Inclusion Criteria:  Respiratory tract infection:  confirmed or highly suspected viral or bacterial pneumonia meeting ARF or ARDS criteria as outlines in the most recent Berlin ARDS consensus statement age 18 years or above arterial and central venous catheters have been inserted or will be inserted as part of routine clinical management mechanical ventilation via an endotracheal tube is expected to continue for the day beyond day of admission adequate transthoracic echocardiographic winds are available to measure the velocity time integral in the left ventricular outflow tract analgosedation is administered as part of routine management of residual neuromuscular blockaded initiated outside ICU OR the administration of analgosedation and/or neuromuscular blockers at doses that achieve full patient-ventilator synchrony are considered part of routine clinical management  Sepsis:  admitted to ICU with a provisional or established diagnosis of septic shock as defined by the Sepsis-3 criteria age 18 years or above arterial and central venous catheters have been inserted or will be inserted as part of routine clinical management mechanical ventilation via an endotracheal tube is expected to continue for at least another two hours adequate transthoracic echocardiographic winds are available to measure the velocity time integral in the left ventricular outflow tract analgosedation is administered as part of routine management of residual neuromuscular blockaded initiated outside ICU OR the administration of analgosedation and/or neuromuscular blockers at doses that achieve full patient-ventilator synchrony are considered part of routine clinical management the administration of a fluid bolus (250 ml or 500 ml) is indicated as judged by the medical officer supervising routine management  Cardiac surgery:  admitted to ICU following cardiac surgery using cardiopulmonary bypass age 18 years and above arterial, central venous and pulmonary arterial catheters have been inserted or will be inserted as part of routine clinical management mechanical ventilation via an endotracheal tube is expected to continue for at least another two hours analgosedation is administered as part of routine management of residual neuromuscular blockade initiated intraoperatively OR the administration of analgosedation and/or neuromuscular blockers at doses that achieve full patient-ventilator synchrony are considered part of routine clinical management the administration of a fluid bolus (250 ml or 500 ml) is indicated as judged by the medical officer supervising routine postoperative management  Exclusion Criteria:  In all cohorts:  age under 18 years known pregnancy arterial and central venous catheters are not indicated as part of routine care known severe valvulopathy ongoing or imminent need for mechanical circulatory support severe haemodynamic instability with imminent transfer for intervention(s) outside ICU patient is not for full active management in ICU patient is not expected to live beyond the day of admission patient is re-admitted to ICU within the same index hospital admission it is not possible to achieve full patient-ventilator synchrony