Objective:To explore the value of critical ultrasound in evaluating the fluid responsiveness of small dose volume challenge in patients with septic shock.Methods:Thirty-six patients with septic shock admitted to the Third People's Hospital of Datong from January 2021 to December 2021 were enrolled, and the patients were randomly divided into control group and observation group, with 18 patients in each group. The control group was treated with traditional fluid challenge (500 mL of crystalloid injected within 30 minutes); the observation group received a small dose fluid challenge (100 mL of crystalloid injected within 1 minute). The hemodynamic indexes [central venous pressure (CVP), invasive mean arterial pressure (MAP), velocity-time integra (VTI)] and bilateral lung ultrasound scores were measured by critical ultrasound in both groups. The outcome related indicators of patients in the two groups were observed. The correlation between the above indexes and the fluid challenge was evaluated.Results:Compared with the control group, the heart rate (HR) and CVP of patients in the observation group after the challenge were significantly lower than those in the control group [HR (times/min): 99.74±3.22 vs. 107.65±3.14, CVP (mmHg, 1 mmHg ≈ 0.133 kPa): 7.55±0.22 vs. 10.26±0.52, both P < 0.05], invasive MAP and VTI were significantly higher than those in the control group [invasive MAP (mmHg): 77.36±2.14 vs. 69.81±2.56, VTI (cm/s): 68.85±1.26 vs. 44.71±1.28, both P < 0.05]. The ultrasonic score of the observation group was significantly better than those of the control group (all P < 0.05). In terms of outcome, the length of intensive care unit (ICU) stay, mechanical ventilation time and the time for urine volume more than 0.5 mL·kg -1·h -1 of the observation group were significantly shorter than those in the control group [the length of ICU stay (hours): 138.26±1.25 vs. 205.73±1.26, mechanical ventilation time (hours): 36.80±0.25 vs. 47.65±0.36, time to reach urine volume more than 0.5 mL·kg -1·h -1 (hours): 27.38±1.25 vs. 38.61±1.30, all P < 0.05], The dosage of norepinephrine was significantly decreased in the observation group compared with the control group (mg: 45.26±1.85 vs. 53.73±1.92, P < 0.05), and the amount of resuscitation crystalloid was significantly reduced compared with the control group (mL: 1 532.62±12.38 vs. 1 755.52 ± 12.30, P < 0.05). Correlation analysis showed that the volume of crystalloid was highly consistent with M-BLUE pulmonary ultrasound (zone 2, 4 and 5), mechanical ventilation time, norepinephrine dose, time to reach the standard of urine volume and ΔVTI (all P < 0.05). Conclusions:Small dose fluid challenge evaluated by critical ultrasound in septic shock patients has a high value for fluid responsiveness, which can better reduce the risk of obvious tissue edema caused by fluid overload, organ damage and even life-threatening, make fluid challenge more reasonable and appropriate, thereby improving the success of treatment.