Over the last 50 years progress has been made in treating childhood and adult solid and hematological tumors. One commonly used chemotherapeutic agent is doxorubicin (DOX), an anthracycline antitumor antibiotic, which was discovered in early 1960s. Anthracycline (ANT) chemotherapy alone or in combination with other chemotherapeutic agents markedly improves the survival rate of cancer patients, particularly among children, where survival rates have more than doubled over 5 decades. However, the use of ANT chemotherapy is a double‐edged sword in that it can produce chemotherapy induced cardiac dysfunction (CRCD), which can cause death in 50% of the patients with who present with the cardiomyopathy. We hypothesized, that one of the causes for CRGD is a reduction in myocardial blood flow (MBF) that leads to cardiac myocyte hypoxia, producing small areas of ischemia and myocyte damage. We also hypothesize that increasing MBF by the coronary vasodilator chromonar prevents cardiomyocytes from hypoxic damage. To test these hypotheses, we treated mice genetically engineered to show cardiac myocyte fluorescence if the cells are hypoxic (hypoxia fate mapping [HFM]) with DOX for 6 weeks (group 1, N=8) and Dox + Chromonar for 6 weeks (group 2 N=8). Five weeks after treatment we started tamoxifen for 5 days to activate the HFM protocol. Tamoxifen induces Cre in cardiac myocytes and hypoxia activates a program that results in the expression of td‐tomato only in hypoxic cardiac myocytes. In tissue sections, hypoxic cardiac myocytes are visualized by fluorescence microscopy. Cardiac function was measured by ultrasound and MBF was measured with contrast echocardiography. Six weeks after DOX treatment many hypoxic myocytes were observed in the myocardium. Cardiac function was significantly decreased at the end of DOX treatment (% ejection fraction EF=39±3) compared to baseline (62±5.8%). Simultaneous administration of chromonar prevents the cardiomyocytes from hypoxic damage and prevents cardiac dysfunction development. %EF was 58±6 at the end of the chemotherapy. Chromonar also significantly increased MBF in group 2 mice compared to group 1 (P<0.05). Based on these data CRCD may be a type of ischemic heart disease where the treatment reduces MBF, which leads cardiomyocyte hypoxia, and causes progressive, accumulative cardiac injury. Moreover, increasing myocardial blood flow by chromonar prevents the cardiomyocytes from hypoxic damage and development of CRCD. Support or Funding Information NIH, NHLBI Panel A shows the relationship between myocardial blood flow and cardiac work 6 weeks after DOX and DOX + Chromonar treatment. Panel B shows ejection fraction changes in DOX +Chromonar (Group 2 mice)

New Findings What is the central question of this study?Is cerebrovascular reactivity affected by isocapnic changes in breathing pattern? What is the main finding and its importance?Cerebrovascular reactivity does not change with isocapnic variations in tidal volume and frequency. Abstract Deviations of arterial carbon dioxide tension from resting values affect cerebral blood vessel tone and thereby cerebral blood flow. Arterial carbon dioxide tension also affects central respiratory chemoreceptors, adjusting respiratory drive. This coincidence raises the question: does respiratory drive also affect the cerebral blood flow response to carbon dioxide? A change in cerebral blood flow for a given change in the arterial carbon dioxide tension is defined as cerebrovascular reactivity (CVR). Two studies have reached conflicting conclusions on this question, using voluntary control of breathing as a disturbing factor during measurements of CVR. Here, we address some of the methodological limitations of both studies by using sequential gas delivery and targeted control of carbon dioxide and oxygen to enable a separation of the effects of carbon dioxide on CVR from breathing vigour. We confirm that there is no detectable superimposed effect of breathing efforts on CVR.