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This pilot study was designed to determine the feasibility of measuring cerebral blood flow noninvasively after an intravenous bolus of indocyanine green using near-infrared spectroscopy and pulse dye-densitometry. Feasibility aside, this study did not attempt to validate the measured values of cerebral blood flow against an established method of measurement. Twelve healthy volunteers were investigated after peripheral intravenous injection of indocyanine green. Arterial and cerebral changes in indocyanine green concentration were measured using pulse dye-densitometry and near-infrared spectroscopy, respectively. Two methods of calculating cerebral blood flow were used, and a blood flow index was also estimated. Absolute cerebral blood flow was calculated using a modification of the Fick principle and a deconvolution algorithm to derive the impulse residue function. Mean (range) estimated cerebral blood flow for the Fick method was 8.2 mL/100 g/min (4.2-16.2 mL/100 g/min) and 8.3 mL/100 g/min (4.7-15.3 mL/100 g/min) for the impulse residue function method. The impulse residue function method provided a more precise intrasubject estimation of cerebral blood flow compared with the modified Fick principle, with a coefficient of variation of 10.1% versus 25.5%. The blood flow index was 8.6 mg/sec (range: 5.6-17.3 mg/sec) with an intrasubject coefficient of variation of 12.0%. Estimation of cerebral blood flow using near-infrared spectroscopy and pulse dye-densitometry can be made at the bedside after intravenous injection of indocyanine green, and the precision can be improved using a deconvolution algorithm. Notwithstanding the low values obtained for absolute cerebral blood flow, further investigation and validation of this bedside technique is warranted.

Background -The maximum relaxation rate (MRR; percentage fall in pressure/ 10 ms) of oesophageal (POES) and transdiaphragmatic (PDI) pressure slows under conditions of loaded breathing, and has been measured previously in normal subjects. MRR has not been measured in intubated patients weaning from mechanical ventilation. Methods -Five postoperative patients who were expected to wean and nine patients who had previously failed were studied. POES and PDI MRR, peak oesophageal pressure during spontaneous breathing, maximum oesophageal pressure, and the inspiratory duty cycle were measured at rest during mechanical ventilation, in the first two minutes of spontaneous breathing, and after reventilation in those patients who failed, or before extubation in those patients who succeeded. Results -At rest POES MRR in intubated patients had a range of 5-6-11 and PDI MRR 6 9-10 0, with a coefficient of variation of 9-9% and 7-3% respectively. POES and PDI MRR were similar before and after extubation in five postoperative patients, and POES MRR was reflected by endotracheal MRR measured at the airway. In five patients who failed to wean POES and PDI MRR slowed by 47% and 44%, and fully recovered after 10 minutes reventilation. In four patients who were successfully weaned MRR was unchanged during spontaneous breathing. At the time when MRR decreased, the respiratory muscles were heavily loaded in relation to their strength. Conclusions -Weaning failure occurs when the applied load exceeds the capacity of the respiratory muscles, and this is associated with a slowing of respiratory muscle MRR. (Thorax 1994;49:54-60) The ability to sustain ventilation is dependent on central nervous system drive, the strength of the respiratory muscles, and the load applied to them. Available data suggest that central drive to the respiratory muscles is elevated in ventilatory failure,' and in patients who are difficult to wean those who eventually fail have a greater drive than patients who succeed.2 Attention has therefore focused on respiratory muscle function where investigators have sought to establish the importance of respiratory muscle fatigue during weaning.Respiratory muscle fatigue has been defined as an acute failure to generate the required or expected force or velocity which is reversible with rest.3 It can be induced in normal subjects during loaded breathing through an inspiratory resistance.4 The degree and duration of loading needed to produce fatigue has been extensively investigated5 and can be numerically quantified to predict when force loss will occur.' By their nature, these studies have focused on the end point of loaded breathing, usually failure to achieve target pressures. Clinically, most attention is directed at observing the performance of patients at the beginning of a trial of spontaneous breathing in order to predict eventual weaning failure.Under conditions of heavy load the rate of contraction and relaxation of muscle slows. The relaxation rate of the respiratory muscles has been measured from t...

Near-infrared spectroscopy, a bedside technique, has been used to measure changes in cerebral blood volume in normal subjects. We have used the same technique in anesthetized patients and have shown that, when a patient is placed in the head up position, the decrease in cerebral blood volume is attenuated, relative to normal subjects.

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