Effects of hyperventilation on systemic and coronary hemodynamics

Hyperventilation testing in the diagnosis of vasospastic angina: A clinical review and meta-analysis

BACKGROUND

Given the limited access to invasive vasospastic reactivity testing in Western Countries, there is a need to further develop alternative non-invasive diagnostic methods for vasospastic angina (VSA). Hyperventilation testing (HVT) is defined as a class IIa recommendation to diagnose VSA by the Japanese Society of Cardiology.

METHODS

In this systematic review and meta-analysis reported according to the PRISMA statement, we review the mechanisms, methods, modalities and diagnostic accuracy of non-invasive HVT for the diagnostic of VSA.

RESULTS

A total of 106 articles published between 1980 and 2022 about VSA and HVT were included in the systematic review, among which 16 were included in the meta-analysis for diagnostic accuracy. Twelve electrocardiogram-HVT studies including 804 patients showed a pooled sensitivity of 54% (95% confidence intervals [CI]; 30%-76%) and a pooled specificity of 99% (95% CI; 88%-100%). Four transthoracic echocardiography-HVT studies including 197 patients revealed a pooled sensitivity of 90% (95% CI; 82%-94%) and a pooled specificity of 98% (95% CI; 86%-100%). Six myocardial perfusion imaging-HVT studies including 112 patients yielded a pooled sensitivity of 95% (95% CI; 63%-100%) and a pooled specificity of 78% (95% CI; 19%-98%). Non-invasive HVT resulted in a low rate of adverse events, ventricular arrhythmias being the most frequently reported, and were resolved with the administration of nitroglycerin.

CONCLUSIONS

Non-invasive HVT offers a safe alternative with high diagnostic accuracy to diagnose VSA in patients with otherwise undiagnosed causes of chest pain.

Hypocapnia Alone Fails to Provoke Important Electrocardiogram Changes in Coronary Artery Diseased Patients

BACKGROUND

There is still an urgent clinical need to develop non-invasive diagnostic tests for early ischemic heart disease because, once angina occurs, it is too late. Hypocapnia has long been known to cause coronary artery vasoconstriction. Some new cardiology tests are accompanied by the claim that they must have potential diagnostic value if hypocapnia enhances their cardiac effects in healthy subjects. But no previous study has tested whether hypocapnia produces bigger cardiac effects in patients with angina than in healthy subjects.

METHODS

Severe hypocapnia (a PetCO2 level of 20 mmHg) lasting >15 min was mechanically induced by facemask, while conscious and unmedicated, in 18 healthy subjects and in 10 patients with angina and angiographically confirmed coronary artery disease, awaiting by-pass surgery. Each participant was their own control in normocapnia (where CO2 was added to the inspirate) and the order of normocapnia and hypocapnia was randomized. Twelve lead electrocardiograms (ECG) were recorded and automated measurements were made on all ECG waveforms averaged over >120 beats. 2D echocardiography was also performed on healthy subjects.

RESULTS

In the 18 healthy subjects, we confirm that severe hypocapnia (a mean PetCO2 of 20 ± 0 mmHg, P < 0.0001) consistently increased the mean T wave amplitude in leads V1-V3, but by only 31% (P < 0.01), 15% (P < 0.001) and 11% (P < 0.05), respectively. Hypocapnia produced no other significant effects (p > 0.05) on their electro- or echocardiogram. All 10 angina patients tolerated the mechanical hyperventilation well, with minimal discomfort. Hypocpania caused a similar increase in V1 (by 39%, P < 0.05 vs. baseline, but P > 0.05 vs. healthy controls) and did not induce angina. Its effects were no greater in patients who did not take β-blockers, or did not take organic nitrates, or had the worst Canadian Cardiovascular Society scores.

CONCLUSION

Non-invasive mechanical hyperventilation while awake and unmedicated is safe and acceptable, even to patients with angina. Using it to produce severe and prolonged hypocapnia alone does produce significant ECG changes in angina patients. But its potential diagnostic value for identifying patients with coronary stenosis requires further evaluation.

Influence of myocardial oxygen demand on the coronary vascular response to arterial blood gas changes in humans

It remains unclear if the human coronary vasculature is inherently sensitive to changes in arterial Po₂ and Pco₂ or if coronary vascular responses are the result of concomitant increases in myocardial O₂ consumption/demand ([Formula: see text]). We hypothesized that the coronary vascular response to Po₂ and Pco₂ would be attenuated in healthy men when [Formula: see text] was attenuated with β₁-adrenergic receptor blockade. Healthy men (age: 25 ± 1 yr, n = 11) received intravenous esmolol (β₁-adrenergic receptor antagonist) or volume-matched saline in a double-blind, randomized crossover study and were exposed to poikilocapnic hypoxia, isocapnic hypoxia, and hypercapnic hypoxia. Measurements made at baseline and after 5 min of steady state at each gas manipulation included left anterior descending coronary blood velocity (LAD_V; Doppler echocardiography), heart rate, and arterial blood pressure. LAD_V values at the end of each hypoxic condition were compared between esmolol and placebo. The rate-pressure product (RPP) and left ventricular mechanical energy (ME_LV) were calculated as indexes of [Formula: see text]. All gas manipulations augmented RPP, ME_LV, and LAD_V, but only RPP and ME_LV were attenuated (4-18%) after β₁-adrenergic receptor blockade ( P < 0.05). Despite attenuated RPP and ME_LV responses, β₁-adrenergic receptor blockade did not attenuate the mean LAD_V vasodilatory response compared with placebo during poikilocapnic hypoxia (29.4 ± 2.2 vs. 27.3 ± 1.6 cm/s) and isocapnic hypoxia (29.5 ± 1.5 vs. 30.3 ± 2.2 cm/s). Hypercapnic hypoxia elicited a feedforward coronary dilation that was blocked by β₁-adrenergic receptor blockade. These results indicate a direct influence of arterial Po₂ on coronary vascular regulation that is independent of [Formula: see text]. NEW & NOTEWORTHY In humans, arterial hypoxemia led to an increase in epicardial coronary artery blood velocity. β₁-Adrenergic receptor blockade did not diminish the hypoxemic coronary response despite reduced myocardial O₂ demand. These data indicate hypoxemia can regulate coronary blood flow independent of myocardial O₂ consumption. A plateau in the mean left anterior descending coronary artery blood velocity-rate-pressure product relationship suggested β₁-adrenergic receptor-mediated, feedforward epicardial coronary artery dilation. In addition, we observed a synergistic effect of Po₂ and Pco₂ during hypercapnic hypoxia.

