The role aortic chemoreceptors during severe CO hypoxia

The importance of aortic chemoreceptors in the circulatory responses to severe carbon monoxide (CO) hypoxia was studied in anesthetized dogs. The aortic chemoreceptors were surgically denervated in eight dogs prior to the induction of CO hypoxia, with nine other dogs serving as intact controls. Values for both whole body and hindlimb blood flow, vascular resistance, and O2 uptake were determined prior to and at 30 min of CO hypoxia in the two groups. Arterial O2 content was reduced 65% using an in situ dialysis method to produce CO hypoxia. At 30 min of hypoxia, cardiac output increased but limb blood flow remained at prehypoxic levels in both groups. This indicated that aortic chemoreceptor input was not necessary for the increase in cardiac output during severe CO hypoxia, nor for the diversion of this increased flow to nonmuscle tissues. Limb O2 uptake decreased during CO hypoxia in the aortic-denervated group but remained at prehypoxic levels in the intact group. The lower resting values for limb blood flow in the aortic-denervated animals required a greater level of O2 extraction to maintain resting O2 uptake. When CO hypoxia was superimposed upon this compensation, an O2 supply limitation occurred because the limb failed to vasodilate even as maximal levels for O2 extraction were approached.     

The effects of hyperoxia on oxygen uptake during acute anemia

The effects of normobaric hyperoxia on the oxygen uptake (VO2) and cardiovascular responses of the whole body and hindlimb during anemia were investigated. Anesthetized, paralyzed dogs were ventilated for 20-min periods with room air (normoxia), 100% O2 (hyperoxia), and returned to room air. Anemia (hematocrit = 15%) was then induced by isovolemic dextran-for-blood exchange and the normoxia, hyperoxia, normoxia sequence was repeated. Whole body VO2 and cardiac output rose following anemia, and then fell (p less than 0.05) with hyperoxia during anemia. These responses were not abolished by beta-blockade with propranolol (1 mg/kg, iv) or bilateral vagotomy. The hindlimb data for blood flow and VO2 were similar in direction to those of the whole body but were more variable. Section of the sciatic and femoral nerves did not appear to have significant effect on the limb responses to hyperoxia. The decrease in whole body and hindlimb VO2 with hyperoxia during anemia may have resulted from a redistribution of capillary blood flow away from exchange vessels in response to the elevated PO2.     

The role of D-dimers in the diagnosis of acute aortic dissection

Acute aortic dissection (AAD) is a life threatening cardiovascular medical emergency with a poor prognosis. To explore the utility of D-dimers (DD) in the diagnosis of AAD, we performed a prospective study and conducted a meta-analysis of previous studies. 368 suspected patients were enrolled, including AAD n = 89, PE n = 12, AMI n = 167, normal controls n = 100. All patients had a DD test immediately after admission. We then performed a comprehensive computer search to identify studies investigating using DD as a screening tool for AAD. Finally, we pooled these data to estimate sensitivity, specificity, positive and negative likelihood ratios (LRs) by using DerSimonian–Laird random-effects models. The DD concentrations in the AAD group were significantly higher than those in the AMI and normal control groups. However, the DD level of 500 ng/ml had a poor sensitivity of 51.7 % and specificity of 89.2 % in the diagnosis of AAD. Subgroup analyses found that DD only showed a well discriminative ability of distinguishing AAD patients from normal controls (specificity and positive LR was 97 % and 17.2, respectively). The pooled sensitivity, specificity, positive and negative LR in our meta-analysis was 89, 68 %, 2.71, 0.07, respectively. In conclusion, our results suggest that plasma DD levels cannot add to the certainty of AAD diagnosis and it is not a good biomarker for AAD. In the future, prospective research on patients from many parts of the world is warranted to validate our findings. In addition, different controls, methods of plasma DD assays and other factors should be considered.     

The essential role of carotid body chemoreceptors in sleep apnea

Sleep apnea is attributable, in part, to an unstable ventilatory control system and specifically to a narrowed "CO2 reserve" (i.e., the difference in P(a)CO2 between eupnea and the apneic threshold). Findings from sleeping animal preparations with denervated carotid chemoreceptors or vascularly isolated, perfused carotid chemoreceptors demonstrate the critical importance of peripheral chemoreceptors to the ventilatory responses to dynamic changes in P(a)CO2. Specifically, (i) carotid body denervation prevented the apnea and periodic breathing that normally follow transient ventilatory overshoots; (ii) the CO2 reserve for peripheral chemoreceptors was about one half that for brain chemoreceptors; and (iii) hypocapnia isolated to the carotid chemoreceptors caused hypoventilation that persisted over time despite a concomitant, progressive brain respiratory acidosis. Observations in both humans and animals are cited to demonstrate the marked plasticity of the CO2 reserve and, therefore, the propensity for apneas and periodic breathing, in response to changing background ventilatory stimuli.     

