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Repeated 24 h blood sampling, which is required for time series analyses of metabolites and/or hormones that show strong fluctuations in blood concentration over time, has a higher failure rate in older adults. We tailored existing venipuncture protocols toward use for 24 h blood sampling (sampling frequency of 10 min) in older adults. The following modifications were made: •Pre-sampling: evidence based risk assessment of older adults.•During sampling:•Ultrasound-guided identification and characterisation of veins.•Use of 20-gauge arterial catheter with guide wire for venous access.•Measures to prevent and/or reduce unidirectional blood flow (fluid flow into but not out of the vein) included:•Use of hot water bottles to dilate veins.•Use of small gauge syringes, shortening of the extension line, and slowing of the blood withdrawal rate to reduce pressure on veins.•Stimulation of movement of the arm or retraction of the IV cannula to relieve mechanical flow obstruction.•Post-sampling: prevention of bruising and prolonged bleeding.

Background: Cardiac output by modelflow pulse contour method can be monitored quantitatively and continuously only after an initial calibration, to adapt the model to an individual patient. The modelflow method computes beat-to-beat cardiac output (COmf) from the radial artery pressure, by simulating a three-element model of aortic impedance with post-mortem data from human aortas., Methods: In our improved version of modelflow (COmfc) we adapted this model to a real time measure of the aortic cross-sectional area (CSA) of the descending aorta just above the diaphragm, measured by a new transoesophageal echo device (HemoSonic 100). COmf and COmfc were compared with thermodilution cardiac output (COtd) in 24 patients in the intensive care unit. Each thermodilution value was the mean of four measurements equally spread over the ventilatory cycle., Results: Least squares regression of COtd vs COmf gave y=1.09x[95% confidence interval (CI) 0.96-1.22], R2=0.15, and of COtd vs COmfc resulted in y=1.02x(95% CI 0.96-1.08), R2=0.69. The limits of agreement of the un-calibrated COmf were -3.53 to 2.79, bias=0.37 litre min(-1) and of the diameter-calibrated method COmfc, -1.48 to 1.32, bias=-0.08 litre min(-1). The coefficient of variation for the difference between methods decreased from 28 (un-calibrated) to 12% after diameter-calibration., Conclusions: After diameter-calibration, the improved modelflow pulse contour method reliably estimates cardiac output without the need of a calibration with thermodilution, leading to a less invasive cardiac output monitoring method.