The effects of graded changes in oxygen and carbon dioxide tension on coronary blood velocity independent of myocardial energy demand

In humans, coronary blood flow is tightly regulated by microvessels within the myocardium to match myocardial energy demand. However, evidence regarding inherent sensitivity of the microvessels to changes in arterial partial pressure of carbon dioxide and oxygen is conflicting because of the accompanied changes in myocardial energy requirements. This study aimed to investigate the changes in coronary blood velocity while manipulating partial pressures of end-tidal CO2 (Petco2) and O2 (Peto2). It was hypothesized that an increase in Petco2 (hypercapnia) or decrease in Peto2 (hypoxia) would result in a significant increase in mean blood velocity in the left anterior descending artery (LADVmean) due to an increase in both blood gases and energy demand associated with the concomitant cardiovascular response. Cardiac energy demand was assessed through noninvasive measurement of the total left ventricular mechanical energy. Healthy subjects (n = 13) underwent a euoxic CO2 test (Petco2 = -8, -4, 0, +4, and +8 mmHg from baseline) and an isocapnic hypoxia test (Peto2 = 64, 52, and 45 mmHg). LADVmean was assessed using transthoracic Doppler echocardiography. Hypercapnia evoked a 34.6 ± 8.5% (mean ± SE; P < 0.01) increase in mean LADVmean, whereas hypoxia increased LADVmean by 51.4 ± 8.8% (P < 0.05). Multiple stepwise regressions revealed that both mechanical energy and changes in arterial blood gases are important contributors to the observed changes in LADVmean (P < 0.01). In summary, regulation of the coronary vasculature in humans is mediated by metabolic changes within the heart and an inherent sensitivity to arterial blood gases.

Carbon Dioxide and the Heart: Physiology and Clinical Implications

Carbon dioxide (CO2) is an end product of aerobic cellular respiration. In healthy persons, PaCO2 is maintained by physiologic mechanisms within a narrow range (35-45 mm Hg). Both hypercapnia and hypocapnia are encountered in myriad clinical situations. In recent years, the number of hypercapnic patients has increased by the use of smaller tidal volumes to limit lung stretch and injury during mechanical ventilation, so-called permissive hypercapnia. A knowledge and appreciation of the effects of CO2 in the heart are necessary for optimal clinical management in the perioperative and critical care settings. This article reviews, from a historical perspective: (1) the effects of CO2 on coronary blood flow and the mechanisms underlying these effects; (2) the role of endogenously produced CO2 in metabolic control of coronary blood flow and the matching of myocardial oxygen supply to demand; and (3) the direct and reflexogenic actions of CO2 on myocardial contractile function. Clinically relevant issues are addressed, including the role of increased myocardial tissue PCO2 (PmCO2) in the decline in myocardial contractility during coronary hypoperfusion and the increased vulnerability to CO2-induced cardiac depression in patients receiving a β-adrenergic receptor antagonist or with otherwise compromised inotropic reserve. The potential use of real-time measurements of PmO2 to monitor the adequacy of myocardial perfusion in the perioperative period is discussed.

Prinzmetal's variant angina)(PVA). Circadian variation in response to hyperventilation

The study reports on the outcome of hyperventilation tests in a 57-year-old male with Prinzmetal's variant angina, formerly often complicated by ventricular fibrillation. It was found that hyperventilation for a period of 6 min after a delay of 4 to 6 min was followed by the development of ST-elevation and pain, but only when the test was performed in the morning, whereas the outcome of tests performed later in the day were negative. Pretreatment with calcium blockers, nifedipine or verapamil proved effective in preventing the anginal response to the test, also when it was performed in the morning. It is concluded that hyperventilation performed in the early morning, but not later in the day, may prove to be an effective and safe procedure for provoking Prinzmetal's variant angina, and that hyperventilation may be useful in the evaluation of the efficacy of drug therapy.

Heart and brain circulation and CO2 in healthy men

AIM

To compare blood flow response to arterial carbon dioxide tension change in the heart and brain of normal elderly men.

METHODS

Thirteen healthy elderly male volunteers were studied. Hypercapnea was induced by carbon dioxide inhalation and hypocapnea was induced by hyperventilation. Myocardial blood flow [mL min(-1) x (100 g of perfusable tissue)(-1)] and cerebral blood flow [mL min(-1) x (100 g of perfusable tissue)(-1)] were measured simultaneously at rest, under carbon dioxide gas inhalation and hyperventilation using the combination of two positron emission tomography scanners.

RESULTS

Arterial carbon dioxide tension increased significantly during carbon dioxide inhalation (43.1 +/- 2.7 mmHg, P < 0.05) and decreased significantly during hyperventilation (29.2 +/- 3.4 mmHg, P < 0.01) from baseline (40.2 +/- 2.4 mmHg). Myocardial blood flow increased significantly during hypercapnea (88.7 +/- 22.4, P < 0.01) from baseline (78.2 +/- 12.6), as did the cerebral blood flow (baseline: 39.8 +/- 5.3 vs. hypercapnea: 48.4 +/- 10.4, P < 0.05). During hypocapnea cerebral blood flow decreased significantly (27.0 +/- 6.3, P < 0.01) from baseline as did the myocardial blood flow (55.1 +/- 14.6, P < 0.01). However, normalized myocardial blood flow by cardiac workload [100 mL mmHg(-1) x (heart beat)(-1) x (gram of perfusable tissue)(-1)] was not changed from baseline (93.4 +/- 16.6) during hypercapnea (90.5 +/- 14.3) but decreased significantly from baseline during hypocapnea (64.5 +/- 18.3, P < 0.01).

