Pre-oxygenation in healthy volunteers: a comparison of the supine and 45° seated positions*.

Pre-oxygenation in the seated (sitting) position has been associated with better oxygenation. This randomised, cross over study compared oxygenation in the supine position with that in the 45° seated position in 40 young, healthy volunteers. Oxygen was administered through a circle system and tight fitting facemask. Transcutaneous P(O)2 levels were recorded at 10-s intervals from two measurement points during 4 min of oxygenation in the two positions. The mean (SD) values of 12 measurements taken between the third and fourth minute were recorded. There was no difference in the increase in tissue oxygenation when comparing the supine and seated positions (32.7 (7.3) vs 32.6 (6.7) kPa, respectively). We conclude that there is no evidence that pre-oxygenation in the 45° seated position improves tissue oxygenation in young healthy volunteers compared with the supine position.

The cardiovascular effects of normobaric hyperoxia in patients with heart rate fixed by permanent pacemaker.

To investigate whether the established reductions in heart rate and cardiac output with hyperoxia in humans are primary effects or secondary to increases in systemic vascular resistance, we paced the hearts of nine patients with permanent pacemakers at a fixed rate when breathing either medical air (inspired O(2) fraction 0.21) or oxygen (inspired O(2) fraction 0.80) in a randomised, double-blind fashion. A thoracic bio-impedance machine was used to measure heart rate, stroke volume and blood pressure and calculate cardiac index and systemic vascular resistance index. Oxygen caused no change in cardiac index (p = 0.18), stroke index (p = 0.44) or blood pressure (p = 0.52) but caused a small (5.5%) increase in systemic vascular resistance index (p = 0.03). This suggests that hyperoxia has no direct myocardial depressant effects, but that the changes in cardiac output reported in previous studies are secondary to changes in systemic vascular resistance.

Cardiovascular changes after achieving constant effect site concentration of propofol.

Target controlled infusions of propofol use a pharmacokinetic/pharmacodynamic model to calculate an effect site concentration of the drug. We assessed the cardiovascular stability of 10 healthy patients using non-invasive thoracic bioimpedance and their 'depth' of anaesthesia using the Bispectral index after they had been anaesthetised to a constant effect site concentration of propofol (6.5 min from starting). Each patient had no surgical stimulus and received no intravenous fluid during the study period. The patients also received effect site target controlled remifentanil (steady state 2.5 min from starting) and a bolus of vecuronium to facilitate intubation and ventilation. We mathematically calculated when each measured parameter would reach a state of stability (i.e. with 95% certainty). Heart rate levelled off at 20 min, Bispectral index at 32 min, and cardiac index and mean arterial pressure at 47 min after achieving effect site stability, the final levels being, respectively, 21%, 47%, 14% and 28% lower than those at effect site stability. We conclude that cardiovascular parameters continue to change to a clinically significant degree after achieving a constant effect site concentration of propofol via target controlled infusions.

Effects of normobaric hyperoxia on haemodynamic parameters of healthy full-term parturients.

Fifteen healthy, full-term women with singleton pregnancies were exposed to an increased F(I)o(2) of 0.4 and their haemodynamic responses measured with a non-invasive transthoracic bio-impedance monitor. There was a mean reduction in cardiac index from 3.18 to 3.03 l x min(-1) x m(-2) (4.7%, p = 0.004). The mean indexed systemic vascular resistance increased from 2049 to 2178 dynes x cm(-5) x m(-2) (5.7%, p = 0.005). There were no significant changes in stroke index, heart rate or mean arterial pressure. This study demonstrates that even a moderate increase in inspired oxygen fraction has significant effects on the cardiovascular system of the term parturient.

The cardiovascular effects of inspired oxygen fraction in anaesthetized patients.

