Studies have shown that the clinical application of ultrasound to measure large blood vessels related parameters has been widely used to assess the blood volume status of patients, with the advantages of simple, non-invasive and non-radioactive, etc. However, these current methods using by ultrasound technology such as diameter and collapsibility index of the inferior vena cava parameters cannot fully reflect the blood volume, the sensitivity and specificity of predicting hypotension after induction are not very satisfied. Recent reports have demonstrated that respirophasic variation in carotid artery blood flow peak velocity (ΔVpeak) and carotid corrected flow time (FTc) can noninvasively assess patient responsiveness to fluids. Thus, we speculate that the combined measurement of the parameters of the carotid artery and the inferior vena cava may more comprehensively and accurately assess the patient's blood volume status, thereby accurately predicting the occurrence of hypotension after induction of anesthesia.

Hypotension is common after induction of general anesthesia, with an incidence of 9% to 90%. Hypotension after induction of anesthesia may increase the risk of cardiovascular and cerebrovascular accidents, postoperative cognitive dysfunction, as well as increase the risk of postoperative acute myocardial injury and increase the mortality rate during hospitalization. The aim of this study is to establish an assessment method that can effectively predict hypotension after induction of anesthesia, so as to prevent possible harm to patients from perioperative hypotension. The study is planned to measure the peak flow velocity variation and corrected blood flow time of the carotid artery, the diameter and cross-sectional area of the inferior vena cava by applying ultrasound before anesthesia, and record the blood pressure and heart rate before induction of anesthesia, and the blood pressure and heart rate per minute within 10 minutes after induction of tracheal intubation. The mean arterial pressure (MAP) was recorded as the basal value before induction of anesthesia. After induction of anesthesia, MAP < 60 mmHg, or a decrease of more than 30% of the basal value, was defined as the occurrence of hypotension.

Inclusion Criteria:

1. Patients who are scheduled to undergo surgery under general anesthesia
2. Age ≥ 18 years old
3. ASA I or II
4. The patient is clearly conscious and has the ability to cooperate with the ultrasound measurement
5. The patient signed the informed consent

Exclusion Criteria:

1. Emergency surgery
2. Consciousness disorder or mental disorder before surgery
3. Sudden emergency vital sign changes before surgery, life-threatening patients
4. Patients with preoperative respiratory distress, elevated intra-abdominal pressure, and expected difficult airway
5. Patients with preoperative autonomic nervous system disease
6. Patients with implantable pacemaker or cardioverter
7. Patients with preoperative peripheral vascular disease, severe cardiovascular disease, unstable angina or cardiac ejection fraction less than 40%
8. Pregnant women
9. Carotid artery stenosis
10. Severe carotid plaque

Conscious adult patients with normal intra-abdominal pressure; no patients with severe diabetes, no severe carotid stenosis and plaque