Intraoperative PTH monitoring: a new approach based on the identification of the "true" time origin of the decay curve.

Some published criteria for intraoperative monitoring of PTH serum concentrations may cause misleading results, since the timing of samples measured between the pre-incision and pre-excision phase of surgery is often unrecorded. In our opinion this information is critical, as the time of an intermediate sample during surgical manipulation may represent the "true" beginning of the PTH decay. We modified the usual criterion of monitoring (cut-off at 10 minutes after clamping) proposing a further check at manipulation in case the primary check at clamping produces an apparently negative result. On the basis of a mathematical model, false negative curves were simulated by means of a time shift. This shift was assumed to be the interval between manipulation and clamping. Analysing the decay curve, we used the 50% cut-off at 10 minutes after the supposed "true" origin (clamping or manipulation). Using a rapid immunochemiluminometric assay (ICMA), data were collected from 22 patients successfully operated for parathyroid adenoma. The check at clamping correctly diagnosed 13 patients. Among the 9 false negative cases, 6 were correctly diagnosed considering the manipulation as the baseline value. In the remaining 3 patients, diagnosis required prolonged observation of the curves. In case the iPTH decay does not follow the expected curve, it can be useful to check the decay normalising to a pre-excision value. The advantages of our criterion are both the prompt recognition of false negative results and the construction of a "true" decay curve for each patient, supporting the surgeon during the excision of hyperfunctioning parathyroid tissue.

On the interaction between respiratory compartments during passive expiration in ARDS patients.

Relaxed expiratory volume-time profile has been frequently analysed by fitting exponential functions of time to one- or two-compartment models. In the latter case, the two exponential constants are assumed as representing the time constants of both compartments. Least-square fittings on the experimental data of five consecutive mechanically ventilated supine patients with acute respiratory distress syndrome (ARDS) were performed using rate-constants (flow/volume ratio) as parameters in order to obtain the model matching. Passive expiratory volume-time curves were recorded under PEEP = 0 and 13.6 +/- 3.3 S.D. cmH2O conditions. Model matching was optimal with significant, reliable parameter values. As a result, the use of a PEEP in ARDS patients: (a) delayed expiration; (b) decreased the percentage initial volume contribution of the slow-emptying compartment; and, (c) modified the interaction between compartments. The volume-time profile of the second compartment was found to increase at the beginning of expiration, and, then, progressively decayed towards zero, showing a maximum, although the overall curve decreased throughout expiration.

Determination of rate-constants as a method to describe passive expiration.

To describe the relaxed expiration by a two-compartment model, we introduced a gas/energy transfer between the lung compartment ( V(1)) and a second one ( V(2)). If V(2) were a real volume, the rate-constants (i.e. the flow/volume ratios) of the compartments would describe a real gas-exchange. Alternatively, if a viscoelastic behaviour of the lung or an energy-exchange between compartments was simulated, V(2) would become a "pseudo-volume". We studied nine mechanically ventilated subjects. Changes in volume were transduced by respiratory inductive plethysmography. The rate-constants were assumed (together with the initial volumes of the compartments) as parameters to fit the total volume [ V(1)( t)+ V(2)( t)]. Once the best fitting was performed using these "physiological" parameters, the system was directly identified and the compartments were independently analysed. The time profile of the second compartment showed a maximum that depended on the value of the rate-constants. Appropriate tests confirmed the reliability of our procedure. In conclusion, our analysis demonstrated that the energy/volume of the second compartment may increase at the beginning of expiration and then decrease, showing a maximum, even though the total curve can only be a decreasing one. In other words, the slowing down of the curve representing expiratory volume is due not only to the longer emptying of the second compartment, but also to the interaction between the two compartments. As presently proposed, this interaction can be represented by either a gas exchange between two actual volumes, or a mechanical energy transfer between the lung and the tissue compartment.