2), accompanied by a decrease of paO(2) and pH. There was a significant decrease of blood pressure, while heart rate did not change. A decrease in the left ventricular blood pool oxygenation was observed, which was similar to drop in SaO(2). Oxygenation in the myocardial tissue however, was maintained throughout the period. Changes in myocardial oxygenation were strongly correlated with the change in paCO(2) during the breath-hold (r = 0.90, p = 0.010).

Conclusion: Despite a drop in blood oxygen levels, myocardial oxygenation is maintained throughout long breath-holds and is linearly correlated with the parallel increase of arterial CO(2), a known coronary vasodilator. Breathing maneuvers in combination with oxygenation-sensitive CMR may be useful as a diagnostic test for coronary artery function.

Conflict of interest statement

Competing Interests: Matthias G. Friedrich is advisor and shareholder of Circle Cardiovascular Imaging Inc., Calgary, AB, Canada. There is a pending patent (US Patent Pending 61_680,981), which protects the use of breathing maneuvers to induce changes of myocardial oxygenation for diagnostic purposes. This does not alter the authors' adherence to all the PLOS ONE policies on sharing data and materials.

Figures

Figure 1. Myocardial and blood pool SI throughout the cardiac cycle.

The area under the curve was calculated from the absolute BOLD-SI values of the 20 phases of the cardiac cycle from images obtained at the start and at the end of a breath-hold of both the myocardium (A) and blood pool (B) of one subject.

Figure 2. Change in arterial blood pool and myocardial tissue oxygenation during a 60 seconds breath-hold.

Phase 11 of the cardiac cycle representing end-systole is presented for both the image from baseline (B) and the end of the breath-hold (C). The analyzed myocardial region is outlined on the BOLD-CMR image by the endocardial (red) and epicardial (green) contours resulting in a trend toward a mean SI increase (+4.8%; p = 0.077, n = 6) (A). The orange contour depicts a region of interest in the blood pool, with a significant mean SI decrease (-8.0%; p = 0.047).

Figure 3. Correlation of myocardial SI in oxygenation-sensitive CMR images to changes in paCO2.

Differences in paCO2 (mmHg) are plotted against the % change in myocardial SI (n = 6). The data shows excellent linear correlation (r = 0.90, p = 0.010). The dashed lines represent the 95% confidence interval.