CONCLUSION

In normal elderly men, hypocapnea produces similar vasoconstriction both in the heart and brain. Mild hypercapnea increased cerebral blood flow but did not have an additional effect to dilate coronary arteries beyond the expected range in response to an increase in cardiac workload.

Hypocapnia reduces the T wave of the electrocardiogram in normal human subjects

During voluntary hyperventilation in unanesthetized humans, hypocapnia causes coronary vasoconstriction and decreased oxygen (O2) supply and availability to the heart. This can induce local epicardial coronary artery spasm in susceptible patients. Its diagnostic potential for detection of early heart disease is unclear. This is because such hypocapnia produces an inconsistent and irreproducible effect on electrocardiogram (ECG) in healthy subjects. To resolve this inconsistency, we have applied two new experimental techniques in normal, healthy subjects to measure the effects of hypocapnia on their ECG: mechanical hyperventilation and averaging of multiple ECG cycles. In 15 normal subjects, we show that hypocapnia (20 ± 1 mmHg) significantly reduced mean T wave amplitude by 0.1 ± 0.0 mV. Hypocapnia also increased mean heart rate by 4 beats/min without significantly altering blood pressure, ionized calcium or potassium levels, or the R wave or other features of the ECG. We therefore provide the first unequivocal demonstration that hypocapnia does consistently reduce T wave amplitude in normal, healthy subjects.

Comparison of hyperventilation and inhaled nitric oxide for pulmonary hypertension after repair of congenital heart disease

BACKGROUND

Pulmonary hypertension is associated with congenital heart lesions with increased pulmonary blood flow. Acute increases in pulmonary vascular resistance (PVR) occur in the postoperative period after repair of these defects. These increases in PVR can be ablated by inducing an alkalosis with hyperventilation (HV) or bicarbonate therapy. Studies have shown that these patients also respond to inhaled nitric oxide (iNO), but uncertainty exists over the relative merits and undesirable effects of HV and iNO.

HYPOTHESIS

Alkalosis and iNO are equally effective in reducing PVR and pulmonary artery pressure (PAP) in children with pulmonary hypertension after open heart surgery.

SETTING

Critical care unit of a tertiary care pediatric hospital.

DESIGN

Prospective, randomized, crossover design.

PATIENTS

Twelve children with a mean PAP > 25 mm Hg at normal pH after biventricular repair of congenital heart disease.

INTERVENTIONS

Patients were assigned to receive iNO or HV (pH > 7.5) in random order, and the effect on hemodynamics was measured. Each treatment was administered for 30 mins with a 30-min washout period between treatments. Finally, both treatments were administered together to look for a possible additive effect.

MEASUREMENTS AND MAIN RESULTS

Cardiac output and derived hemodynamic parameters using the dye dilution technique. Hyperventilation, achieved by an increase in ventilator rate without a change in mean airway pressure, decreased Pa(CO2) from a mean (SD) of 43.7+/-5.3 to 32.3+/-5.4 mm Hg and increased pH from 7.40+/-0.04 to 7.50+/-0.03. This significantly altered both pulmonary and systemic hemodynamics with a reduction in PAP, PVR, central venous pressure, and cardiac output and an increase in systemic vascular resistance. In comparison, iNO selectively reduced PAP and PVR only. The reduction in PVR was comparable between treatments, although addition of iNO to HV resulted in a small additional reduction in PVR. An additional decrease in PAP was seen when HV was added to iNO, attributable to a reduction in cardiac output rather than a further decrease in PVR.

CONCLUSIONS

Inhaled NO and HV are both effective at lowering PAP and PVR in children with pulmonary hypertension after repair of congenital heart disease. The selective action of iNO on the pulmonary circulation offers advantages over HV because a decrease in cardiac output and an increase in SVR are undesirable in the postoperative period.

Effect of acute moderate changes in PaCO2 on global hemodynamics and gastric perfusion

OBJECTIVE

To describe global hemodynamics and splanchnic perfusion changes in response to acute modifications in Paco2 in hemodynamically stable patients.

DESIGN

Prospective, randomized crossover study.

SETTING

Medical-surgical intensive care unit at a community hospital (400,000 inhabitants).

PATIENTS

Ten critically ill patients who were sedated, paralyzed, and mechanically ventilated.

INTERVENTIONS

Hypercapnia and hypocapnia were obtained by increasing and reducing instrumental deadspace in random order. After each intervention, patients returned to the basal condition. Each period lasted 80 min: 20 min to achieve stable Paco2 and 60 min for tonometer equilibration. In each period, global hemodynamic variables and tonometric data were collected. The periods were compared using analysis of variance.

MEASUREMENTS AND MAIN RESULTS

Acute hypercapnia (Paco2 from 40+/-3 to 52+/-3 torr, p<.05) increased cardiac index (3.43+/-0.37 vs. 3.97+/-0.43 mL/min/m2, p<.05), heart rate (95+/-6 vs. 105+/-3 beats/min, p<.05), and mean pulmonary artery pressure (21+/-1 vs. 24+/-1 mm Hg, p<.05) and reduced systemic vascular resistance (992+/-98 vs. 813+/-93 dyne x sec/ cm5, p<.05) and oxygen extraction ratio (27+/-3% vs. 22+/-2%, p<.05). Standardized intramucosal Pco2 increased from 49+/-2 to 61+/-3 torr (p<.05) with an associated decrease in calculated intramucosal pH ([pHi] 7.35+/-0.03 vs. 7.25+/-0.02, p<.05), but the gastro-arterial Pco2 gradient (deltaPco2) did not change. Acute hypocapnia (Paco2 from 41+/-3 to 34+/-3 torr, p<.05; pH 7.41+/-0.01 to 7.47+/-0.02, p<.05) induced slight increments in systemic vascular resistance (995+/-117 vs. 1088 +/- 160 dyne x sec/cm5, p<.05) and oxygen extraction ratio (28+/-2% vs. 30+/-2%, p<.05). Standardized intramucosal Pco2 decreased (50+/-4 vs. 44+/-3 torr, p<.05), pHi increased (7.33+/-0.03 vs. 7.36+/-0.02; p<.05), but deltaPco2 did not change.

