The impact of insulin-dependent diabetes on ventilatory control in the mouse

Insulin-dependent diabetes mellitus (IDDM) can lead to ventilatory depression and decreased sensitivity to hypercapnia. We examined relationships between ventilation, plasma insulin, leptin, ketones, and blood glucose levels in two mouse models of IDDM: (1) streptozotocin-induced diabetes in C57BL/6J mice on a regular diet or with induced obesity from a high fat diet; and (2) spontaneous diabetes mellitus in NOD-Ltj mice. In both mouse models, IDDM resulted in depression of the hypercapnic ventilatory response (HCVR). This ventilatory depression was not associated with decreases in plasma insulin or leptin levels. There was, however, a strong association between the duration of hyperglycemia, the decline in HCVR, and increased glycosylation of the diaphragm. Hyperventilation was observed in only six of 14 C57BL/6J obese wild-type mice, despite a significant degree of diabetic ketoacidosis (DKA) in all 14 animals. In mice with DKA, there was a significant correlation between the increase in baseline minute ventilation (V E) and hyperleptinemia (r = 0.77, p < 0.01). In leptin-deficient C57BL/6J-Lep(ob) mice, low levels of both V E and ketones were observed. These results suggest that: (1) depression of the HCVR in IDDM is associated with hyperglycemia and glycosylation of the diaphragm; and (2) the hyperventilation of DKA is leptin dependent.

Abbreviated method for assessing upper airway function in obstructive sleep apnea

STUDY OBJECTIVES

Previous studies have shown that the level of flow through the upper airway in patients with obstructive sleep apnea (OSA) is determined by the critical closing pressure (Pcrit) and the upstream resistance (RN). We developed a standardized protocol for delineating quasisteady-state pressure-flow relationships for the upper airway from which these variables could be derived. In addition, we investigated the effect of body position and sleep stage on these variables by determining Pcrit and RN, and their confidence intervals (CIs), for each condition.

DESIGN

Pressure-flow relationships were constructed in the supine and lateral recumbent positions (nonrapid eye movement [NREM] sleep, n = 10) and in the supine position (rapid eye movement [REM] sleep, n = 5).

SETTING

University Hospital Antwerp, Belgium.

PATIENTS

Ten obese patients (body mass index, 32.0+/-5.6 kg/m(2)) with severe OSA (respiratory disturbance index, 63.0+/-14.6 events/h) were studied.

INTERVENTIONS

Pressure-flow relationships were constructed from breaths obtained during a series of step decreases in nasal pressure (34.1+/-6.5 runs over 3.6+/-1.2 h) in NREM sleep and during 7.8+/-2.2 runs over 0.8+/-0.6 h in REM sleep.

RESULTS

Maximal inspiratory airflow reached a steady state in the third through fifth breaths following a decrease in nasal pressure. Analysis of pressure-flow relationships derived from these breaths showed that Pcrit fell from 1.8 (95% CI, -0.1 to 2.7) cm H(2)O in the supine position to -1.1 cm H(2)O (95% CI, -1.8 to 0.4 cm H(2)O; p = 0.009) in the lateral recumbent position, whereas RN did not change significantly. In contrast, no significant effect of sleep stage was found on either Pcrit or RN.

CONCLUSIONS

Our methods for delineating upper airway pressure-flow relationships during sleep allow for multiple determinations of Pcrit within a single night from which small yet significant differences can be discerned between study conditions.

A model of obstructive sleep apnea in normal humans. Role of the upper airway

We determined whether upper airway obstruction in normal individuals with intact reflexes could produce the syndrome of obstructive sleep apnea. Upper airway obstruction was produced in 12 normal individuals by lowering nasal pressure to -10 cm H(2)O during sleep. Full night polysomnography was performed during two consecutive nights of sleep with subatmospheric nasal pressure and compared with control nights before and after the negative pressure nights. We found that the application of negative pressure was associated with the development of recurrent obstructive apneas (non-REM-disordered breathing rate, 32.6 +/- 34.8 and 37.8 +/- 29.1 events/h during each of two negative pressure nights; p < 0.001) that were associated with oxyhemoglobin desaturation, arousals from sleep, and alterations in sleep stage distribution. Moreover, the median daytime sleep latency after two nights of sleep with subatmospheric pressure fell from 6.9 +/- 1.1 to 3.4 +/- 0.6 min, and rose significantly again to 8.1 +/- 1.5 min (p < 0.03) after the control night following subatmospheric pressure nights. Our findings suggest that a decrease in the pharyngeal transmural pressure alone is a sufficient condition for the production of the sleep apnea syndrome in normal individuals.

