Mazzeo AT, La Monaca E, Di Leo R, Vita G, Santamaria LB. Heart rate variability: a diagnostic and prognostic tool in anesthesia and intensive care. Acta Anaesthesiol Scand. 2011 Aug;55(7):797-811. doi: 10.1111/j.1399-6576.2011.02466.x. Epub 2011 Jun 9. PMID: 21658013.