CONCLUSIONS

In this small group of stable patients, moderate acute variations in Paco2 had a significant effect on global hemodynamics, but splanchnic perfusion, assessed by deltaPco2, did not change. In these conditions, the use of pHi to evaluate gastric perfusion appears unreliable.

Investigation of the most effective provocation test for patients with coronary spastic angina: usefulness of accelerated exercise following hyperventilation

This study sought to compare the clinical usefulness of the hyperventilation plus cold stress test or the hyperventilation combined with accelerated exercise test with other single tests in patients with coronary spastic angina. The study examined 24 patients (23 men, mean age 66 years) with angiographically confirmed coronary spastic angina and less than 50% stenosis. Moreover, none had spontaneous ST segment elevation before the study. Under no medication for at least 24 h prior, 4 procedures were performed from 09.00 h to 11.00 h: (i) a hyperventilation test for 5 min (HV(5)); (ii) HV(5) combined with a cold stress test for the last 2 min (HV(5)+CS(2)); (iii) a treadmill exercise test based on Bruce's protocol (TM(3)); and (iv) a treadmill exercise test accelerated at 1 min intervals according to Bruce's protocol immediately after HV(5) (HV(5)+TM(1)). The rate of appearance of chest pain and ischemia-induced ECG changes due to HV(5)+TM(1) were significantly higher than the other 3 tests. HV(5)+CS(2) was not superior to HV(5) alone. The incidence of provoked ST segment elevation due to HV(5)+TM(1) was higher than with the other 3 procedures. Thus, in patients with coronary spastic angina, no spontaneous ST segment elevation and near normal coronary arteries, HV(5)+CS(2) was no more useful than HV(5) alone. It is recommended that the newly designed HV(5)+TM(1) combination test be used for documenting evidence of ischemia in patients with coronary spastic angina, low disease activity and near normal coronary arteries.

Plasma endothelin-1 levels in patients with angina pectoris and normal coronary angiograms

BACKGROUND

Some patients with typical angina and electrocardiographic evidence of ischemia have normal coronary angiograms. These patients have a reduced coronary flow reserve and abnormal endothelium-dependent vasodilator responses; this syndrome is known as microvascular angina. Among endothelium-derived peptides, endothelin-1 (ET-1) is a potent vasoconstrictor and an important modulator of microvascular function.

METHODS

Plasma ET-1 was measured in 13 patients with typical angina, instrumental evidence of ischemia, and normal arteriograms and in 20 normal control subjects.

RESULTS

Mean concentration of ET-1 was 2.89+/-1.24 pmol/L in patients with angina and normal angiograms and 1.99+/-0.81 pmol/L in normal control subjects (p < 0.02). Plasma levels of ET-1 values were significantly higher in patients with angina, positive exercise test results for ischemia, and normal coronary arteriograms compared with the group of patients with no clinical or instrumental evidence of ischemia.

CONCLUSIONS

This is consistent with the hypothesis that in patients with microvascular angina, an endothelial dysfunction in the coronary vascular area caused by impaired endothelium-derived ET-1 could play an active role in the disease process.

Hemodynamic response to hyperventilation test in healthy volunteers

Hyperventilation is well known to affect the electrocardiogram (ECG) in subjects without heart disease and produce spasm in patients with variant angina. The autonomic nervous system is thought to play a significant role in these effects. However, the normal hemodynamic response to hyperventilation is not well defined. We subjected 369 healthy volunteers (200 men, 169 women) to prolonged hyperventilation (30 respirations for 5 min and 10 min recovery) under continuous ECG monitoring and to exercise testing. Heart rate (HR), systolic and diastolic blood pressures (SBP, DBP) and rate-pressure product were recorded. Hyperventilation resulted in an immediate (within the first min), significant increase in HR by 27.4%, a further small increase at min 2 of hyperventilation, and a subsequent small decrease in HR at mins 3-5. An immediate drop of HR by 20.1% was observed with discontinuation of hyperventilation. Apart from a slightly higher HR increase in men, a similar pattern of HR changes was found in both genders. On multivariate analysis, younger age, absence of smoking, and male gender were associated with a higher HR increase with hyperventilation (p < 0.0001, p < 0.0001, and p < 0.001, respectively). SBP and DBP increased with hyperventilation, with their highest value at min 5 of hyperventilation and a subsequent drop to baseline levels. Age and gender did not affect the degree and pattern of BP changes. Absence of smoking and the presence of hypertension were associated with a higher SBP with hyperventilation (p < 0.003 and p < 0.007). The rate-pressure product increased by 43.6% with hyperventilation, a change that was only 19.1% of the respective rate-pressure product observed with exercise. Hyperventilation results in significant HR and BP increases, changes that are influenced by age, gender, smoking, and hypertension. Our study could serve as a standard for comparison of the hyperventilation effects in different disease states.

Pulmonary vascular resistance in infants after cardiac surgery: role of carbon dioxide and hydrogen ion

OBJECTIVE

The objective of this study was to describe the effects of altering arterial PCO2 and pH on pulmonary vascular resistance in infants after cardiopulmonary bypass for cardiac surgery.

DESIGN

Prospective study (with each patient as his or her own control).

SETTING

Cardiac intensive care unit in a children's hospital.

PATIENTS

We studied 15 infants (ages ranging from 0.4 to 15.6 months; median 5.7) who were mechanically ventilated during the immediate postoperative period after corrective cardiac surgery.

INTERVENTIONS

The study was designed to have the following three stages: In the baseline stage, the initial postoperative hemodynamic parameters were stabilized and body temperature was normalized. In the hypercarbia stage, with FIO2 at 0.40, the rate of ventilation was decreased to produce an end-tidal CO2 level of > 55 torr (> 7.3 kPa). This stage established a clinical model of increased pulmonary vascular resistance. With the minute ventilation held constant in order to maintain a constant PaCO2, the arterial pH was increased by administration of a 4-mEq/kg iv dose of sodium bicarbonate (sodium bicarbonate stage). Arterial blood gas and hemodynamic determinations were obtained after a 10-min stabilization period at each stage. Drug infusions were not altered during the study period.