High-flow transtracheal insufflation treats obstructive sleep apnea. A pilot study

To determine the effect of transtracheal insufflation (TTI) on obstructive sleep apnea (OSA), we examined breathing patterns in five tracheostomized patients with OSA at varying TTI flow rates when breathing with a closed tracheostomy. The breathing patterns and polysomnographic responses to air insufflation were studied as TTI was increased from 0 to 15 L/min for brief periods of non-rapid eye movement (NREM) sleep (Experiment 1). The frequency of sleep-disordered breathing episodes remained high at 0 and 5 L/min (87.0 +/- 33.7 and 79.4 +/- 24.4 episodes per hour NREM) and decreased significantly to 41.3 +/- 31.5 and 43.4 +/- 31.4 episodes/h NREM sleep at rates of 10 and 15 L/min, respectively (p = 0.003). At high levels of TTI (10 and 15 L/min), obstructive apneas and hypopneas decreased but periodic laryngeal obstructions were induced during stage 1 NREM sleep. To prevent laryngeal obstructions, a servo-control system was used to briefly interrupt TTI during these events. When this system was implemented for more prolonged periods of sleep (Experiment 2, total sleep time 176.6 +/- 12.5 min), high-flow TTI (hf-TTI, 15 L/min) led to an overall reduction in the combined frequency of obstructive apneas and laryngeal obstructions from 63.8 +/- 21.8 to 10.7 +/- 9.1 (p < 0.03) and was associated with a marked reduction in arousal frequency from 60.0 +/- 26.0 to 8. 3 +/- 5.4/h in NREM sleep, and from 67.5 +/- 3.5 to 0 +/- 0/h in rapid eye movement (REM) sleep. Our findings demonstrate that hf-TTI stabilized breathing patterns in apneic patients, and was safe and efficacious for prolonged periods of sleep.

Neural and local effects of hypoxia on cardiovascular responses to obstructive apnea

Obstructive sleep apnea (OSA) acutely increases systemic (Psa) and pulmonary (Ppa) arterial pressures and decreases ventricular stroke volume (SV). In this study, we used a canine model of OSA (n = 6) to examine the role of hypoxia and the autonomic nervous system (ANS) in mediating these cardiovascular responses. Hyperoxia (40% oxygen) completely blocked any increase in Ppa in response to obstructive apnea but only attenuated the increase in Psa. In contrast, after blockade of the ANS (20 mg/kg iv hexamethonium), obstructive apnea produced a decrease in Psa (-5.9 mmHg; P < 0.05) but no change in Ppa, and the fall in SV was abolished. Both the fall in Psa and the rise in Ppa that persisted after ANS blockade were abolished when apneas were induced during hyperoxia. We conclude that 1) hypoxia can account for all of the Ppa and the majority of the Psa response to obstructive apnea, 2) the ANS increases Psa but not Ppa in obstructive apnea, 3) the local effects of hypoxia associated with obstructive apnea cause vasodilation in the systemic vasculature and vasoconstriction in the pulmonary vasculature, and 4) a rise in Psa acts as an afterload to the heart and decreases SV over the course of the apnea.

Effects of arousal and sleep state on systemic and pulmonary hemodynamics in obstructive apnea

During obstructive sleep apnea (OSA), systemic (Psa) and pulmonary (Ppa) arterial pressures acutely increase after apnea termination, whereas left and right ventricular stroke volumes (SV) reach a nadir. In a canine model (n = 6), we examined the effects of arousal, parasympathetic blockade (atropine 1 mg/kg iv), and sleep state on cardiovascular responses to OSA. In the absence of arousal, SV remained constant after apnea termination, compared with a 4.4 +/- 1.7% decrease after apnea with arousal (P < 0.025). The rise in transmural Ppa was independent of arousal (4.5 +/- 1.0 vs. 4.1 +/- 1.2 mmHg with and without arousal, respectively), whereas Psa increased more after apnea termination in apneas with arousal compared with apneas without arousal. Parasympathetic blockade abolished the arousal-induced increase in Psa, indicating that arousal is associated with a vagal withdrawal of the parasympathetic tone to the heart. Rapid-eye-movement (REM) sleep blunted the increase in Psa (pre- to end-apnea: 5.6 +/- 2.3 mmHg vs. 10.3 +/- 1.6 mmHg, REM vs. non-REM, respectively, P < 0.025), but not transmural Ppa, during an obstructive apnea. We conclude that arousal and sleep state both have differential effects on the systemic and pulmonary circulation in OSA, indicating that, in patients with underlying cardiovascular disease, the hemodynamic consequences of OSA may be different for the right or the left side of the circulation.

