For Olena Isayenko, the beeping sound her oxygen machine makes when disconnected from power is far scarier than the screeching of the air raid sirens now commonly heard throughout Kyiv.

She suffers from respiratory failure, meaning she can’t breathe adequately on her own and must receive a constant flow of oxygen through an electrical ventilator just to stay alive.

But the repeated Russian assaults on Ukraine’s power grid have left her gasping for air at times as the capital city continues to experience long blackouts. Other Ukrainians who require a constant power supply to keep vital medical devices running suffer similar fear each time the lights go out.

Green tubes carrying oxygen run across Isayenko’s face as she speaks with CNN at the home she shares with her husband, on the 15th floor of a residential block in Kyiv. Her portable oxygen machine is her lifeline. When the air raid sirens sound during blackouts, putting the elevator out of use, Isayenko, 49, is unable to get down to the block’s bomb shelter — but this worries her less than the lack of power for her ventilator.

“When there is no power, this machine makes a long beep and it reminds me of when I was in intensive care, surrounded by many machines. It sounds like a flatline,” she told CNN.