Noncontact holmium:YAG laser thermal keratoplasty for hyperopia.

PURPOSE: To describe our experience with noncontact holmium:YAG laser thermal keratoplasty (Ho:YAG LTK). SETTING: Refractive Surgery and Cornea Department, Clinica de Ojos Dr. Nano, Buenos Aires, Argentina. METHODS: In this retrospective study, 182 hyperopic eyes from 116 patients were treated with noncontact Ho:YAG LTK. Mean age was 50 years +/- 7 (SD), and the spherical equivalent (SE) of the subjective mean refraction (SMR) was +2.50 +/- 0.87 diopters (D). Eyes with corneal power of more than 45 D, corneal pachymetry thicker than 550 microns, and previous corneal surgery or disease were excluded. Holmium:YAG laser parameters include one to three rings of eight spots arranged in a radial and symmetrical array. Pulse energy was 240 mJ, with five pulses per spot. Patients were followed for 12 months. RESULTS: The mean uncorrected visual acuity from 3 to 12 months was 20/40 (20/200 to 20/25), improving an average of three Snellen lines. Mean best spectacle-corrected visual acuity was 20/25 at all postoperative examinations, the same as preoperatively. Mean SE of the SMR was +1.50 +/- 0.98 D (range -0.75 to +4.50 D) at 6 months and +1.25 +/- 0.96 D (+0.25 to +3.25 D) at 1 year. After 9 months, 17% of operated eyes were retreated. CONCLUSION: In this study, Ho:YAG LTK was safe and effective, provided satisfactory correction of low hyperopia, and had a low complication rate. Good patient selection is the key to obtaining good results.

Excimer laser photorefractive keratectomy in pediatric patients.

PURPOSE: To evaluate the results of excimer laser photorefractive keratectomy (PRK) for myopia in pediatric eyes with amblyopia resulting from anisometropia. SETTING: Clinica de Ojos Dr. Nano, Buenos Aires, Argentina. METHODS: Five children with amblyopia resulting from anisometropia in whom conventional therapy was unsuccessful had PRK followed immediately by photoastigmatic refractive keratectomy. Mean patient age was 12.4 years, and follow-up was 12 months. RESULTS: Postoperatively, all eyes had reduced anisometropia and a significant improvement in uncorrected and corrected visual acuities. Sixty percent of patients had no haze 12 months after PRK, 20% had trace haze, and 20%, mild. No patient had decreased best spectacle-corrected visual acuity. CONCLUSION: The results of PRK for myopia were good in this small sample of children. Longer follow-up with more patients should be done.

Entrainment of respiratory rhythm by periodic lung inflation: effect of airflow rate and duration.

In anesthesized, paralyzed, and mechanically ventilated rabbits, the rate and duration of airflow were varied to investigate the effect on the central respiratory rhythm during 1:1 entrainment (1 phrenic nerve burst for 1 pump inflation). Our results showed that 1) the largest range of 1:1 entrainment was obtained with the longest inflation durations associated with the lowest flow rates and 2) both inspiratory (TI) and expiratory (TE) durations (as estimated by the period of phrenic nerve activity and phrenic nerve silence, respectively) increased when total respiratory duration (TT = TI+TE) increased. The phase relationships we studied were DI (the time elapsing between the onset of phrenic activity and the beginning of pump inflation) and DE (the time between the end of phrenic activity and the end of pump inflation). Both DI and DE increased when the total ventilatory period increased. However, for constant ventilatory period, DI changed with the rate and duration of airflow, whereas DE remained constant. In addition, under these conditions, TI, TE, and the integrated phrenic nerve activity remained constant independently of airflow rate and duration. The present results are discussed with regard to the pulmonary receptors (slowly and rapidly adapting receptors and C fibers) that are stimulated during mechanical ventilation, and hypotheses are drawn regarding their possible contribution to rhythm generation and the mechanism through which they may act.

Volume detection during voluntary and passive breathing.

The ability to detect small changes in tidal volume (VT) during either volitional or passive breathing was compared in seven normal subjects. Passive breathing was achieved with positive pressure applied at the mouth by a ventilator. Although baseline breathing pattern was similar for each subject during the two types of breathing, the ability of the subjects to detect changes in VT was at least as good, and in general better, during passive as compared to volitional breathing. This suggests that the generation of a motor cortical command to inspire and the resultant respiratory muscle contraction are not essential to the perception of a change in lung volume. An increase in information from receptors in the mouth, pharynx and extrathoracic airways sensitive to positive pressure may be responsible for the increased ability of most subjects to detect changes in VT during passive breathing.

Entrainment of the respiratory rhythm by periodic lung inflation during vagal cooling.

The aim of this study was to determine whether pulmonary receptors other than slowly adapting stretch receptors are capable of entraining the respiratory rhythm when periodically stimulated during artificial ventilation. Experiments were performed on anaesthetised (urethane, 1.5 g/kg) and paralysed (pancuronium bromide, 0.1 mg/kg) rabbits. Vagi were cooled in order to block conduction in the myelinated fibres innervating slowly adapting receptors. The effectiveness of this cooling was assimilated to the absence of the Hering-Breuer inflation reflex and the presence of the deflation reflex. Our results indicate that under such conditions: (1) harmonic entrainment (one phrenic burst for one pump period) can be observed, (2) the range of harmonic entrainment is more limited when the vagi are cooled, and (3) during harmonic entrainment the inspiratory duration and phrenic activity are similar to what is observed at the same period with intact vagi, whereas vagal cooling modifies the phase difference between the phrenic burst and the pump. We have concluded that periodic input from rapidly adapting receptors and/or vagal C-fibres can entrain the respiratory rhythm as does input from slowly adapting receptors but with different patterns as evidenced by phase relationship.