Leptin, obesity, and respiratory function

Leptin is a protein produced by adipose tissue that circulates to the brain and interacts with receptors in the hypothalamus to inhibit eating. The importance of this single peptide is vividly demonstrated by the profound obesity exhibited by the ob/ob mouse (C57BL/6J-Lep(ob)) which is unable to produce functional leptin. The measurement of respiratory function in the ob/ob mouse shows that the profound obesity is associated with impaired respiratory mechanics and depressed respiratory control, particularly during sleep. Longitudinal studies and leptin replacement studies in the ob/ob mouse indicate that leptin may act as both as a growth factor in the lung and as a neurohumoral modulator of central respiratory control mechanisms. Moreover, wildtype mice with diet-induced obesity have normal respiratory function associated with markedly elevated leptin levels. Human obesity, similar to obesity in wildtype mice, also causes an elevation in circulating leptin. However, unlike the tight relationship between obesity and elevated leptin present in an inbred strain of wildtype mice, human obesity is associated with more variable leptin levels for a given degree of adiposity. Thus, the possibility exists that a relative deficiency in leptin, or a leptin resistance, may play a role in obesity-related breathing disorders such as obesity hypoventilation syndrome (OHS) or obstructive sleep apnea (OSA).

Subclinical aortic perforation with the infant double-button patent ductus arteriosus occluder

Modification of the double-button (Sideris) patent ductus arteriosus (PDA) occluder has resulted in a single-strut aortic component rather than the conventional cross-strut design. We report the use of this infant PDA occluder for transcatheter closure in three patients with PDA measuring 2 mm, 3.7 mm, and 4 mm. Subclinical aortic perforation with a small aortic aneurysm developed in two patients 1 year after occluder implantation. The third patient had developed a small aortic aneurysm without perforation at 3-month follow-up. All three patients had a residual shunt and underwent successful PDA surgical closure with aortic aneurysmal repair. Single-strut umbrella designs are not recommended for PDA transcatheter closure.

Effect of sleep/wake state on arterial blood pressure in genetically identical mice

Genetic determinants may contribute to the large variability in arterial blood pressure responses to changes in sleep/wake state in humans. In this study, we developed techniques to examine the relationship between sleep/wake state and mean arterial pressure (MAP) in unrestrained, genetically identical mice (C57BL/6J; n = 9). The left common carotid artery was catheterized, and arterial blood gases were analyzed 24-48 h postsurgery to verify normal respiratory and metabolic function. The animals were then allowed to cycle naturally through sleep/wake states over a 3- to 4-h period while continuous polysomnography and arterial pressure measurements were made. The MAP decreased from quiet wakefulness to non-rapid-eye-movement sleep (9.8 +/- 1.3 mmHg; P < 0.001) and further decreased from non-rapid-eye-movement to rapid-eye-movement sleep (9.7 +/- 1.8 mmHg; P < 0.001). We conclude that the inbred strain of C57BL/6J mice exhibits significant and consistent changes in MAP related to sleep/wake state. Future studies can compare responses in this strain of mice with those in other inbred or transgenic mice to determine whether specific genes regulate arterial blood pressure responses to sleep/wake state.