MEASUREMENTS AND MAIN RESULTS

In the hypercarbia stage (stage 2), the mean PaCO2 increased from 36 +/- 5 torr (4.8 +/- 0.7 kPa) (at baseline) to 55 +/- 16 torr (7.3 +/- 2.1 kPa) (p < .01). As a result, the mean arterial pH decreased from 7.48 +/- 0.05 to 7.31 +/- 0.03 (p < .01). During this stage, the mean pulmonary arterial pressure increased from 21 +/- 6 to 30 +/- 8 mm Hg (p < .01) but the cardiac index remained unchanged (3.7 +/- 1.2 to 3.8 +/- 1.2 L/min/m2). Pulmonary vascular resistance index increased from 4.1 +/- 2.0 to 6.0 +/- 3.1 U.m2 (p < .01). After the administration of sodium bicarbonate (stage 3), the arterial pH increased to 7.44 +/- 0.06 (p < .05), while the PaCO2 was unchanged. The pulmonary vascular resistance index decreased to 3.1 +/- 1.5 U.m2 (from 6.0 +/- 3.1 U.m2; p < .01) as a result of both a decrease in mean pulmonary arterial pressure (to 26 +/- 6 mm Hg; p < .01) and a concomitant increase in cardiac index to 5.1 +/- 1.6 L/min/m2 (p < .01).

CONCLUSIONS

Increasing the arterial pH by the administration of sodium bicarbonate both lowers the pulmonary arterial pressure and increases the cardiac index, resulting in a decrease in pulmonary vascular resistance. These changes were observed without alteration in PaCO2. Metabolic alkalosis may have a role in the treatment of increased pulmonary vascular resistance in infants after cardiopulmonary bypass for cardiac surgery.

Effect of hyperventilation and mental stress on coronary blood flow in syndrome X

OBJECTIVES

To assess the effect of hyperventilation and mental stress on coronary blood flow and symptom production in patients with syndrome X.

DESIGN

A prospective study. Hyperventilation and mental stress tests were performed on the ward and were repeated in the cardiac catheter laboratory where coronary blood flow velocity was also measured with an intracoronary Doppler catheter in the left anterior descending coronary artery. Oesophageal manometry studies were also performed.

PATIENTS

29 patients with syndrome X (typical anginal chest pain, a positive exercise test, and normal coronary angiogram).

SETTING

A regional cardiothoracic centre.

RESULTS

Hyperventilation produced typical chest pain in 16 patients on the ward. 13 patients experienced their typical chest pain with mental stress test 5. Ten patients experienced chest pain with both hyperventilation and mental stress tests. This pattern was reproduced exactly when the tests were repeated in the cardiac catheter laboratory. Hyperventilation produced a significant increase in the rate-pressure product during ward and laboratory testing. There was, however, no significant change in the rate-pressure product on mental stress tests. The mean (SEM) coronary flow velocity decreased significantly on hyperventilation in the catheter laboratory from 10.0 (0.92) cm/s to 5.9 (0.72) cm/s (p < 0.001). There was also a significant reduction in the mean (SEM) coronary blood flow velocity on mental stress tests from 9.8 (0.86) cm/s to 7.4 (0.6) cm/s (p < 0.001). This reduction in flow velocity occurred in the absence of any changes in diameter of the left anterior descending artery. Further analysis showed that the coronary flow velocity was reduced significantly in only that group of patients in which hyperventilation and mental stress provoked chest pain. There was a significant increase in the arterial concentrations of noradrenaline on both hyperventilation and mental stress testing. Oesophageal manometry showed abnormalities in 17% of patients.

CONCLUSIONS

Both hyperventilation and mental stress can produce chest pain in patients with syndrome X and this is associated with a reduction in coronary blood flow velocity. The results of this study suggests that this reduction in coronary flow occurs as a result of increased microvascular resistance.

Doppler continuous-wave analysis of grafted mammary artery as a non-invasive technique for static and dynamic assessment of coronary flow in man

In this paper, Doppler continuous-wave analysis of blood velocity in the internal mammary artery, anastomosed to the left coronary vascular bed in humans who have undergone myocardial revascularization, is proposed as a non-invasive technique to study coronary blood flow during physiological procedures which cause it to change. Blood velocity curves obtained in normal and anastomosed internal mammary arteries were compared during hyperventilation and the Valsalva manoeuvre. During hyperventilation, blood velocity increased in the normal mammary but not in the anastomosed artery. During the expiratory effort of the Valsalva manoeuvre, the mean blood velocity decreased in the normal mammary artery but it did not change significantly in the anastomosed artery. Variations in the mean velocity were largely prevented by simultaneous and well-balanced increases and decreases in the diastolic and systolic velocities, respectively.

Increased coronary tone in exertional angina: the beneficial effects of calcium antagonists

Coronary vasoconstriction may play a relevant role in the pathogenesis of exercise-induced myocardial ischemia, not only in patients with Prinzmetal's angina, but also in patients with chronic stable angina. In these patients the use of calcium antagonists, namely, dihydropyridine derivatives, may be beneficial. Hyperventilation is a simple and sensitive test to discriminate patients with effort angina who will improve their exercise capacity after administration of these drugs.

Metabolic and hemodynamic consequences of sodium bicarbonate administration in patients with heart disease

PURPOSE

The use of sodium bicarbonate (NaHCO3) in cardiopulmonary arrest has been questioned, but the effects of NaHCO3 in patients with heart disease are not known. We therefore prospectively evaluated the effects of NaHCO3 in patients with congestive heart failure.

PATIENTS AND METHODS

Ten patients received NaHCO3 and control infusions of equimolar sodium chloride (NaCl). Measurements were made of blood gases, 2,3-diphosphoglyceric acid (2,3-DPG), glucose, lactate, cardiac hemodynamics, and oxygen consumption.