BACKGROUND AND OBJECTIVE: Increased inspired oxygen fractions (FiO2) have significant haemodynamic effects in awake volunteers. We sought to establish whether these effects are also present in anaesthetized patients. METHODS: We prospectively studied 30 ASA I-II patients, 15 in each of a propofol and sevoflurane group. Their haemodynamic responses, awake and anaesthetized, when the FiO2 was changed between 0.3 and 1.0 were measured with a non-invasive transthoracic bio-impedance monitor. RESULTS: While preoxygenating awake patients in both groups the FiO2 was increased from 0.21 to 1.0. This reduced the mean cardiac index (3.38 +/- 0.5 to 3.03 +/- 0.5 L min(-1) m(-2); P < 0.001); reduced the heart rate (HR) (68.1 +/- 10.4 to 62.8 +/- 9.4 beats per minute (bpm); P < 0.001); and reduced the stroke index (50.4 +/- 9.6 to 48.5 +/- 8.6; P = 0.02). It increased the systemic vascular resistance index (2060 +/- 319 to 2220 +/- 382 dyn s(-1) cm(-5) m(-2); P = 0.002); but did not change mean arterial pressure. In the anaesthetized patients, when decreasing the FiO2 from 1.0 to 0.3, mean cardiac index (L min(-1) m(-2) increased (3.06 +/- 0.57 to 3.25 +/- 0.56, P = 0.008 for sevoflurane; 2.76 +/- 0.46 to 2.89 +/- 0.42, P = 0.002 for propofol). The mean HR (bpm) increased (65.1 +/- 7.8 to 69.1 +/- 7.5, P < 0.001 for sevoflurane; 67.5 +/- 11.8 to 72.7 +/- 11.6, P = 0.001 for propofol). The mean systemic vascular resistance (dyn s(-1) cm(-5) m(-2)) decreased (1883 +/- 329 to 1735 +/- 388, P = 0.008 for sevoflurane; 2015 +/- 369 to 1771 +/- 259, P = 0.003 for propofol). Mean arterial pressure (mmHg) decreased (74.8 +/- 8.7 to 71.4 +/- 8.7, P < 0.001 for sevoflurane; 72.1 +/- 8 to 66.5 +/- 6.8, P = 0.002 for propofol). CONCLUSION: O2 has haemodynamic effects in awake and anaesthetized patients. These effects were of overall similar magnitude for patients anaesthetized with propofol and sevoflurane.

Effects of a height and weight adjusted dose of local anaesthetic for spinal anaesthesia for elective Caesarean section.

In this prospective, randomised, double-blind study, we compared the effects of two dosage regimens. Pregnant patients at term were randomly assigned to two groups to be given diamorphine 0.4 mg in hyperbaric bupivacaine 0.5% 2.4 ml or diamorphine 0.4 mg in a volume of hyperbaric bupivacaine 0.5% adjusted according to the patient's height and weight. Adequate anaesthesia was provided in all patients in both groups. The onset of the sensory block for cold and pinprick was faster with the fixed dose regimen (p = 0.01). There were more spinal blocks to above the first thoracic dermatome in the fixed dose group (17.1% vs. 2.2%, p = 0.022). Hypotension occurred in 71.7% vs. 50.0% of patients in the fixed dose and adjusted dose groups respectively (p = 0.035). In the fixed dose group, more patients required ephedrine to treat hypotension (79.5% vs. 56.8%, p = 0.022) and a larger median dose was administered (9 mg vs. 6 mg, p = 0.042). The decrease in mean (SD) arterial pressure was less in the adjusted group (35.0 (16.4) mmHg vs. 28.0 (13.5) mmHg, p = 0.036).

The effect of normobaric hyperoxia on cardiac index in healthy awake volunteers.

Fifteen healthy volunteers were exposed to a stepwise increase in FIO2 between 0.21 and 1.0, and their haemodynamic responses were measured with a non-invasive transthoracic bio-impedance monitor. There was mean reduction in cardiac index from 3.44 to 3.08 l.min-1.m-2 (10.7%, p < 0.001). The mean reduction in heart rate was from 77.3 to 69.1 beats.min-1 (10.5%, p < 0.001) and the mean systemic vascular index increased from 2062 to 2221 dyne.s-1.cm-5.m-2 (7.7%, p < 0.025). There were no significant changes in stroke index or mean arterial pressure. These changes are similar quantitatively and qualitatively to those previously reported by dye dilution techniques.

Post partum pneumothorax: two case reports and discussion.

Pneumothorax is a rare event during pregnancy. We present two cases of pneumothorax occurring after caesarean section under general anaesthesia, including one tension pneumothorax. We summarise risk factors for developing a pneumothorax during pregnancy and discuss differential diagnosis and the anaesthetic management in the labour ward.