Myocardial oxygenation is maintained during hypoxia when combined with apnea - a cardiovascular MR study

Abstract

Oxygenation-sensitive (OS) cardiovascular magnetic resonance (CMR) is used to noninvasively measure myocardial oxygenation changes during pharmacologic vasodilation. The use of breathing maneuvers with OS CMR for diagnostic purposes has been recently proposed based on the vasodilatory effect of Co2, which can be enhanced by the additive effect of mild hypoxia. This study seeks to investigate this synergistic concept on coronary arteriolar resistance with OS CMR. In nine anesthetized swine, normoxemic and mild hypoxemic arterial partial pressure of oxygen (Pao2) levels (100 and 80 mmHg) were targeted with three arterial partial pressure of carbon dioxide (Paco2) levels of 30, 40, and 50 mmHg. During a 60-sec apnea from the set baselines, OS T2*-weighted gradient echo steady-state free precession (SSFP) cine series were obtained in a clinical 1.5T magnetic resonance imaging (MRI) system. Arterial blood gases were acquired prior to and after apnea. Changes in global myocardial signal intensity (SI) were measured. Although a greater drop in arterial oxygen saturation (SaO2) was observed in the hypoxemic baselines, myocardial SI increased or was maintained during apnea in all levels (n = 6). An observed decrease in left ventricular blood pool SI was correlated with the drop in SaO2. Corrected for the arterial desaturation, the calculated SI increase attributable to the increase in myocardial blood flow was greater in the hypoxemic levels. Both the changes in Paco2 and Pao2 were correlated with myocardial SI changes at normoxemia, yet not at hypoxemic levels. Using OS CMR, we found evidence that myocardial oxygenation is preserved during hypoxia when combined with Co2-increasing maneuvers, indicating synergistic effects of hypoxemia and hypercapnia on myocardial blood flow.

Keywords: BOLD; CMR; carbon dioxide; hypoxia; myocardial oxygenation.

Figures

Figure 1

Baseline blood gas values (bold lines) and changes of Pao2 and Paco2 (n = 6). After a 60-sec breath-hold, Pao2 (blue) decreased significantly for all levels while the significant change in Paco2 (red) was positive (P < 0.05).

Figure 2

Percent change signal intensity (SI) of the myocardium (green circles) during apnea. Myocardial SI (oxygenation) was maintained or increased from baseline in all levels. At hypoxemic states, the percent change SI of the blood pool (orange squares) was greater and shows a significant decrease. The corrected hyperemia-induced signal was therefore greater in the hypoxemic states. (n = 6, *P < 0.05, from baseline image).

Figure 3

Relationship between signal intensity changes with arterial blood gases at normoxemic baseline states: A positive correlation was observed between the myocardial SI/oxygenation changes and the change in Paco2 (r = 0.50, P = 0.03, n = 18; left panel), while a negative correlation was encountered with the change in Pao2 (r = 0.57, P = 0.01, n = 18; right panel).

Figure 4

Relationship of Hb Saturation and Blood Pool SI: A moderate correlation was observed between the percent change in SI of the left ventricular arterial blood (LVbpSI) and the calculated changes in SaO2 (r = 0.46, P < 0.01, n = 35; upper panel). The measured baseline and post–breath-hold oxygen tension for both the normoxemic (100/50, dark gray) and the hypoxemic (80/50, light gray) levels are simulated on a Hb saturation curve shifted for the hypercapnic states (lower panel) showing the hypoxemic baseline's position on the steeper slope of the curve with its greater decrease in SaO2 after a similar decrease in Pao2 in comparison to normoxemia.