Deliberate arterial hypotension does not reduce intraocular pressure in pigs

Changes of intraocular pressure and ocular perfusion pressure during controlled hypotension in patients undergoing arthroscopic shoulder surgery: A prospective, randomized, controlled study comparing propofol, and desflurane anesthesia

PURPOSE

The aim of the present study is to evaluate the effects of deliberate hypotensive anesthesia on intraocular pressure (IOP) and ocular perfusion pressure (OPP) and compare the effects of propofol total intravenous anesthesia (TIVA) and desflurane anesthesia on IOP and OPP.

METHODS

A total of 50 patients undergoing arthroscopic shoulder surgery in the lateral decubitus position were randomized to receive desflurane or propofol anesthesia. Mean arterial pressure (MAP) was maintained in the range of 60-75 mm Hg during hypotensive anesthesia. IOP was measured using a handheld tonometer at 7 time points: before induction (T1, baseline); immediately after endotracheal intubation (T2); 10 minutes after position change to lateral decubitus (T3); 10, 30, and 50 minutes after the start of hypotensive anesthesia (T4-T6); and at the end of surgery (T7).

RESULTS

MAP decreased about 35% to 38% during hypotensive anesthesia. Compared to baseline values, the IOP at T6 in dependent and non-dependent eyes decreased by 0.43 and 2.74 mm Hg, respectively in desflurane group; 3.61 and 6.05 mm Hg, respectively in the propofol group. IOP of both eyes in the propofol group was significantly lower than in the desflurane group from T2 to T7. OPP of both eyes in both groups was significantly lower than at baseline, except at T2 in the desflurane group. OPP of both eyes in the propofol group was significantly higher than that in the desflurane group at T5 and T6.

CONCLUSIONS

Hypotensive anesthesia reduced IOP and OPP, but propofol TIVA maintained higher OPP than desflurane anesthesia. These findings suggest that propofol TIVA can help mitigate the decrease of OPP during hypotensive anesthesia.

Changes in intraocular pressure due to surgical positioning: studying potential risk for postoperative vision loss

STUDY DESIGN

Parallel group design.

OBJECTIVE

Compare the intraocular pressure responses in the prone flat versus prone Trendelenburg's position.

SUMMARY OF BACKGROUND DATA

Postoperative vision loss (PVL) complicates approximately 0.05% of spine surgeries. Prone positioning is considered a risk factor because it increases intraocular pressure, which may decrease perfusion pressure to the optic nerve (perfusion pressure = mean arterial pressure - intraocular pressure [IOP]). The prone Trendelenburg's position is often used during spine surgery; however, its effect on optic nerve perfusion is unknown. The purpose of this study is to compare the IOP responses in the prone flat versus prone Trendelenburg's positions to determine if prone Trendelenburg's position also risks PVL.

METHODS

Twenty subjects randomized into 2 groups. Group 1 lay in the prone flat position (0 degrees). Group 2 lay in the prone Trendelenburg's position (-7 degrees). IOPs were measured with a hand-held applanation tonometer while seated, 1 minute after assuming the group's position (Time 0), and at 10-minute intervals for 60 minutes.

RESULTS

The differences in mean IOPs with respect to positions and time were significant (P = 0.0001, P = 0.000). There was a significant difference between sitting and all other times for both groups. In Group 1, there was a significant difference in IOP between Time 0 and all other times prone flat (P < 0.05). In Group 2, there was a significant difference in IOP between Time 0 all other times prone Trendelenburg (P < 0.05).

CONCLUSION

IOP increases in the prone Trendelenburg's position, and when combined with other factors, may be a risk factor for PVL. The pathophysiology is discussed and suggestions for clinicians are made.