RESULTS

The arterial oxygen tension (pO2) fell an average of 10 mm Hg after NaHCO3 administration in patients with congestive heart failure, whereas it rose with NaCl (p less than 0.005). Myocardial oxygen consumption decreased by 17% (p less than 0.002) without an accompanying change in oxygen demand. Systemic oxygen consumption fell by 21%. Red blood cell 2,3-DPG levels were elevated at baseline, but did not change with NaHCO3 administration. The oxygen pressure at 50% hemoglobin saturation (P50) was correspondingly elevated at baseline in these patients and decreased significantly with NaHCO3 (Bohr effect) (p less than 0.003). The arterial and mixed venous carbon dioxide tensions increased with NaHCO3 but decreased with NaCl administration (p less than 0.05). Blood glucose concentrations fell by 1.7 mmol/L with NaHCO3 (p less than 0.003) and blood lactate concentrations increased uniformly (p less than 0.001). Three patients developed net myocardial lactate generation during NaHCO3 administration; two of these three developed symptoms of angina. Coronary blood flow did not change with NaHCO3 but increased with NaCl (p less than 0.04). Two patients developed transient pump failure.

CONCLUSION

These data demonstrate that NaHCO3 impairs arterial oxygenation and reduces systemic and myocardial oxygen consumption. The decrease in oxygen utilization is associated with anaerobic metabolism, enhanced glycolysis, and elevation of the blood lactate level, and may lead to transient myocardial ischemia in some patients. Thus, the use of NaHCO3 in such patients warrants re-evaluation.

Effect of pH and PCO2 on pulmonary and systemic hemodynamics after surgery in children with congenital heart disease and pulmonary hypertension

Fourteen children with congenital heart disease and associated pulmonary hypertension (preoperative mean pulmonary artery pressure (MPAP) 48 mm Hg +/- 1 SEM were examined to determine the effect of arterial carbon dioxide tension (PaCO2) and pH on pulmonary and systemic hemodynamics after surgical repair. Baseline measurements were obtained with hyperventilation to PaCO2 20 to 30 mm Hg (pH 7.56 +/- 0.01 mm Hg). The addition of carbon dioxide to inspired gas to achieve a PaCO2 40 to 45 mm Hg (pH 7.35 +/- 0.01) resulted in a significant increase in MPAP, from 32 +/- 5 mm Hg to 47 +/- 8 mm Hg (p less than 0.05). An increase in mean cardiac index (CI) from 2.7 +/- 0.3 L/min/m2 to 3.3 +/- 0.3 L/min/m2 (p less than 0.05) explained in part the associated increase in MPAP. For a subgroup of eight patients with postoperative MPAP greater than 30 mm Hg (at pH 7.35 to 7.40), pulmonary vascular resistance index (PVRI) also significantly increased (p less than 0.05) as PaCO2 was increased, implying a direct pulmonary vasodilating effect of alkalosis. Removal of carbon dioxide from inspired gas returned hemodynamic values to baseline. The higher the MPAP at physiologic pH the greater the absolute amount of MPAP reduction and PVRI reduction (p less than 0.05) with alkalosis. No complications from alkalosis were seen. We suggest that a trial of hypocarbic alkalosis in the child with severe residual pulmonary hypertension after surgical repair of congenital heart disease is warranted to reduce right ventricular afterload.

Hyperventilation and myocardial infarction

Chest pain that is associated with hyperventilation is often considered to be benign and noncardiac in nature. While not commonly recognized, hyperventilation can provoke coronary vasospasm. We report a man who presented with hyperventilation and developed myocardial infarction. In the setting of hyperventilation, chest pain and ST segment elevation, coronary vasospasm must be considered.

Hypocapnia and hypercapnia in the dog: effects on myocardial blood-flow and haemodynamics during beta- and combined alpha- and beta-adrenoceptor blockade

We have recently demonstrated an increase in myocardial blood-flow (MBF) during systemic hypercapnia independent of myocardial oxygen consumption. During hypocapnia no significant decrease of MBF was observed; however, a significant increase of myocardial oxygen extraction resulted. The present study was undertaken to examine whether these effects were caused by changes in the sympathoadrenal activity. During beta-adrenoceptor blockade and combined alpha- and beta-blockade the effects of hypocapnia and hypercapnia on MBF and haemodynamics were examined. Closed-chest dogs were anaesthetized with pentobarbital and hypocapnia was induced by hyperventilation. Carbon dioxide was added to the inspiratory gas to create normocapnia and hypercapnia. Hypocapnia did not result in any changes of MBF, coronary vascular resistance or myocardial oxygen extraction, neither during beta-adrenoceptor blockade nor during combined alpha- and beta-blockade. Hypercapnia did not increase MBF, neither during beta-blockade nor during combined alpha- and beta-blockade. However, both myocardial oxygen consumption and mechanical performance of the heart were reduced during hypercapnia. Thus, a relative myocardial overperfusion, indicated by decreased coronary vascular resistance and myocardial oxygen extraction, was observed during hypercapnia. In conclusion, the unaltered MBF during hypocapnia and the coronary overperfusion during hypercapnia were not related to changes in the sympathoadrenal activity.

Effects of carbon dioxide and pH on myocardial blood-flow and metabolism in the dog

The relative importance of pCO2 versus pH in regulating myocardial blood-flow (MBF) is not settled. Therefore, the influence of hypocapnia, hypercapnia and sodium carbonate infusion, on MBF and myocardial metabolism, has been investigated in 10 closed-chest pentobarbital anaesthetized dogs. The animals were hyperventilated, and CO2 was added to the inspiratory gas to induce normocapnia and hypercapnia. A mass spectrograph continuously measured the ventilatory gas components, and MBF was measured by the hydrogen desaturation technique with a catheter positioned in the coronary sinus. During the experiments, there were no significant alterations in heart rate, mean aortic blood-pressure, myocardial oxygen consumption or uptake of glucose and free fatty acids. During hypocapnia MBF was insignificantly reduced, while myocardial oxygen extraction increased significantly. During hypercapnia, however, MBF increased more than 40%. This increase in MBF was abolished following an infusion of sodium carbonate. Thus, in the present study, increased MBF, observed during hypercapnia, was due to the reduction in pH and not to the increase in pCO2.