The hypotonic upper airway in obstructive sleep apnea: role of structures and neuromuscular activity

The structural properties of the upper airway determine its collapsibility during periods of muscle hypotonia. Both rapid-eye-movement (REM) sleep and increases in nasal pressure (PN) produce hypotonia, which persists even after nasal pressure is abruptly reduced. To determine the factors that influence the collapsibility of the hypotonic airway, the critical pressure (Pcrit) and nasal resistance upstream to the site of pharyngeal collapse (RN) were measured in the first three breaths after abrupt reductions in PN during non-REM and REM sleep. PN was reduced abruptly from 15.2+/-3.2 cm H2O (mean +/- SD) for three breaths in 19 apneic patients. Upper-airway pressure-flow relationships were analyzed to determine Pcrit for each breath in non-REM and REM sleep. We found that Pcrit rose (collapsibility increased, p < 0.001) and RN fell (p = 0.02) between the first and third breath after the decrease in PN, whereas no difference in Pcrit was detected between sleep stages. In six patients, genioglossus-muscle electromyograms (EMGs) were recorded. Peak phasic activity rose between the first and third breath (p = 0.03), but tonic and peak phasic EMG activity fell in REM as compared with non-REM sleep (p < 0.001). We conclude that the hypotonic upper airway becomes most collapsible by the third breath after an abrupt decrease in PN, regardless of sleep stage and despite an increase in genioglossus-muscle activity. Our findings suggest that predominantly mechanical rather than neuromuscular factors modulate the properties of the pharynx after abrupt reductions in nasal pressure.

Systemic and pulmonary hemodynamic responses to normal and obstructed breathing during sleep

We examined the hemodynamic responses to normal breathing and induced upper airway obstructions during sleep in a canine model of obstructive sleep apnea. During normal breathing, cardiac output decreased (12.9 +/- 3.5%, P < 0.025) from wakefulness to non-rapid-eye-movement sleep (NREM) but did not change from NREM to rapid-eye-movement (REM) sleep. There was a decrease (P < 0.05) in systemic (7.2 +/- 2.1 mmHg) and pulmonary (2.0 +/- 0.6 mmHg) arterial pressures from wakefulness to NREM sleep. In contrast, systemic (8.1 +/- 1.0 mmHg, P < 0.025), but not pulmonary, arterial pressures decreased from NREM to REM sleep. During repetitive airway obstructions (56.0 +/- 4.7 events/h) in NREM sleep, cardiac output (17.9 +/- 3.1%) and heart rate (16.2 +/- 2.5%) increased (P < 0.05), without a change in stroke volume, compared with normal breathing during NREM sleep. During single obstructive events, left (7.8 +/- 3.0%, P < 0.05) and right (7.1 +/- 0.7%, P < 0.01) ventricular outputs decreased during the apneic period. However, left (20.7 +/- 1.6%, P < 0.01) and right (24.0 +/- 4.2%, P < 0.05) ventricular outputs increased in the post-apneic period because of an increase in heart rate. Thus 1) the systemic, but not the pulmonary, circulation vasodilates during REM sleep with normal breathing; 2) heart rate, rather than stroke volume, is the dominant factor modulating ventricular output in response to apnea; and 3) left and right ventricular outputs oscillate markedly and in phase throughout the apnea cycle.

Endothelin causes portal and pulmonary hypertension in porcine endotoxemic shock

A porcine model of endotoxemic shock was used to test the hypothesis that endothelins (ET) mediate the sustained increases in portal and pulmonary vascular resistances. Anesthetized pigs (n = 18) were instrumented and pretreated with 1) saline as a control; 2) indomethacin (Idm), a cyclooxygenase (Cox) inhibitor; or 3) Idm + bosentan (Bos), a mixed ET-receptor antagonist, and then were treated with endotoxin to produce shock and followed for 240 min. Global and regional hemodynamic parameters and plasma levels of ET-1 and thromboxane B2 were measured. The results show that 1) ET is independently responsible for the sustained increase in pulmonary vascular resistance; 2) ET and Cox products combine to increase portal venous resistance; 3) ET independently reduces cardiac output and attenuates or negates global systemic arterial vasodilation (presumptively mediated by nitric oxide) and exhibits regional differences, having little if any influence on the gut arterial bed. When considered with our prior study of nitric oxide regulation of the same beds in endotoxemic shock (N. Brienza, T. Ayuse, J. P. Revelly, C. P. O'Donnell, and J. L. Robotham, J. Appl. Physiol. 78: 784-792, 1995), the similarities between the portal venous and pulmonary arterial beds suggest that these two beds reflect phenomena occurring in microvascular and/or venous beds in multiple organs. The overall results suggest that a dynamic balance exists between NO and ET regulating arterial and microvascular and/or venous vasomotor activity during the evolution of endotoxemic shock.

