Sick Heart River by John Buchan — Read

Next day Leithen collapsed utterly, for the strength went from his legs, and his difficult breathing became almost suffocation. The business of filling the lungs with air, to a healthy man an unconscious function, had become for him a desperate enterprise where every moment brought the terror of failure. He felt every part of his decrepit frame involved, not lungs and larynx only, but every muscle and nerve from his brain to his feet. The combined effort of all that was left of him to feed the dying fires of life. A rough sledge was made and Lew and the Hare dragged him laboriously through the drifts. Fortunately they had reached the wind-swept ridges, where the going was easier. Twenty-four hours later there was delivered at Johnny's camp a man who looked to be in the very article of death.