Effects of alpha -adrenergic blockade during acute anemia

Studies were carried out in seven anesthetized paralyzed dogs to examine the importance of alpha -adrenergic tone in the cardiovascular responses during acute anemia. Data were obtained 1) at normal hematocrit (Hct), 2) during anemia produced by isovolemic hemodilution with dextran (Hct, 13-15%), 3) during anemia after alpha -blockade (alpha -bl) with phenoxybenzamine (3 mg/kg), and 4) following volume expansion during anemia with a red blood cell dextran solution. Cardiac output (QT), limb and total body oxygen uptake (VO2), and limb blood flow (QL) were determined. Both QT and QL increased during anemia (P less than 0.01), whereas limb resistance (RL) and total peripheral resistance (TPR) were decreased (P less than 0.01). No further change in either RL or TPR occurred with alpha -blockade anemia, but both QT and QL decreased (P less than 0.01). Whole-body VO2 increased during anemia and then declined with alpha -bl and anemia. Following volume expansion during anemia with alpha -bl, QT, QL, and whole-body VO2 increased. We conclude that alpha -adrenergic sympathetic tone to capacitance vessels is essential for the cardiac output increased during anemia, but has little or no effect on resistance vessels and hence distribution of peripheral blood flow.     

Hindlimb vascular responses to sympathetic augmentation during acute anemia

The effect of increased sympathetic activity on skeletal muscle blood flow during acute anemic hypoxia was studied in 16 anesthetized dogs. Sympathetic activity was altered by clamping the carotid arteries bilaterally below the carotid sinus. One group (n = 8) was beta blocked by administration of propranolol (1 mg/kg); a second group (n = 8) was untreated. Venous outflow from the left hindlimb was isolated for measurement of blood flow and O2 uptake (VO2). After a 20-min control period, both carotid arteries were clamped (CC) for 20 min followed by a 20-min recovery period. The sequence was repeated after hematocrit was lowered to about 15% by dextran exchange for blood. Prior to anemia, CC did not alter cardiac output or limb blood flow in either group. After induction of anemia, hindlimb resistance was higher with CC in the beta block than in the no block group. Both limb blood flow and VO2 fell in the beta-block group with CC during anemia. Beta block also prevented the additive increases in whole body VO2 seen with CC and induction of anemia. The data showed that the increased vasoconstrictor tone that was obtained with beta block during anemia was successful in redistributing the lower viscosity blood away from resting skeletal muscle, even to the point that muscle VO2 was decreased.     

Re-setting of the hypoxic sensitivity of aortic chemoreceptors in the new-born lamb

We tested to see whether the steady-state hypoxic sensitivity of aortic chemoreceptors was re-set during the first 2-3 weeks of post-natal life. Aortic chemo-receptor activity was recorded from the distal end of the cut aortic branch of the cervical vagus in pentobarbitone - anesthetized, new-born lambs. Two groups were studied, the first aged 1-4 days and the second aged 10-19 days. Chemoreceptor discharge increased as hyperbolic function with increasing isocapnic hypoxia in both groups and we quantified the position and the shape of this response curve. It was shifted to the right significantly in the older group of lambs, the mean vertical asymtote increasing from 10.00 to 27.95 torr PO2. No significant difference was found in the horizontal asymotote or in the 'shaping term' between the two groups. The greatest differences between the stimulus-response curves of the two groups of animals with respect to the mean level of discharge and the slope of the curve occurred when PaO2 was below ca. 50 torr. The aortic chemoreceptors of older lambs were unable to maintain a sustained discharge at arterial PO2 values below ca. 30 torr. In contrast, in the younger group PO2 often had to be reduced below this level before discharge increased significantly. We conclude that, like the carotid chemoreceptors, aortic chemoreceptor sensitivity is re-set over the first few weeks of life. The re-setting may contribute to the increase in the ventilatory response to hypoxia which occurs over this period.