Cardiac inotropic responses from changes in carbon dioxide tension in the cephalic circulation of anaesthetized dogs

Influence of hypercapnia on cardiovascular responses to tracheal intubation

BACKGROUND

Laryngoscopy and tracheal intubation are often associated with tachycardia, hypertension, and arrhythmias. There is a risk of hypercapnia in the case of difficult mask ventilation. The circulatory response to hypercapnia is increases in arterial pressure and heart rate. We evaluated the difference of cardiovascular responses to tracheal intubation between normocapnia and hypercapnia during mask ventilation before tracheal intubation.

METHODS

We studied 40 ASA physical status I to II patients under general anesthesia. Induction of anesthesia was achieved with midazolam 0.05 mg/kg, propofol 1.5 mg/kg, alfentanil 10 microg/kg, and rocuronium 0.6 mg/kg IV. The lungs were mechanically ventilated with a tidal volume of 10 mL/kg and 6 to 10 bpm in the hypercapnia group (n = 20) or 12 to 15 bpm in the normocapnia group (n = 20) during the induction period. Intubation was performed 3 minutes after the induction, and anesthesia was maintained using 1.5% sevoflurane (inspired) and 75% N(2)O in oxygen. Heart rate, systolic arterial pressure (SAP), and diastolic arterial pressure were recorded every minute throughout the study.

RESULTS

The proportion of the patients whose increase of SAP between just before intubation and 1 minute after intubation was more than 30 mm Hg in the hypercapnia group (40%) was greater than that in the normocapnia group (9.5%) (P = .0325). There were no differences in heart rate and diastolic arterial pressure between hypercapnia and normocapnia groups. For the SAP of the patients, the trend of changes was increased (P = .024).

CONCLUSIONS

Hypercapnia during mask ventilation before tracheal intubation may exaggerate the increase of SAP during intubation compared to normocapnia. Ventilation was important in minimizing hemodynamic responses during induction regardless of using drugs.

The effects of a respiratory acidosis on human heart rate variability

Human heart rate variability (HRV) was examined during a mild respiratory acidosis induced by inhalation of a normoxic hypercapnic gas mixture. On two separate occasions, separated by 72 but not more than 120hr, ECG was recorded from 9 normotensive subjects during supine rest. ECG was recorded for 20 min breathing room air or a 5% CO2 in normoxic air mixture. Expired V'E, O2 and CO2 were measured with breath-by-breath mass spectrometry. HRV spectra were calculated using a Welch averaged periodogram method and banded as Very Low Frequency (VLF: 0 - 0.04 Hz), Low Frequency (LF: 0.04 - 0.15 Hz), and High Frequency (HF: 0.15 - 0.4Hz). Student paired samples t-tests were used to compare room air (RA) versus inhaled 5% CO2 in air (5% CO2) data. All results reported as mean +/- SD. In the HRV time domain, hypercapnic normoxia reduced mean r-r intervals (5% CO2: 956.1 +/- 149.2 vs. RA: 1035 +/- 146.8 ms, p = 0.022) and median r-r intervals (5% CO2: 942.6 +/- 153.1 vs. RA: 1047.8 +/- 157.3 ms, p = 0.010), and increased heart rates (5% CO2: 64.4 +/- 12 vs. RA: 59.3 +/- 10.1 bpm, p = 0.019). In the HRV frequency domain, hypercapnic normoxia increased the high frequency component of HRV (5% CO2: 9799 +/- 7649 vs. RA: 4399 +/- 3857 ms2, p = 0.004) and reduced the LF/HF ratio (5% CO2: 0.243 +/- 0.145 vs. RA: 0.906 +/- 0.672, p = 0.017). An incomplete ventilatory compensation probably accounts for the increased HF contribution. An increased heart rate with a mild respiratory acidosis may be caused by a vasodilator effect of elevated arterial PCO2, stimulating the increase in heart rate to maintain blood pressure.

Resting and Postexercise Cardiac Autonomic Control in Trained Masters Athletes

This study used measures of heart rate variability during recovery from high-intensity exercise in trained Master athletes to examine postexercise cardiac autonomic regulation. Seven males (mean age 52.1 +/- 3.3 yr; mass 85.1 +/- 18.0 kg) and 6 females (mean age 50.5 +/- 2.9 yr; mass 63.1 +/- 6.0 kg) performed incremental exercise to an intensity that induced a >4.5 mmol capillary blood lactate concentration, followed by incremental exercise to volitional exhaustion (VO2(max)). A 6 min ECG recording before (Pre) and after (Post) exercise was analyzed in the time (mean rr interval, sd rr) and frequency domains (total power, very low frequency [VLF: 0-0.04 Hz], low frequency [LF: 0.04-0.15 Hz], high frequency [HF: 0.15-0.4 Hz]). VO2(max) for males and females was 49.4 +/- 7.1 ml kg(-1) min(-1) and 45.1 +/- 10.1 ml kg(-1) min(-1), respectively. Lower mean rr interval (Pre: 1,048 +/- 128 ms; Post: 730 +/- 78 ms; P < 0.001) and lower sd rr (Pre: 77 +/- 30 ms; Post: 43 +/- 17 ms; P < 0.001) were recorded following exercise, with no differences based on gender. Total power decreased following exercise (Pre: 6,331 +/- 6,119 ms; Post: 1,921 +/- 1,552 ms). When normalized for changes in total power, a decreased HF component (Pre: 34.52 +/- 14.79 n.u.; Post: 18.49 +/- 13.64 n.u.; P < 0.05) with no change in LF component (Pre: 61.00 +/- 18.66 n.u.; Post: 69.63 +/- 23.97 n.u.; P = 0.34) was recorded. No gender differences in HRV in the frequency domain were recorded. Decreased heart rate variability in both time and frequency domains suggested an increased parasympathetic withdrawal during the autonomic control of postexercise tachycardia in trained Master athletes.

