Officials told the court the 70-year-old Pakistan’s former military ruler was transferred to the Armed Forces Cardiology Hospital in Rawalpindi.
It is the third time Pervez Musharraf has failed to appear in court following two previous security scares.
The treason charges relate to his decision in 2007 to suspend the constitution and impose emergency rule.
Pervez Musharraf denies the charges and says all the accusations against him are politically motivated.
He is the first Pakistani former military ruler to face trial for treason.
If found guilty, Pervez Musharraf could be sentenced to death or life in prison.
He also faces separate charges of murder and restricting the judiciary.
The nature of Pervez Musharraf’s reported heart condition is not yet clear.
One of Pervez Musharraf’s aides told the AFP news agency that he was in “bad shape”.
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