Reflex stimulation of renal sympathetic nerve activity and blood pressure in response to apnea

The purpose of this study was to examine the role of afferent input in the reflex modulation of renal sympathetic nerve activity (SNA) in response to apnea. Apneas of 20-, 40-, and 60-s duration were induced in the anesthetized, paralyzed cat (n = 7) ventilated with either room air or 100% oxygen. While receiving room air, there were increases (p < 0.005) in renal SNA of 34.5 +/- 4.2%, 53.3 +/- 6.4%, and 59.9 +/- 7.2% of maximum during the 20-, 40-, and 60-s apneas, respectively. There were corresponding increases (p < 0.025) in mean arterial pressure (Pa) of 9 +/- 3, 30 +/- 9, and 45 +/- 12 mm Hg during the 20-, 40-, and 60-s apneas while receiving room air, respectively. The effect of 100% oxygen was to reduce (p < 0.0001) the renal SNA response to apnea, at a matched level of PaCO2, by at least 80%, and to eliminate any increase in Pa. During the first breath of the postapneic period, there was a partial inhibition of renal SNA. During the second and third breaths of the postapneic period, there was a marked fall in renal SNA that was associated with a precipitous decline in directly recorded carotid chemoreceptor activity (n = 2). The magnitude of the fall in renal SNA after apnea was related to the degree of postapneic hypertension. We conclude that hypoxic chemoreceptor stimulation is the predominant factor generating the renal SNA response to apnea, with modulating inputs from thoracic afferents and arterial baroreceptors likely contributing to the marked inhibition of renal SNA immediately after the apnea.

Airway obstruction during sleep increases blood pressure without arousal

Recent studies suggest that arousal is the dominant factor acutely increasing blood pressure in obstructive sleep apnea and that neither stimulation of chemoreceptors nor mechanical factors associated with large negative swings in intrapleural pressure substantially contribute to the rise in blood pressure associated with each obstructive apneic event. A canine model of obstructive sleep apnea was used to examine the relative contributions of these mechanisms in the blood pressure response to induced airway obstruction during non-rapid-eye-movement sleep. In part A of the study, the arousal response was eliminated from an obstructive event by restoring airway patency just before the expected arousal, allowing blood pressure responses to be compared between obstructive events with and without arousal. In part B of the study, the protocol of Part A was repeated after pharmacological blockade of the autonomic nervous system with hexamethonium (20 mg/kg iv), eliminating neurally mediated responses due to arousal, stimulation of chemoreceptors, or other reflexes, while maintaining any mechanical effects on blood pressure related to swings in intrapleural pressure. The results of part A (n = 4 dogs) show that obstructive apneic events of 28.5 +/- 3.1 s duration, with arterial hemoglobin desaturation to 92.9 +/- 0.8% and airway pressure swings of -37.6 +/- 6 mmHg, significantly increased mean arterial pressure (MAP) by 13.8 +/- 1.5 mmHg in the absence of arousal (P < 0.005). In comparison, when arousal was allowed to occur, MAP increased by a further 11.8 +/- 1.2 mmHg (P < 0.01). In part B (n = 3 dogs), there was no change in MAP during the obstructive apneic event, and MAP fell by > 10 mmHg in the postobstruction period whether or not arousal occurred (P < 0.05). We conclude that neural reflexes, but not mechanical factors, substantially contribute to the acute blood pressure response to an obstructive apneic event and that arousal produces a separate, additional acute hypertensive response.

Distinct behavior of portal venous and arterial vascular waterfalls in porcine liver

PURPOSE

Hepatic dysfunction is associated with morbidity and mortality in critically ill patients. Understanding liver hemodynamics in pathological states requires characterization of the normal portal venous and hepatic arterial circulations. Using pressure flow analysis, we tested the hypothesis that vascular waterfalls determine blood flows in the normal liver.

METHODS

In 14 vascularly isolated porcine livers, steady-state pressure-flow relationships, which defined a slope (incremental resistance) and a zero flow pressure intercept (Po), were generated for each vessel over a range of hepatic venous pressures (Phv).

RESULTS

Critical closing pressures occurred in the portal venous circulation (Po = 3.8 +/- 0.4 mm Hg) with classical waterfall physiology observed as Phv was raised. The hepatic arterial critical closing pressure (Po = 8.3 +/- 1 mm Hg) showed a constant positive pressure difference of mm Hg versus Phv as the latter was increased from 0 to 28 mm Hg (P < .05). Portal venous resistance decreased when Phv was greater than Po (P < .05), but no effect on hepatic arterial resistance was seen as Phv was increased.

