Phototherapy in a new light

Standardized bench method for evaluating the efficacy of phototherapy devices

AIM

As phototherapy (PT) devices employ a variety of broadband light sources, we developed and tested a standardized bench method for evaluating the efficacy of some devices.

METHODS

To evaluate efficacy, we quantified the in vitro photodegradation rate (expressed as t1/2) of unconjugated bilirubin in solution at 37 degrees C during exposure to a given light source at its mean delivered irradiance to the 2D body surface area (BSA) of newborn models. Reproducibility (between-day variation) of the method was determined at irradiance levels from 10 to 70 microW/cm2/nm on three different days.

RESULTS

Between-day t1/2 measurements had coefficients of variation from 3% to 10%. When t1/2 values were normalized to the exposable 2D horizontal BSA, halogen lamp devices, without and with fiberoptics, were least effective (t1/2=60-108 min and 100-126 min for preterm and term models, respectively). Fluorescent tube devices had t1/2=19-78 min and 25-78 min, for preterm and term models, respectively. Light-emitting diode (LED)-based devices yielded the shortest t1/2 values (16-24 min) for preterm and term newborn models.

CONCLUSIONS

We demonstrated the applicability of the method through the determination of the efficacy of several commercially available PT devices. This standardized method is reproducible and effectively evaluates the relative in vitro efficacy of various devices and may guide further in vitro and in vivo evaluations of devices.

Dermal bilirubin kinetics during phototherapy in term neonates

AIM

To investigate dermal bilirubin kinetics during phototherapy in the presence of neonatal indirect hyperbilirubinaemia.

METHODS

33 neonates with non-haemolytic indirect hyperbilirubinaemia, who required phototherapy, were included in the study. Phototherapy modules containing four normal and four blue fluorescent lamps were used during the study. The transcutaneous bilirubin index (TcBI) was measured in an area of the forehead covered by a 2.5 cm diameter opaque patch and a nearby exposed site. The TcBI obtained from patched and unpatched areas and simultaneous serum bilirubin (SB) concentrations were measured before the start of phototherapy and after 6, 12, 18, 30, 42 and 66 h of phototherapy.

RESULTS

SB concentration and the TcBI from the unpatched area decreased significantly during the first 6 h of exposure, while the TcBI obtained from the patched area decreased significantly after 12 h. The TcBI from the unpatched area was consistently lower than that from the patched area during phototherapy. After the onset of phototherapy, there was a weak, non-significant correlation between SB concentrations and the TcBI from patched and unpatched areas.

CONCLUSION

Phototherapy was effective for both patched and unpatched areas, but the rate of decline was slower in patched areas, only becoming significant in the second 6 h of treatment. There was no significant correlation between the levels of SB and TcBI after the onset of phototherapy, and therefore the use of TcBI cannot be recommended as a surrogate measure of SB.

Phototherapy units in Brazil: are they effective?

The irradiance delivered by a phototherapy unit is an important contributing factor to its efficacy. However, measurement of irradiance during phototherapy requires the use of a photometer which is rarely available in the nurseries of developing countries. We assessed the irradiance of phototherapy units both in the laboratory and in all public hospitals of the City of Rio de Janeiro and found that: a) there is no consensus among companies manufacturing phototherapy units as to the number of fluorescent lights to be used in an equipment since this number varied from 4 to 8, b) over 1/3 of the phototherapy units analysed had 1 or more burnt out lamps, c) the mean irradiance delivered by phototherapy units was 2.36 +/- 0.75 muw/cm2/nm (0.6 to 4.4 muw/cm2/nm) which is considerably lower than the dose currently recommended for clinical efficacy. These results may be relevant to other developing countries where phototherapy meters are seldomly available and irradiance is not routinely measured.

