Intraocular pressure and pulse rate measurements by the OBF tonograph--comparison to reference instruments

The purpose of this study was to evaluate the accuracy of intraocular pressure (IOP) and ocular pulse rate (OPR) measurements obtained by the Ocular Blood Flow (OBF) tonograph (OBF Labs, Wiltshire, UK). Measurements of IOP and OPR by the OBF tonograph were compared to those of reference instruments. For IOP evaluation, measurements were obtained on patients with normal and abnormal pressures using the OBF tonograph and the Goldmann applannation tonometer in random alternate order. For the OPR evaluation, measurements were obtained using the OBF tonograph with simultaneous heart rate monitoring by ECG on patients with normal IOP. The validity of the OBF tonograph measurements was quantified in terms of 95% limits of agreement and their relationships to measurements by reference instruments was determined by linear regression analyses. 102 patients were recruited for IOP measurements. Mean IOP obtained by the Goldmann tonometer was 20.7 mmHg (7-42 mmHg, SD 6.98) whilst mean IOP obtained by the OBF tonograph was 20.1 mmHg (8.1-40.2, SD 6.1). Goldmann IOP and OBF tonograph IOP readings were well correlated (r = 0.945). Analysis of the difference in IOP measurements between two instruments (tonograph minus Goldmann tonometer) showed the mean bias to be 0.26 mmHg (-7.8 to +6.1 mmHg) and the 95% limits of agreement to be -4.35 to +4.87 mmHg. Agreement between two instruments appeared to be dependent on the IOP; at IOP lower than 20.6 mmHg there was an overall tendency for the tonograph IOP to be higher than Goldmann IOP and vice versa when IOP was above 20.6 mmHg. 47 patients were recruited for OPR and ECG measurements. Mean pulse rates were 74.8 beats per min (mean 43-110) by ECG and 73.9 beats per min (43-110) by tonography. Analysis of the difference in pulse rate between instruments (tonograph OPR minus ECG pulse rate) against the average pulse rate showed the mean bias to be -0.8 beats and the 95% limits of agreement to be between -7 to +5 beats. Ocular pulse rate values obtained by the OBF tonograph were very accurate when compared to ECG pulse rate. This indicates that there is unlikely to be a systematic lag in continuous ocular pulse waveform recording. Intraocular pressure measurements by the OBF tonograph correlated very well with Goldmann readings over a wide range of pressures and should be valid in the clinical setting.