Different susceptibility to myocardial ischemia provoked by hyperventilation and cold pressor test in exertional and variant angina pectoris

Coronary constriction at the site of atherosclerotic stenoses has been suggested to play an important role in modulating the frequency of symptoms in patients with exertional angina. To investigate whether stimuli triggering coronary constriction have similar effects in patients with exertional and variant angina, responses to hyperventilation (HV) and cold pressor test (CPT) were evaluated. Twenty patients with chronic exertional angina, positive exercise test results and coronary heart disease were compared with 14 patients with variant angina and ST-segment elevation during an ergonovine test. In patients with exertional angina, the CPT produced diagnostic ST-segment depression in 6 of 20 patients (30%) at levels of rate-pressure product much lower than those during the exercise test; all patients had low effort tolerance and severe coronary artery disease. HV produced diagnostic ST-segment depression in only 1 of 20 patients (5%) (p less than 0.05 compared to that with CPT). Conversely, in patients with variant angina, HV produced ST-segment elevation in 11 of 14 patients (78%) and CPT produced elevation in only 2 of 14 (14%) (p less than 0.01). Thus, coronary constriction can provoke myocardial ischemia not only in patients with variant angina but also in some patients with exertional angina. Furthermore, the 2 groups of patients have a different susceptibility to stimuli known to produce coronary constriction.

Local vasoactivity of oxygen and carbon dioxide in the right coronary circulation of the dog and pig

1. Eight mongrel dogs were anaesthetized with sodium thiamylal and chloralose-urethane, ventilated, vagotomized and heparinized. Five Poland-China pigs were anaesthetized with sodium thiamylal and nitrous oxide, ventilated, vagotomized and heparinized. 2. Extracorporeal perfusion of the right coronary artery at constant pressure (100 mmHg) was instituted. A lung from a donor animal was interposed in the coronary perfusion circuit to effect changes in CO2 and O2 tensions in the coronary arterial blood while systemic blood gases were maintained at normal levels. 3. Local hypoxia (PO2 range 17-22 mmHg) produced a 25-75% decrease in coronary vascular resistance (P less than 0.05) and a 0-24% (not significant) decrease in right ventricular dP/dt. 4. Local changes in PCO2 over the range 8-105 mmHg were associated with a 17-58% decrease in coronary vascular resistance (P less than 0.05), a 19-24% decrease in right ventricular dP/dt (P less than 0.05) with no change in right ventricular end-diastolic pressure, and a 1-18% (not significant) decrease in heart rate. 5. These studies suggest that local decreases in O2 or increases in CO2 tensions produce decreases in right coronary vascular resistance that are in the opposite direction to those that would be expected from the observed changes in heart rate and contractility (two primary determinants of myocardial oxygen consumption). 6. These data support the hypothesis that CO2 and O2 are locally vasoactive in the coronary circulation.

The hyperventilation test as a method for developing successful therapy in Prinzmetal's angina

In 10 cases of Prinzmetal's angina in which episodes of myocardial ischemia were easily and reproducibly induced by hyperventilation, this test was performed 111 times, 41 times under control conditions and 70 times during treatment with one or more of the following drugs: phentolamine, isosorbide dinitrate, propranolol, verapamil, nifedipine and amiodarone. Seventeen of 18 negative tests performed under the influence of a long-acting drug coincided with total remission of the patient's anginal episodes when this drug was administered on a short- or long-term basis. No patient died or sustained infarction during a follow-up period of 10.9 months. A negative test was thus a good indication that the clinical response to the corresponding drug would be favorable. The electrocardiographic changes and chest pain provoked by hyperventilation occurred not when alkalosis was greatest (hydrogen ion [pH] change from 7.42 to 7.58, p less than 0.001), but when pH was approaching normal or control values. The onset of electrocardiographic changes occurred an average of 175 seconds after the end of hyperventilation and, in two cases, the time lag was as much as 480 and 705 seconds, respectively. This raises several questions regarding the true mechanism triggering coronary spasm under such conditions. The hyperventilation test appears to be a useful and safe procedure for selecting the best possible drug for long-term treatment of Prinzmetal's angina as well as for comparing the relative efficacy of different drugs.

Effects of respiratory alkalosis on coronary vascular dynamics and myocardial energetics in patients with coronary artery disease

To determine if respiratory alkalosis produces hemodynamically significant coronary vasoconstriction in coronary artery disease (CAD), we studied the effects of hyperventilation on coronary sinus blood flow (CSBF), myocardial O2 uptake, and lactate extraction in 13 CAD patients. No patient developed chest pain or ischemic ECG changes during hyperventilation. Hyperventilation increased pressure-rate product (myocardial O2 consumption index, MVO2) minimally did not change global CSBF, coronary vascular resistance or lactate extraction. However, hyperventilation increased global myocardial O2 uptake from 14.5 plus or minus 3.2 to 18.7 plus or minus 17.2 ml/min (p less than 0.01) principally due to increased myocardial O2 extraction (65.0 plus or minus 7.4 to 71.6 plus or minus 6.2%, p less than 0.01). The increased pressure-rate product was not sufficient to account for increased myocardial O2 uptake. We conclude that respiratory alkalosis increases myocardial O2 extraction but does not produce hemodynamically significant coronary vasoconstriction in CAD patients.

Sequence of events in angina at rest: primary reduction in coronary flow

To investigate the events that lead to acute myocardial ischemia we monitored continuously the ECG, the left ventricular (four patients) or aortic (two patients) pressure and the great cardiac vein oxygen saturation (CSO2S) by a fiberoptic catheter in six patients with frequent anginal attacks at rest. We recorded 137 transient ischemic episodes (10 with chest pain) characterized by ST-segment elevation in 28 episodes, depression in three episodes and by pseudonormalization of previously inverted or flat T waves in 106 episodes. The onset of electrocardiographic and hemodynamic changes was preceded by a large drop in CSO2S in all 135 episodes with ST-T changes in the anterior leads but not in two episodes with ST elevation on inferior leads. The fall in CSO2S, consistently followed by signs of left ventricular function impairment and never preceded by any detectable increase in the hemodynamic determinants of myocardial oxygen consumption, probably reflects a reduction in regional perfusion. Thus, a reduction in coronary flow may cause transient ischemia in patients with angina at rest. These episodes may be associated with variable, often minor electrocardiographic changes and occasionally with anginal pain.