CONCLUSION

Both critical closing pressures and incremental resistances showed markedly different responses to increased outflow pressures in the portal venous and hepatic arterial circulations. The results provide the physiological basis to analyze hemodynamic changes in the liver under normal and pathological conditions.

Regional control of venous return: liver blood flow

The aim of the study was to determine whether closing pressures or vascular distensibility can be used to describe liver venous hemodynamics when right atrial pressure is raised. The study was performed using a vascularly isolated pig liver preparation that allowed the independent control of portal vein and hepatic artery inflows and of outflow pressure (Pout). Pressure-flow (P-Q) relationships of both liver vessels were generated at multiple levels of Pout. At Pout of 0 mm Hg, the portal vein P-Q relationship was linear, with a convexity toward the pressure axis at low flows (5 to 10 ml/min/kg). The zero flow pressure was 1.5 +/- 0.2 mm Hg, greater than Pout (p < 0.05). On raising Pout from 0 to 15 mm Hg, the shape of the portal vein P-Q relationships became progressively more linear, with a decrease in slope; no difference between zero flow pressure and Pout was observed. At Pout of 0 mm Hg, the hepatic artery presented a zero flow pressure > Pout. Raising Pout from 0 to 15 and 30 mm Hg resulted in a zero flow pressure always > Pout (p < 0.05). The behavior of the liver vein system is characterized by a zero flow pressure mimicking a classic vascular waterfall and by distensibility, once the waterfall is exceeded. Both factors act to minimize the reduction in venous return with an increased central venous pressure. Flow through the hepatic artery is affected by an increase in backpressure occurring upstream from the sinusoids, reducing arterial inflow for a constant perfusion pressure.

A dog model to investigate the relationship between obstructive sleep apnoea and blood pressure regulation

Patients exhibiting obstructive sleep apnoea (OSA) do not display a normal circadian pattern of blood pressure. It is not clear whether this disruption of the circadian blood pressure pattern is a result of the intermittent airway obstruction during sleep or is the result of confounding factors, such as obesity and age, which are common in OSA and may independently affect blood pressure. To determine if a cause and effect relationship exists between repetitive airway obstruction during sleep and blood pressure regulation a chronically instrumented canine model of OSA has been developed. This canine model has been shown to reproduce the characteristic apnoea and hypersomnolence of human OSA. Furthermore, in this model a 12-h nocturnal period of repetitive airway obstruction during sleep caused an increase in baseline blood pressure of more than 10 mmHg that was sustained for at least two hours following the restoration of normal airway patency. These results imply that there is a cause and effect relationship between intermittent airway obstruction during sleep and elevated blood pressure.

Alternations in liver hemodynamics in an intact porcine model of endotoxin shock

Septic shock decreases preload, increases splanchnic blood pooling and edema formation, and induces hepatic dysfunction. We hypothesized that the hemodynamic effects of endotoxemic shock on the portal venous (PV) and hepatic arterial (HA) vascular beds contribute to this picture. Multipoint pressure-flow relationships were generated to evaluate the slope (resistance or conductance) and effective back pressure (Pback) in each bed in an intact porcine model of endotoxemia. Slope and Pback were determined during endotoxemia over 300 min (n = 8) and compared with sham-treated control studies (n = 5). At time (t) = 60 min, HA slope significantly decreased (P < 0.05) without a change in Pback. The HA buffer response (HABR), defined as a decrease in HA resistance produced by reduction in PV flow (Qpv), was abolished at t = 90 min. The PV Pback significantly increased without a change in PV slope. At t = 300 min, HA slope returned to baseline, and the HABR was present while PV slope and Pback increased (P < 0.05). Fractional flow (flow relative to cardiac output) was constant except for a transient increase in HA fractional flow at t = 60 min. Histological studies showed focal necrosis and hemorrhage without evidence of vasoconstriction or thrombosis. In conclusion, endotoxic shock leads to time-dependent impairment of Qpv with increased PV resistance, causing an increase in splanchnic blood pooling and subsequent decrease in venous return. The HA bed is dilated early with an absent HABR. Later an HABR is present but defined by increased HA resistance for a given Qpv.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of endotoxin on isolated porcine liver: pressure-flow analysis