Interaction of chemoreceptor and baroreceptor reflexes by hypoxia and hypercapnia - a mechanism for promoting hypertension in obstructive sleep apnoea

Asphyxia, which occurs during obstructive sleep apnoeic events, alters the baroreceptor reflex and this may lead to hypertension. We have recently reported that breathing an asphyxic gas resets the baroreceptor-vascular resistance reflex towards higher pressures. The present study was designed to determine whether this effect was caused by the reduced oxygen tension, which affects mainly peripheral chemoreceptors, or by the increased carbon dioxide, which acts mainly on central chemoreceptors. We studied 11 healthy volunteer subjects aged between 20 and 55 years old (6 male). The stimulus to the carotid baroreceptors was changed using graded pressures of -40 to +60 mmHg applied to a neck chamber. Responses of vascular resistance were assessed in the forearm from changes in blood pressure (Finapres) divided by brachial blood flow velocity (Doppler) and cardiac responses from the changes in RR interval and heart rate. Stimulus-response curves were defined during (i) air breathing, (ii) hypoxia (12% O(2) in N(2)), and (iii) hypercapnia (5% CO(2) in 95% O(2)). Responses during air breathing were assessed both prior to and after either hypoxia or hypercapnia. We applied a sigmoid function or third order polynomial to the curves and determined the maximal differential (equivalent to peak sensitivity) and the corresponding carotid sinus pressure (equivalent to 'set point'). Hypoxia resulted in an increase in heart rate but no significant change in mean blood pressure or vascular resistance. However, there was an increase in vascular resistance in the post-stimulus period. Hypoxia had no significant effect on baroreflex sensitivity or 'set point' for the control of RR interval, heart rate or mean arterial pressure. Peak sensitivity of the vascular resistance response to baroreceptor stimulation was significantly reduced from -2.5 +/- 0.4 units to -1.4 +/- 0.1 units (P < 0.05) and this was restored in the post-stimulus period to -2.6 +/- 0.5 units. There was no effect on 'set point'. Hypercapnia, on the other hand, resulted in a decrease in heart rate, which remained reduced in the post-stimulus period and significantly increased mean blood pressure. Baseline vascular resistance was significantly increased and then further increased in the post-control period. Like hypoxia, hypercapnia had no effect on baroreflex control of RR interval, heart rate or mean arterial pressure. There was, also no significant change in the sensitivity of the vascular resistance responses, however, 'set point' was significantly increased from 74.7 +/- 4 to 87.0 +/- 2 mmHg (P < 0.02). This was not completely restored to pre-stimulus control levels in the post-stimulus control period (82.2 +/- 3 mmHg). These results suggest that the hypoxic component of asphyxia reduces baroreceptor-vascular resistance reflex sensitivity, whilst the hypercapnic component is responsible for increasing blood pressure and reflex 'set point'. Hypercapnia appears to have a lasting effect after the removal of the stimulus. Thus the effect of both peripheral and central chemoreceptors on baroreflex function may contribute to promoting hypertension in patients with obstructive sleep apnoea.

Urinary bladder distension: its effects on carotid baroreceptor reflex left ventricular inotropic response in the dog

1. The effects of distension of the urinary bladder on heart rate, maximum rate of change of left ventricular pressure (dP/dt max) and hindlimb vascular resistance together with their modulation at different carotid sinus pressures were studied in dogs anaesthetized with a mixture of chloralose and urethane and artificially ventilated. 2. When the carotid sinus mean perfusion pressure was raised in randomly selected steps from 60 to 210 mmHg, it caused a progressive bradycardia, and a reduction in left ventricular dP/dt max and in arterial blood pressure, together with vasodilatation in the perfused hindlimb. Distension of the bladder at each level of carotid sinus pressure resulted in tachycardia, a small but significant increase in left ventricular dP/dt max (160 +/- 30 mmHg s-1) and hindlimb vasoconstriction. 3. When heart rate and arterial blood pressure were held constant to exclude these secondary effects on left ventricular dP/dt max, raising the carotid sinus pressure caused a progressive reduction in left ventricular dP/dt max and hindlimb vasodilatation. Superimposition of tests of bladder distension at each level of sinus pressure resulted in variable responses, but overall there was a significant increase in left ventricular dP/dt max of 190 +/- 54 mmHg s-1. Hindlimb vasocontriction, however, was a consistent finding. 4. The gain of the relationship between the carotid sinus perfusion pressure and left ventricular dP/dt max was unaffected by distension of the bladder. 5. It is concluded that, when changes secondary to increases in heart rate and blood pressure are prevented, distension of the bladder causes a small but significant reflex increase in left ventricular dP/dt max. The responses, however, are variable and the possible reasons for this are discussed.