Coronary arterial spasm and Prinzmetal's variant form of angina induced by hyperventilation and Tris-buffer infusion

Vigorous hyperventilation was induced for five minutes immediately after a five-minute infusion of 100 ml of Tris-buffer (pH 10) in nine patients with Prinzmetal's variant angina. In eight of the patients, chest pain with ischemic changes in the electrocardiogram occurred during this procedure or within five minutes after it ended. Coronary arterial spasm appeared after the procedure and disappeared after the administration of nitroglycerin in all four patients in whom coronary cinearteriography was performed. This was evident both before and after the procedure and after sublingual administration of nitroglycerin (0.6 mg). The oral administration of 90 mg of diltiazem, a calcium antagonistic drug, two hours before, completely suppressed the attack induced by the procedure in all of the five patients who received this drug. We conclude that hyperventilation plus Tris-buffer infusion induces coronary arterial spasm and anginal attack in patients with Prinzmetal's variant angina and that diltiazem suppresses these reactions.

Relative effect of CO2 on canine coronary vascular resistance

We determined the effect of alterations in coronary arterial PCO2 on coronary vascular resistance (CVR) at a constant coronary sinus (CS) PO2 and the effect of coronary arterial PO2 variation on CVR at a constant CS PCO2. A linear but opposing effect on CVR was found for both gases. The sensitivity of CVR to O2 change, represented as CVR/CS PO2, was approximately twice that of the ratio CVR/CS PCO2. (0.0852 +/- 0.006 vs. -0.0362 +/- 0.005). The entire range of CVR variation obtainable through CO2 variation was as great as that resulting from O2 variation. During randomized variation of O2 and CO2, CVR can be mathematically related in a multiple linear expression to CS PO2 and CS PCO2.

Severe bronchial asthma: factors influencing intensive care management and outcome

Over a seven year period 2933 patients were admitted with asthma. Respiratory failure occurred on 106 occasions. A total of 26 patients died including 11 of the 106 who developed respiratory failure. The intra-hospital management of asthma is outlined. The experience is analysed to indicate those patients particularly at risk. These include those who develop pneumothorax, have associated chest infections or have recently ceased steroid therapy.

The response of canine coronary vascular resistance to local alterations in coronary arterial P CO2

The effect of hypercapnia on coronary vascular resistance (CVR) was studied in seven open-chest dogs. Coronary blood flow was supplied to the cannulated left main coronary artery from the femoral artery by a precision pump. Coronary arterial PCO2 was locally controlled with a small membrane oxygenator in the coronary perfusion circuit. Each PCO2 change was made at a constant coronary flow, and CVR was calculated from the ratio of perfusion pressure to flow. Coronary sinus (CS) PCO2 and PO2 were recorded continuously from blood withdrawn through a CS catheter. Normocapnia (PCO2 = 42.3 +/- 2.8 mm Hg) was obtained with a membrane oxygenator gas composition of 95% O2-5% CO2, and hypocapnia was produced with 100% O2-0% CO2. In addition to physiology normal coronary flow (determined by a CS PO2 of 20-30 mm Hg) relatively high and low flow states were studied. At a normal control CS PO2, a decrease in coronary arterial PCO2 from 42.3 +/- 2.8 to 23.8 +/- 1.3 mm Hg caused CVR to increase by 84.2%, from 1.27 +/- 0.06 to 2.30 +/- 0.04 units. Since pH was inversely related to PCO2, the effect on CVR may have been mediated through a pH change. CS PCO2 decreased from 65.2 +/- 1.9 to 39.4 +/- 1.3 mm Hg. myocardial oxygen consumption was unchanged. Increases in CVR of 74.5, 119.5, and 69.3% occurred during hypocapnia in three additional experiments in which control arterial PO2 was maintained at 52-90 mm Hg. When CS PO2 was greater than 30 mm Hg, the normocapnic CVR was high, and was only minimally increased by hypocapnia. When coronary flow was reduced to an ischemic level there was little response in CVR to hypocapnia. Thus the level of arterial PCO2 can have an important effect on CVR independent of changes in O2 consumption. Myocardial PCO2, derived from metabolically produced CO2 and contributed to by arterial CO2, may be a major factor in normal control of coronary flow.

Comparison of the coronary collateral circulation in dogs and baboons after coronary occlusion

The relevance to man of experimental observations on coronary collateral blood flow (CCBF) in dogs has been questioned. The effect of 2 to 3 hour coronary occlusions in the anesthetized dog and a primate, the baboon, were therefore compared, with CCBF measured by injections of 85Kr distal to occlusion with precordial counting. Before killing, additional isotope was infused to compare inner/outer wall flow distribution and myocardial tissue samples were analyzed for electrolyte content. Effects of nitrates on hemodynamics and metabolism were also compared in dog and baboon. Similar values for CCBF and resistance following occlusions were found in dog and baboon (flow approximately 25 per cent control, calculated resistance increase four- to sevenfold). Greater subendocardial ischemia in both species was indicated by isotope distribution less to the inner wall, but electrolyte changes (k+ less and Na+ greater in the ischemic area compared to nonischemic) were similar transmurally in both species. Hemodynamic responses to nitrate infusion (isosorbide dinitrate) were similar, with increase in CCBF and decrease in resistance. In neither group were inner/outer wall isotope distribution or electrolyte changes influenced by nitrate. The coronary collateral response to occlusion is similar in dog and baboon in terms of both hemodynamics and metabolic changes. After 2 to 3 hours of coronary occlusion some hemodynamic benefit may be demonstrated with nitrates but no metabolic advantage, at least in the central area of ischemia.

Impairment of myocardial O2 supply due to hyperventilation

Thirteen patients with ischemic coronary heart disease purposely hyperventilated for seven minutes in order to induce hypocapnic alkalosis. One patient experienced chest pain, and one exhibited chemical signs of myocardial hypoxia. Heart rate, blood pressure and myocardial O2 consumption did not change significantly. Coronary blood flow decreased and coronary (a-v)O2 difference widened. Since the alkalosis increased the blood O2 affinity, the fall in PO2 in coronary venous blood was proportionately even greater than the fall in O2 concentration. Thus, hypocapnic alkalosis due to hyperventilation interferes with myocardial O2 supply by 1) coronary vasoconstriction and 2) increased O2 affinity of blood.