The peripheral vascular response to sepsis is characterized by a vasodilatation of the systemic arterial vessels. Pulmonary hypertension with an increase in resistance and back pressure to flow defined by pressure-flow (P-Q) relationships has been reported in experimental sepsis. We hypothesized that endotoxin can induce differential alterations in resistance and back pressure to flow in the liver venous and arterial beds. Ninety minutes after endotoxin administration in intact anesthetized pigs (n = 8), the liver was vascularly isolated and perfused. Steady-state P-Q relationships in both the portal vein (PV) and hepatic artery (HA) were generated at multiple outflow pressures (Pout; 0, 5, 10, and 15 mmHg) and compared with those obtained in control livers (n = 6). Extrapolated zero-flow pressure intercepts (Pback) and slopes of the P-Q relationships were obtained by least squares linear regression analysis. Endotoxemia increased PV Pback (P < 0.05), and Pback always exceeded Pout (P < 0.05) when the latter was raised. In contrast, in controls, no difference was observed between Pback and Pout when the latter was raised. Endotoxemia also increased the PV slope compared with control. Raising Pout from 0 to 15 mmHg decreased PV slope in the endotoxin group to a greater degree than in controls (P < 0.05). In the HA, endotoxin caused a decrease in slope but did not alter Pback. The simultaneous increase in the PV Pback and slope that occurs with endotoxemia decreases splanchnic venous return, pooling blood in the splanchnic compartment for a given total blood volume.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of sleep deprivation on responses to airway obstruction in the sleeping dog

The effect of sleep deprivation on sleep architecture and respiratory responses to repetitive airway obstruction during sleep was investigated in four chronically instrumented tracheostomized dogs during 12-h nocturnal experiments. A 24-h period of prior sleep deprivation increased (P < 0.05) the rate at which airway obstruction could be induced from 20 +/- 3 (SE) to 37 +/- 10 times/h compared with non-sleep-deprived dogs. During non-rapid-eye-movement sleep the duration of obstruction, minimum arterial hemoglobin saturation, and peak negative inspiratory effort at arousal were 20.5 +/- 1.0 s, 91.7 +/- 0.5%, and 28.4 +/- 1.8 mmHg, respectively, in non-sleep-deprived dogs. Sleep deprivation increased (P < 0.01) the duration of obstruction to 28.0 +/- 0.9 s, worsened (P < 0.05) the minimal arterial hemoglobin desaturation to 85.4 + 3.1%, and increased (P < 0.025) the peak negative inspiratory effort at arousal to 36.1 +/- 1.6 mmHg. Sleep deprivation also caused increases (P < 0.025) in total sleep time, rapid-eye-movement (REM) sleep time, and percentage of time in REM sleep in a 2-h recovery period without airway obstruction at the end of the study. We conclude that airway obstruction in the sleeping dog can reproduce the disturbances in sleep architecture and respiration that occur in obstructive sleep apnea and that prior sleep deprivation will increase apnea severity, degree of somnolence, and REM sleep rebound independent of change in upper airway collapsibility.

Relationship between blood pressure and airway obstruction during sleep in the dog

The relationship between airway obstruction during sleep and changes in mean arterial pressure (MAP) was investigated in four chronically instrumented tracheostomized dogs during 12-h nocturnal experiments. The MAP response was determined 1) during experimental airway obstruction whenever sleep occurred, 2) over each 12-h experiment, and 3) during a 2-h recovery period at the end of each experiment. The effects of 24 h of sleep deprivation and changes in plasma levels of renin and atrial natriuretic peptide were assessed. In non-rapid-eye-movement sleep, a period of airway obstruction caused MAP to increase (P < 0.002) from 95 +/- 3 (SE) mmHg to 112 +/- 3 mmHg, and this difference was enhanced (P < 0.04) by sleep deprivation. There was an increase of 12 +/- 2 mmHg in the overall MAP over time (P < 0.001) in non-rapid-eye-movement sleep that was sustained in the 2-h recovery period. Plasma levels of renin and atrial natriuretic peptide were constant and unrelated to changes in MAP. We conclude that in the sleeping dog airway obstruction causes an increase in MAP that can be accentuated by prior sleep deprivation and that repetitive airway obstruction will cause an increase in MAP over time that is sustained for > or = 2 h when normal airway patency is restored.

