The investigators studied the predictive value of PPV in the patients with different influencing factors;and the method to improve the predictive value,which can improve the application of PPV in ICU.

PPV can be used to predict fluid responsiveness in patients with circulatory shock.However, it is affected by many factors, which leads to the decrease of the applicability of PPV in ICU.Therefore, the investigators studied the predictive value of PPV in the patients with different influencing factors;and the method to improve the predictive value,which can improve the application of PPV in ICU.

Inclusion Criteria:

• Patients requiring invasive mechanical ventilation
• Acute circulatory failure,defined as:systolic blood pressure <90 mmHg or blood pressure in patients with previous hypertension lower than the baseline value> 40 mmHg; vasoactive drugs are required to maintain blood pressure (mean arterial pressure> 65 mmHg); with or without signs of hypoperfusion (oliguria<0.5ml.kg-1.h-1 for at least 2 hours; arterial blood lactic acid>2.5mmol.L-1; skin mottling)

Exclusion Criteria:

• The age of the patient is less than 18 years old or more than 80 years old
• Refuse to sign the informed consent to join the group
• Atrial fibrillation and other arrhythmias that seriously affect the output per beat;
• With pregnancy
• Other situations where the clinician believes that the patient is not suitable for inclusion in the study

Patients with circulatory failure selected to enter the Department of Critical Care Medicine