During the ultrasound study of carotid and vertebral arteries a 74-year-old patient developed a condition manifested by dizziness, weakness, nausea, transient loss of consciousness. Objectively: pale skin, AP-80/60 mm Hg, Ps- 96/min of poor volume. ECG shows sinus tachycardia, left ventricular hypertrophy. Focal neurological symptoms were not found. What is the provisional diagnosis?