Pressure-flow analysis of portal vein and hepatic artery interactions in porcine liver

Interactions between the hepatic arterial and portal venous circulations were investigated in nine intact and eight isolated perfused porcine livers. Pressure-flow (P-Q) relationships were obtained in either the portal vein or hepatic artery with constant baseline or low flow in the other bed and a stable hepatic venous pressure (Phv). The slope was obtained by linear regression analysis of the P-Q relationship, and effective back pressure (Pback) was obtained from the pressure intercept for the portal vein and the measured zero-flow pressure for the hepatic artery. The Pback in the hepatic artery (13.4 +/- 1.5 mmHg) and the portal vein (4.6 +/- 0.3 mmHg) were higher than Phv (P < 0.05). Reducing portal vein flow (Qpv) produced an increase in hepatic artery flow (Qha) (P < 0.05) due only to a decrease in slope (P < 0.05). Decreasing Qha caused an identical change in Pback of the portal vein (P < 0.05) in the intact and isolated liver preparations. A change in Qpv alters the hepatic arterial resistance upstream from the site of a constant arterial Pback. Changes in total flow through the common sinusoidal compartment appear to alter the Pback of the portal vein via hydraulic mechanisms.

Hepatic and renal blood flow responses to a clinical dose of intravenous cyclosporine in the pig

The immunosuppressant Cyclosporine A (CsA) is considered to induce nephrotoxicity in part by causing vasoconstriction of the glomerular afferent arterioles. Although CsA is widely used in hepatic transplantation, little is known concerning its effects on hepatic blood flow. We used ultrasonic flow probes in an anesthetized swine model to measure the effects of a single 60 min infusion of a clinically comparable dose of CsA (5 mg/kg per h) on hepatic, renal, and supraceliac descending aortic blood flows (n = 7 swine). To account for any change in systemic output or systemic vascular resistance during the 60 min CsA infusion that may non-specifically affect hepatic and renal blood flows, the total hepatic (portal vein plus hepatic artery) and renal blood flows were reported relative to the supraceliac descending aortic blood flow (termed 'fractional' total hepatic and renal blood flows). The fractional total hepatic blood flow decreased significantly (p < 0.05) by 40 min of CsA infusion vs baseline, and continued to decrease throughout the infusion (baseline = 0.38 +/- 0.03 units vs 0.28 +2- 0.05 units by 60 min of CsA infusion). During the recovery period, the fractional total hepatic blood flow increased to a value which was not different from baseline (recovery = 0.38 +2- 0.03 units). Fractional right renal artery blood flow did not change significantly from baseline at any time during the CsA infusion or during the recovery period. We conclude that a single, clinically comparable dose of CsA results in a significant decrease in total hepatic blood flow, and that this decrease is greater than that seen in renal blood flow.

Renin and vasopressin responses to graded reductions in atrial pressure in conscious dogs

Hypovolemia activates reflexes that stimulate secretion of renin and arginine vasopressin (AVP). A large body of evidence, obtained mainly in anesthetized preparations, supports the hypothesis that unloading cardiac receptors stimulates increases in plasma AVP and renin activity (PRA). We have observed significant increases in PRA before any change in either mean arterial pressure (MAP) or pulse pressure in conscious dogs undergoing continuous hemorrhage; however, plasma AVP did not change until there was a significant fall in MAP. These results are compatible with the hypothesis that cardiac receptors cause reflex stimulation of renin but not AVP secretion. The aim of the present study was to test the hypothesis that a decrease in atrial pressure alone is sufficient to stimulate an increase in plasma AVP and PRA. Graded thoracic inferior vena caval constriction (TIVCC) was used to reduce atrial pressure in four steps without altering MAP in conscious dogs. In a fifth step, TIVCC was increased to cause a fall in MAP. A reduction in left atrial pressure (LAP) of 4.2 +/- 0.9 mmHg was accompanied by a significant (P < 0.05) increase in PRA from a control value of 0.4 +/- 0.1 ng angiotensin I (ANG I).ml-1.3 h-1 to 1.1 +/- 0.2 ng ANG I.ml-1.3 h-1 but no change in plasma AVP (from 1.0 +/- 0.1 to 1.2 +/- 0.2 pg/ml) or MAP (from 85 +/- 5 mmHg to 86 +/- 4 mmHg).(ABSTRACT TRUNCATED AT 250 WORDS)