Hypothermia can cause a low volume low pressure cardiac output with a blood pressure un-recordable by non-invasive techniques. (The image shows suprasternal Doppler cardiac output monitoring.) [1]. To avoid missing a weak pulse, standard advanced life support (ALS) technique is modified, in that the pulse check should last 1 minute.
There is a role for invasive monitoring and non-invasive Doppler or ultrasound techniques in assessing cardiac output but do not let prolonged attempts stop CPR.
If there is any doubt about finding a pulse, start CPR as for a normothermic patient.