Mechanical ventilation. American College of Chest Physicians' Consensus Conference.

Arterial baroreflex function in humans is commonly assessed through a number of laboratory tests based on quantification of the reflex responses in heart rate or blood pressure to external stimuli applied to the cardiovascular system. Evidence is available that these laboratory estimates of baroreflex sensitivity have both pathophysiological and clinical relevance. Indeed, a number of studies have shown that the sensitivity of the baroreceptor-heart rate reflex may have a prognostic value in myocardial infarction, heart failure and diabetic patients, where mortality seems to be inversely related to the sensitivity of cardiac baroreflex modulation. A deeper insight into the features of daily-life baroreflex cardiovascular control has been offered more recently by techniques based on computer analysis of spontaneous blood pressure and heart rate fluctuations. This innovative approach allows spontaneous baroreflex sensitivity to be assessed in real life conditions, with no need for external stimulation of the patient as required by the older laboratory techniques. This review will briefly survey the methods most widely used to assess baroreflex function in humans, in the laboratory and in daily life.

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How to measure baroreflex sensitivity: from the cardiovascular laboratory to daily life.

BackgroundCore hypothermia after induction of general anesthesia results from an internal core-to-peripheral redistribution of body heat and a net loss of heat to the environment. However, the relative contributions of each mechanism remain unknown. The authors evaluated regional body heat content a

Heat flow and distribution during induction of general anesthesia.

Effect of long-acting testosterone treatment on functional exercise capacity, skeletal muscle performance, insulin resistance, and baroreflex sensitivity in elderly patients with chronic heart failure a double-blind, placebo-controlled, randomized study.

An improved methodology and systems for delivery of breathing gas such as for the treatment of obstructive sleep apnea through application of alternating high and low level positive airway pressure within the airway of the patient with the high and low airway pressure being coordinated with the spontaneous respiration of the patient, and improved methods and apparatus for triggering and for leak management in such systems.

Breathing gas delivery method and apparatus

We compared two methods of assessment of baroreflex sensitivity in eight supine healthy volunteers during repeated baseline measurements and various conditions of cardiac autonomic blockade. The spontaneous baroreflex method involved computer scanning of recordings of continuous finger arterial pressure and electrocardiogram to locate sequences of three or more beats in which pressure spontaneously increased or decreased, with parallel changes in pulse intervals. The mean regression slope of all these sequences during each study condition was considered to represent the mean spontaneous baroreflex slope. In the drug-induced method, sigmoidal curves were constructed from data obtained by bolus injections of phenylephrine and nitroprusside; the tangents taken at the resting pressure of each of these curves were compared with the mean spontaneous baroreflex slopes. The two methods yielded slopes that were highly correlated (r = .96, P < .001), with significant but similar intraindividual baseline variability. Atropine virtually eliminated the baroreflex slope; subsequent addition of propranolol did not alter it further. Propranolol or clonidine alone increased average baroreflex slope to the extent that they increased resting pulse interval (r = .69 to .83). The spontaneous baroreflex method provides a reliable, noninvasive assessment of human vagal cardiac baroreflex sensitivity within its physiological operating range.

