Problems associated with the measurement of mean circulatory filling pressure by the atrial balloon technique in anaesthetized rats

Autonomic control of the venous system in health and disease: effects of drugs

The venous system contains approximately 70% of the blood volume. The sympathetic nervous system is by far the most important vasopressor system in the control of venous capacitance. The baroreflex system responds to acute hypotension by concurrently increasing sympathetic tone to resistance, as well as capacitance vessels, to increase blood pressure and venous return, respectively. Studies in experimental animals have shown that interference of sympathetic activity by an alpha1- or alpha2-adrenoceptor antagonist or a ganglionic blocker reduces mean circulatory filling pressure and venous resistance and increases unstressed volume. An alpha1- or alpha2-adrenoceptor agonist, on the other hand, increases mean circulatory filling pressure and venous resistance and reduces unstressed volume. In humans, drugs that interfere with sympathetic tone can cause the pooling of blood in limb as well as splanchnic veins; the reduction of cardiac output; and orthostatic intolerance. Other perturbations that can cause postural hypotension include autonomic failure, as in dysautonomia, diabetes mellitus, and vasovagal syncope; increased venous compliance, as in hemodialysis; and reduced blood volume, as with space flight and prolonged bed rest. Several alpha-adrenoceptor agonists are used to increase venous return in orthostatic intolerance; however, there is insufficient data to show that these drugs are more efficacious than placebo. Clearly, more basic science and clinical studies are needed to increase our knowledge and understanding of the venous system.

Measurement of body venous tone

The venous system contains about 70% of the blood volume, and approximately 75% of the venous volume is in the small veins and venules. Veins play an active role in the control of cardiac output (CO) and blood pressure. Drugs that interfere with venous tone have profound effects on CO and blood pressure due to the large venous capacity. Information on body venous tone cannot be obtained from studies using isolated venous preparations and perfused venous beds, which lack modulating cardiovascular reflex mechanisms. In vivo methods used for the assessment of venous function in experimental animals and humans are as follows: the mean circulatory filling pressure (MCFP) method for the determination of body venous tone, constant CO reservoir technique for measuring vascular compliance and unstressed volume, plethysmography or blood-pool scintigraphy along with venous occlusion for measuring the volume and compliance of an organ, linear variable differential transformer (LVDT) technique for estimating the diameter of a human dorsal hand vein, intravascular ultrasound (IVUS) imaging technique to monitor the cross-sectional area of a large vein, and ultrasonic crystals to estimate the dimension of an organ. These methods are described and critically evaluated to disclose their validity, merits and limitations.

Circuit factors in the high cardiac output of sepsis

PURPOSE

The increase of cardiac output (CO) in sepsis must be matched by an increase in venous return. Our goal was to determine which of the determinants of venous return are responsible in volume-loaded and nonvolume-loaded pigs with endotoxemia. The determinants include stressed volume, venous compliance (Cv), venous resistance (RVR) and right atrial pressure (Pra). We also tested the effect of the nitric oxide (NO) synthase inhibitor, N omega-nitro-L-arginine-methyl ester (L-NAME) after the hemodynamics with endotoxin stabilized.

METHODS

Pigs were anesthetized and mechanically ventilated. We measured CO by thermodilution, mean circulatory filling pressure (MCFP) by inflating a balloon in the right atrium, blood volume by dye dilution, and Cv by rapid blood infusions. RVR was calculated from MCFP-Pra/CO). After baseline measurements, we infused 10 micrograms/(kg x h-1) of Escherichia coli endotoxin. Eight animals also received 30 mL x kg-1 of dextran over the 2 hours (volume treated), and seven did not (no volume). After 2 hours we injected 25 mg x kg-1 of the NO synthase inhibitor, L-NAME, and repeated the measurements.

RESULTS

In volume-treated animals, CO increased from 3.9 +/- 0.7 to 5.4 +/- 0.8 L x min-1 (P < .05), and blood pressure (BP) fell from 118 +/- 9 to 76 +/- 12 mmHg. MCFP rose, and there was no change in RVR or Cv, whereas capacitance increased (ie, right shift of pressure-volume curve). Cardiac function (ie, Starling curve) did not change. In no-volume animals, CO fell from 4.47 +/- 0.64 to 2.50 +/- 0.86 L x min-1, BP from 114 +/- 10 to 9 +/- 13 mmHg and MCFP fell. Systemic vascular resistance did not change. Cardiac function was markedly depressed, and the heart rate increased from 143 +/- 13 to 203 +/- 30 beats x min-1. L-NAME restored BP in both groups but also increased RVR and depressed cardiac function.

CONCLUSION

Changes in vascular tone during endotoxemia are dependent on volume status. The increased cardiac output in volume-treated septic animals occurred because of an increase in stressed volume due to the volume given in combination with a dilated vasculature. L-NAME restored arterial tone but decreased CO because of a rise in RVR and decrease in cardiac function.

Renal effects of CO2 insufflation: oliguria and acute renal dysfunction in a rat pneumoperitoneum model

OBJECTIVE

To determine the mechanism by which oliguria develops during raised intra-abdominal pressure secondary to CO2 insufflation, we created a rat pneumoperitoneum model.

METHODS

Male Sprague-Dawley rats (n = 67) were organized into three groups. Each group was subjected to abdominal pressures of 0 (control), 5, or 10 mm Hg, over one, two, and four hours. Fourteen additional rats underwent a two-hour period of 10 mm Hg insufflation pressure followed by desufflation to 0 mm Hg. Urine output (UO) and serum creatinine levels were measured both during insufflation at one, two, and four hours, and two, four, ten, and twenty-two hours following its release. These measurements were compared to control values at each time point. Ultrasonic flow probes placed around both the inferior vena cava (IVC) and abdominal aorta during insufflation characterized the effects of increased abdominal pressure on blood flow. The flow rate was determined at insufflation pressures of 0 (control, 100% flow) to 25 mm Hg.

RESULTS

Rats subjected to 10 mm Hg pressure had significant decreases in UO (oliguria) compared to controls for up to four hours (P < 0.01). There were no significant differences in UO in the control or 5 mm Hg groups over each time interval. While a reduction in UO was observed at two, four, and ten hours postrelease, significance was achieved only at ten hours (P < 0.006). By twenty-two hours postrelease, no differences in UO were observed. Serum creatinine elevations declined two hours postdesufflation. IVC flow was reduced by 92.9 percent at 10 mm Hg, while arterial flow decreased by 46.4 percent. Flow was restored to preinsufflation levels after release of pneumoperitoneum.

CONCLUSIONS

Oliguria can be produced in rats undergoing pneumoperitoneum. The renal effects of pneumoperitoneum are most likely related to renal vascular insufficiency from central venous compression.