A crowdfunding appeal has raised over a million dollars towards the medical expenses of popular preacher and writer Voddie Baucham who is battling "full-blown" heart failure.

Baucham, Dean of Theology at African Christian University in Lusaka, Zambia, told supporters on Facebook: "Brethren, if you have ever considered supporting us in prayer, please do so now.

"We are walking through the darkest valley we have ever faced. Not only have we experienced a great deal of loss in recent weeks, but we are also facing a family health crisis."

He said he started to feel unwell during his last speaking tour, with symptoms including shortness of breath and fatigue. With 17 preaching dates in 18 days, he thought he had "worked too hard."

"However, as it turns out, I was experiencing heart failure!" he said. 

He flew to the US for medical treatment earlier this week, eventually reaching Jacksonville, Florida, after battling through the devastating snow storm that hit Texas.

A GoFundMe campaign set up to help with medical expenses has raised nearly $1.3m in just the first five days, surpassing the estimated $1m cost of his treatment. 

The campaign was launched by Tom Ascol, President of Founders Ministries, of which Baucham is a board member.

"Voddie Baucham is a faithful servant of Christ. He has been a dear friend for nearly twenty years [and he] is facing a serious medical trial. Christians around the world are praying for him and his family," he said. 

Baucham had been planning to go to the Mayo Clinic in Jacksonville but revealed that before he could attend his appointment, "things took a turn for the worse" and he ended up in the ER of a local hospital, where he remains. 

"Up to this point we have been playing defense (i.e. don't die!). Now we go on offense (find the problem and fix it)," he said. 

"Thank you for praying and for your words of encouragement. This journey is a long way from over, but we are grateful for every step."