Spontaneous Cardiac Baroreflex in Humans: Comparison With Drug-Induced Responses

We examined end-tidal CO2 tension (PETCO2) and pulmonary CO2 elimination of CO2 (VECO2) during CO2 insufflation under general anesthesia for three surgical procedures: gynecologic laparoscopy (intraperitoneal CO2 insufflation for 43 +/- 4 min), laparoscopic cholecystectomy (intraperitoneal CO2 insufflation for 125 +/- 14 min), and pelviscopy (extraperitoneal CO2 insufflation for 45 +/- 3 min). All patients (10 in each group) underwent controlled mechanical ventilation. Oxygen consumption (VO2) and VECO2 were measured at 2-min intervals by a system using a mass spectrometer. For the three surgical procedures, VO2 remained stable, whereas VECO2 and PETCO2 increased in parallel from the 8th to the 10th min after the start of CO2 insufflation. A plateau was reached 10 min later in patients having intraperitoneal insufflation, whereas VECO2 and PETCO2 continued to increase slowly throughout CO2 insufflation during pelviscopy. During pelviscopy, the maximum increase in VECO2 and PETCO2 (76 +/- 5% and 71 +/- 7%) was significantly more pronounced than that observed during cholecystectomy (25 +/- 4% and 25 +/- 4%) and gynecologic laparoscopy (15 +/- 3% and 12 +/- 2%). The authors conclude that CO2 diffusion into the body is more marked during extraperitoneal than during intraperitoneal CO2 insufflation but is not influenced markedly by the duration of intraperitoneal insufflation.

Pulmonary CO2 elimination during surgical procedures using intra- or extraperitoneal CO2 insufflation.

To explore the main factors which could be involved in the fluid retention induced by continuous positive pressure ventilation (CPPV), hemodynamics, renal, and hormonal parameters were measured in seven intensive care patients during three consecutive 60-min periods; one of intermittent positive pressure ventilation (IPPV), one of CPPV (PEEP 10 cmH2O), and finally one of IPPV During CPPV, a 15% decrease in cardiac output was observed, without alteration in arterial pressure or right atrial transmural pressure In addition, decreases were observed in urinary output by 34%, glomerular filtration rate by 19%, renal blood flow by 32%, sodium excretion by 33%, and potassium excretion by 26% There was no change in the fractional excretion of sodium and free water Institution of PEEP also led to a significant increase in plasma renin activity, plasma aldosterone, and urinary antidiuretic hormone, without significant variation in plasma neurophysins and urinary prostaglandins E and F alpha All of the changes that occurred during CPPV were reversed when PEEP was withdrawn It is concluded that the short-term antidiuretic effect of PEEP is mainly due to a hemodynamic impairment of renal function The water- and sodium-retaining hormonal systems also are stimulated and could participate in the fluid retention during more prolonged respiratory support with PEEP

Effect of PEEP ventilation on renal function, plasma renin, aldosterone, neurophysins and urinary ADH, and prostaglandins.

Objective
To evaluate a monitor of pulmonary gas exchange (Deltatrac, Datex) in a clinical setting.

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Clinical validation of the Deltatrac monitoring system in mechanically ventilated patients.

To examine the effect of an alpha-2 agonist, clonidine, on oxygen uptake and on the incidence of postoperative shivering, 28 patients presenting for major abdominal surgery were randomly assigned in a double-blind manner to one of two groups. Intraoperatively, 14 patients received 5 micrograms.kg-1 clonidine infused over 3 h (clonidine group), and 14 patients received placebo (placebo group). Oxygen uptake was measured continuously over the first 3 postoperative hours with a mass spectrometer system. Circulatory variables, esophageal temperature, and skin temperature were measured over the first 6 postoperative hours. Heart rate, mean arterial pressure, rate pressure product, and norepinephrine concentration were decreased in the clonidine group (P less than 2 x 10(-4)). There were no differences among groups in the incidence of shivering and in the rate of increase of esophageal temperature. By contrast, oxygen uptake was lower in the clonidine group (P = 4 x 10(-4)). This contrasting pattern may be secondary to a reduction in the intensity of mean muscular tremor in the clonidine group.

J. P. Viale

Papers

Spontaneous Cardiac Baroreflex in Humans: Comparison With Drug-Induced Responses

Pulmonary CO2 elimination during surgical procedures using intra- or extraperitoneal CO2 insufflation.

Effect of PEEP ventilation on renal function, plasma renin, aldosterone, neurophysins and urinary ADH, and prostaglandins.

Clinical validation of the Deltatrac monitoring system in mechanically ventilated patients.

Oxygen uptake after major abdominal surgery: effect of clonidine.