Preoperative Peribulbar Block in Patients Undergoing Retinal Detachment Surgery Under General Anesthesia: A Randomized Double-Blind Study

Retinal detachment surgery is frequently associated with significant postoperative pain and emesis in adults. In this randomized, double-blind, controlled study we sought to demonstrate that 1% ropivacaine peribulbar (PB) block in conjunction with general anesthesia (GA) improves operative conditions and postoperative analgesia compared with GA combined with subcutaneous normal saline injection into the inferior eyelid. Thirty-one patients were included in each group. Anesthesia was performed with target-controlled infusion propofol and continuous remifentanil infusion adjusted to maintain bispectral index values between 40 and 50. Postoperative analgesia included fixed-dose IV infusion of propacetamol and IV injection of nefopam via a patient-controlled analgesia device. Tramadol was infused IV as rescue medication. Demographic data were comparable between the groups and bispectral index values were maintained at the objective target. In the PB group, fewer patients presented an oculocardiac reflex (6 versus 17; P < 0.01); bleeding interfering with the surgical field was reduced (1 versus 11 patients; P < 0.01); mean time to first nefopam request was longer (148 +/- 99 versus 46 +/- 58 min; P < 0.01); mean nefopam consumption was diminished during the first 6 h after tracheal extubation (18.9 +/- 13.9 versus 28.5 +/- 14.7 mg; P < 0.05); immediate postoperative pain scores were lower; and fewer patients required rescue medication (5 versus 23; P < 0.01). The two groups were similar with respect to the incidence of postoperative nausea and vomiting. Overall, PB block combined with GA improved operating conditions and postoperative analgesia in retinal detachment surgery.

Efeitos da infusão contínua de cetamina sobre a pressão intra-ocular em cães hipovolêmicos anestesiados com desflurano

Compararam-se os efeitos de duas doses de cetamina, administradas em infusão contínua, sobre a pressão intra-ocular (PIO) de 18 cães submetidos à hipovolemia e à anestesia com desflurano. Promoveu-se a hipovolemia em todos os cães, retirando-se 40 ml de sangue/kg de peso. A anestesia foi induzida com desflurano, através de máscara facial, até que a intubação orotraqueal fosse permitida. Decorridos 30 minutos, para estabilização dos parâmetros, iniciou-se a infusão contínua de cetamina. Os cães foram distribuídos, aleatoriamente, em três grupos (n= 6). O grupo I (controle) recebeu solução salina estéril; o grupo II (GII) recebeu cetamina, na dose de 100mig/kg/min, e o grupo III (GIII), cetamina na dose de 200mig/kg/min. A PIO foi medida por tonometria de aplanação. Foram mensurados freqüência cardíaca (FC), ritmo cardíaco, pressão arterial média (PAM), débito cardíaco (DC), pressão venosa central (PVC) e pressão parcial de CO2 no final da expiração (ETCO2). O desflurano não influenciou os resultados da PIO, porém observou-se discreta ação da cetamina em todos os grupos. Foi possível estabelecer relação direta entre os valores de PIO e de ETCO2. A PIO apresentou relação direta somente com a ETCO2.

Humidity of anaesthetic gases with respect to low flow anaesthesia

It has been demonstrated in an experimental study in swine using the scanning electron microscope that a rebreathing technique utilising minimal fresh gas flowrates significantly improves climatization of anaesthetic gases. Consequently, effects of various anaesthetic techniques on airway climate must be assessed, which covers the need for suitable measuring devices. Basic principles and methods of humidity measurement in flowing anaesthetic gases include gravimetric hygrometry, dew point hygrometry, wet-dry bulb psychrometry, mass spectrometry, spectroscopic hygrometry and electrical hygrometry. A custom-made apparatus for continuous measurement of humidity and temperature in the inspired and expired gas mixtures of a breathing circuit (separated by a valve system, integrated between the endotracheal tube and the Y-piece) is described. Comparative evaluation of this apparatus and the psychrometer was carried out. It could be demonstrated that the apparatus, measuring with capacitive humidity sensors, is more suitable for prolonged use under clinical conditions than the psychrometer. In the second part of the study, climatization of anaesthetic gases under clinical conditions was investigated using fresh gas flowrates of 0.6, 1.5, 3.0 and 6.0 l/min. In the inspiratory limb of the circuit an absolute humidity of 21.3 mg H2O/l and a temperature of 31.5 degrees C were obtained after 120 minutes of minimal flow. Humidity and temperature of inspired air obtained with fresh gas flowrates of 6.0 and 3.0 l/min were found to be inadequate for prolonged anaesthesia. Reducing the fresh gas flow to 1.5 l/min increases heat and moisture content in the respired gases, but conditions are still inadequate for prolonged anaesthesia. Sufficient moisture (> or 20 mg H2O/l) and temperature are obtained under minimal flow conditions after one hour.