This prospective observational diagnostic accuracy study investigates the efficacy of dynamic central venous oxygen saturation in predicting fluid responsiveness in septic shock, compared to measurements obtained using echocardiography (VTI) and cardiometry. We will correlate the changes in measuring cardiac output by both echocardiography and cardiometry with the changes in dynamic central venous oxygen saturation both at baseline and after fluid challenge to investigate the efficacy of dynamic central venous oxygen saturation in predicting fluid responsiveness in septic shock.

All patients will undergo standardized monitoring to ensure accurate and reproducible hemodynamic assessment throughout the study period.

Blood pressure will be monitored noninvasively using an automated oscillometric cuff applied to the upper arm. MAP will be maintained at ≥65 mmHg with vasopressor support as required, and no changes in vasopressor infusion will be permitted during the measurement period.

A central venous catheter will be inserted in the internal jugular or subclavian vein, with the position confirmed by chest radiography. Central venous blood samples will be collected at baseline and after the fluid challenge for the determination of a Central venous oxygen saturation (ScvO₂).

Echocardiographic Assessment Left ventricular outflow tract (LVOT) diameter will be measured in the parasternal long-axis view by transthoracic echocardiography (Sonoscape S8 Echocardiography), and Doppler assessment of velocity- time integral (VTI) will be obtained in the apical five-chamber view. Stroke volume (SV) will be calculated as LVOT area × VTI, and values will be averaged from three consecutive cardiac cycles over a single respiratory cycle. The echocardiography operator will be an expert intensivist in echo operation (more than 7 years of experience), and the operator will be blinded to ScvO2.

Cardiac Output Monitoring (Electrical Cardiometry) Cardiac output will also be assessed noninvasively using a validated electrical cardiometry device (ICON®, Osypka Medical, Germany).

Parameters including stroke volume, cardiac output, and cardiac index will be documented at baseline and after the fluid challenge. Device calibration will be verified before each study session.

Timing of Measurements

Hemodynamic variables and oxygenation parameters will be recorded at two prespecified time points:

1. Baseline - immediately prior to fluid challenge or passive leg raising maneuver.
2. Post-intervention - within 5 minutes following intervention. Intervention A standardized fluid challenge protocol will be employed. Indications for fluid administration will include: Hypotension, defined as systolic blood pressure (SBP) ≤ 90 mmHg, a reduction in SBP ≥ 40 mmHg in patients with pre-existing hypertension, or mean arterial pressure (MAP) ≤ 65 mmHg, or evidence of tissue hypoperfusion, such as delayed capillary refill time, skin mottling, altered mental status, urine output ≤ 0.5 mL/kg/h for > 2 hours, or hyperlactatemia (> 2 mmol/L).

Eligible patients will receive a rapid infusion of 500 mL isotonic crystalloid solution ( normal saline) administered over a 15-minute period.

Throughout the intervention, all concomitant therapies-including background maintenance fluids, vasopressor or inotrope dosage, and mechanical ventilation settings-will remain unchanged to avoid confounding effects on hemodynamic measurements.

Definition of Fluid Responsiveness Fluid responsiveness will be defined as an increase in stroke volume index (derived from echocardiography or cardiometry) of ≥10- 15% after the intervention. This definition is consistent with established hemodynamic criteria and will serve as the reference standard for comparison with dynamic changes in ScvO₂. Stroke Volume Index (SVI) is the volume of blood pumped by the heart's ventricle with each beat, normalized to the individual's body surface area (BSA).

Data Collection For each patient, the following variables were systematically recorded: Baseline characteristics

• Demographic data (age, sex), and Comorbidities

• Source of infection

• Severity of illness scores:

  o Acute Physiology and Chronic Health Evaluation II (APACHE II) o Sequential Organ Failure Assessment (SOFA)

• Life-support therapies as vasopressor use.

• Clinical outcomes (ICU length of stay, mortality) Hemodynamic and laboratory parameters Measured at two time points: immediately before the fluid challenge and within five minutes after completion.

• Heart rate (beats/min)

• Blood pressure

• Hemoglobin concentration (Hb, g/dL)

• Cardiac output (CO) by echocsardiography and electrical cardiometry

• Central venous blood gas analysis: Central venous oxygen saturation (ScvO₂)

Inclusion Criteria:

• Age > 18 years
• Diagnosis of septic shock according to Sepsis-3 criteria:

Suspected or confirmed infection o Requirement for vasopressors to maintain MAP > 65 mmHg o Serum lactate > 2 mmol/L despite adequate fluid resuscitation

Exclusion Criteria:

• Pregnancy
• Significant valvular heart disease.
• Atrial fibrillation.
• Inadequate echocardiographic window.
• Ongoing bleeding or contraindication to fluid bolus.

This single-center, prospective, observational study will be conducted in the Intensive Care Unit (ICU) at Menoufia University Hospitals. Adults spontaneously breathing patients with septic shock who undergo a fluid challenge as a part of their hemodynamic management in the first 24 hours will be eligible for enrollment. Ethical approval will be obtained from the ethics committee at the Menoufia Faculty of Medicine, and the study will follow ethical guidelines and secure approval from the Institutional Review Board (IRB). Written informed consent will be obtained from each participant or their legal surrogate before study inclusion.