Function of the ventrolateral medulla in the control of the circulation

The CNS control of the cardiovascular system involves the coordination of a series of complex neural mechanisms which integrate afferent information from a variety of peripheral receptors and produce control signals to effector organs for appropriate physiological responses. Although it is generally thought that these control signals are generated by a network of neural circuits that are widely distributed in the CNS, over the last two decades a considerable body of experimental evidence has accumulated suggesting that several of these circuits involve neurons found on or near the ventral surface of the medulla oblongata. Neurons in the VLM have been shown to be involved in the maintenance of vasomotor tone, in baroreceptor and chemoreceptor (central and peripheral) reflex mechanisms, in mediating the CIR and somatosympathetic reflexes and in the control of the secretion of vasopressin. These physiological functions of VLM neurons have been supported by neuroanatomical and electrophysiological studies demonstrating direct connections with a number of central structures previously implicated in the control of the circulation, including the IML, the site of origin of sympathetic preganglionic axons, and the SON and PVH, the site of origin of neurohypophyseal projecting axons containing AVP. Considerable suggestive evidence has also been obtained regarding the chemical messengers involved in transmitting information from VLM neurons to other central structures. There have been developments suggesting a role for monoamines and neuropeptides in mediating the neural and humoral control of SAP by neurons in the VLM. This review presents a synthesis of the literature suggesting a main role for VLM neurons in the control of the circulation.

Abdominal vascular responses to changes in carbon dioxide tension in the cephalic circulation of anaesthetized dogs

Dogs were anaesthetized with chloralose, the regions of both carotid sinuses were vascularly isolated and perfused with arterial blood and both cervical vagosympathetic trunks were cut above the nodose ganglia. The cephalic circulation was perfused through the brachiocephalic and left subclavian arteries with blood which was equilibrated with various levels of CO2. The abdomen was vascularly isolated, perfused through the aorta at constant flow and drained through the inferior vena cava at constant pressure. Changes in vascular resistance were determined from changes in abdominal aortic perfusion pressure and changes in capacitance from the integral of the changes in venous outflow. An increase in PCO2 in the cephalic perfusate resulted in an increase in abdominal vascular resistance and a decrease in capacitance. However, when carotid sinus pressure was high, the response of resistance to an increase in cephalic PCO2 was abolished and that of capacitance was significantly reduced. The reflex responses of both vascular resistance and capacitance to a change in carotid sinus pressure were enhanced when the cephalic PCO2 was raised. However, the effect on the reflex capacitance response from stimulation of baroreceptors was obtained only when PCO2 was changed below 5 kPa whereas the effect on resistance occurred at higher values of PCO2. The interaction between the effects of changes in cephalic PCO2 and the carotid sinus reflex and the differential effect on resistance and capacitance vessels have been explained in terms of the known difference in the sensitivities of these vessels to sympathetic nerve activity.

Effect of cephalic carbon dioxide tension on the cardiac inotropic response to carotid chemoreceptor stimulation in dogs

Dogs were anaesthetized with chloralose and the cephalic circulation was perfused, through the brachiocephalic and left subclavian arteries, with blood equilibrated with various tensions of CO2. The vascularly isolated carotid bifurcations were perfused at a constant pressure with either arterial or venous blood. Inotropic responses were assessed by measuring the maximum rate of change of left ventricular pressure (dP/dt max) with heart rate and aortic pressure held constant. Stimulation of carotid chemoreceptors with venous blood, at all values of cephalic PCO2, always resulted in a decrease in dP/dt max. An increase in cephalic PCO2, during arterial perfusion of chemoreceptors, resulted in an increase in dP/dt max and the response to chemoreceptor stimulation was enhanced. Graded changes in cephalic PCO2 resulted in graded changes in dP/dt max during arterial perfusion of chemoreceptors. However, the value of dP/dt max during venous perfusion was not significantly affected by increases in cephalic PCO2 above normal but it did decrease significantly during cephalic hypocapnia. These results confirm that an increase in cephalic PCO2 and stimulation of carotid chemoreceptors result in opposite responses of the cardiac inotropic state. The responses to chemoreceptor stimulation were enhanced by cephalic hypercapnia but the responses to cephalic hypercapnia, although not to hypocapnia, were suppressed by chemoreceptor stimulation.