Wavelength-dependent quantum yield for Z----E isomerization of bilirubin complexed with human serum albumin

The quantum yield, phi ZE, for configurational photoisomerization (4Z,15Z----4Z,15E) of bilirubin bound non-covalently to human serum albumin was determined (at 23 +/- 2 degrees C) by laser excitation and chromatographic analysis of products. Values obtained for photoexcitation at 465 nm were about one-half those previously reported. The quantum yield was dependent on excitation wavelength, decreasing from a value of 0.109 +/- 0.010 for excitation at 457.9 nm to a value of 0.054 +/- 0.005 for excitation at 514.5 nm. The wavelength dependence is consistent with rapid transfer of excitation energy between the two non-identical pyrromethenone chromophores of bilirubin in the singlet excited state. Since the quantum yields for photoisomerization and luminescence of bilirubin bound to serum albumin at room temperature are both low, internal conversion processes, rather than Z----E configurational isomerizations, are probably the major pathways for deactivation of photo-excited bilirubin.

Quantum yields for the cyclization and configurational isomerization of 4E,15Z-bilirubin

Extraction of a solution of bilirubin configurational isomers in chloroform with an aqueous solution of human serum albumin was found to remove selectively the 4Z,15E-isomer. This phenomenon was used to develop a method for the purification of the 4E,15Z-isomer of bilirubin. The quantum yield for the cyclization and configurational isomerization of the 4E,15Z-isomer bound to a molar excess of human serum albumin was measured at 450 and 510 nm. The quantum yield for cyclization to form lumirubin was 0.12 and 0.19 at 450 and 510 nm, respectively. The quantum yield for configurational isomerization to form 4Z,15Z-bilirubin was 0.03 and 0.05 at 450 and 510 nm. An analysis of previously published data on the quantum yield for the formation of lumirubin from 4Z, 15Z-bilirubin bound to human serum albumin suggests that all of the formation of lumirubin may occur via consecutive photochemical processes with the 4E,15Z-isomer as an intermediate.

Influence of hyperbaric oxygenation on bilirubin and ditaurobilirubin auto-oxidation and porphyrin-sensitized photo-oxidation

Oxygen concentration controls the reaction rate of porphyrin-sensitized photo-oxidation under constant experimental conditions. The rate of bilirubin and ditaurobilirubin auto-oxidation and meso-tetra-(4-sulphonatophenyl) porphine-sensitized photo-oxidation in aqueous solution increases by about 3.5 times in 0.5 MPa of oxygen. Use of hyperbaric oxygenation in the photodynamic therapy of tumours and in the phototherapy of jaundice is proposed.

Quantum yields for laser photocyclization of bilirubin in the presence of human serum albumin. Dependence of quantum yield on excitation wavelength

The quantum yield for laser photocyclization of bilirubin to lumirubin in the presence of human serum albumin (phi LR) was measured at five monochromatic excitation wavelengths in the range 450-530 nm. Solutions used were optically thin throughout the wavelength range and precautions were taken to exclude contributions from photocyclization of bilirubin XIII alpha impurities. The values obtained (7.2-18 x 10(-4] were lower than those previously reported and showed the following wavelength dependence: 457.9 less than 488.0 less than 501.7 less than 514.5 approximately equal to 528.7. However, the rate of lumirubin formation, normalized to constant fluence, decreased with wavelength over the same wavelength range and no evidence was found that photoisomerization of bilirubin to lumirubin is faster with green (514.5 or 528.7 nm) than with blue (457.9 or 488.0 nm) light. The stereoselectivity of the configurational isomerization of bilirubin to 4Z,15E and 4E,15Z isomers also was studied. This reaction became less regioselective for the 4Z,15E isomer with increasing wavelength. The observed wavelength dependence of phi LR and of the [4E,15Z]: [4Z,15E] ratio at photoequilibrium are consistent with an exciton coupling model in which intramolecular energy transfer can occur between the two pyrromethenone chromophores of the bilirubin molecule in the excited state. Relative rates of lumirubin formation in vivo at different excitation wavelengths and constant fluence were estimated for different optical thicknesses and for different skin thicknesses. These estimates suggest that the recently reported clinical equivalence of blue and green phototherapy lights probably reflects the marked variation of skin transmittance with wavelength more than wavelength-dependent photochemistry. The calculations also indicated that the optimal wavelength for phototherapy is probably on the long wavelength side of the bilirubin absorption maximum.