Subtle features of the hemodynamic response to amyl nitrite inhalation: new aspects of an old tool

Does amyl nitrite have a role in the management of pre-hospital mass casualty cyanide poisoning?

CONTEXT

Amyl nitrite has been recommended as a cyanide antidote for several decades. Its antidotal properties were initially attributed to induction of methemoglobin and later to a nitric oxide mediated hemodynamic effect. The ease of administration and alleged rapid clinical effect would recommend its wide use in the pre-hospital management of mass casualty cyanide poisoning; yet there are concerns regarding the use of amyl nitrite for this indication.

OBJECTIVE

Review the data on amyl nitrite in cyanide poisoning and evaluate its efficacy and safety in mass casualty cyanide poisoning.

METHODS

A literature search utilizing PubMed, Toxnet, textbooks in toxicology and pharmacology, and the bibliographies of the articles retrieved identified 17 experimental studies and human reports on the use of amyl nitrite in cyanide poisoning, and 40 additional articles on amyl nitrite's properties and adverse effects. One paper was excluded as it was a conference abstract with limited data.

MECHANISMS OF ACTION

The antidotal properties of amyl nitrite were attributed initially to induction of methemoglobinemia and later to nitric oxide mediated vasodilation.

EFFICACY

EXPERIMENTAL STUDIES

Animal studies on the use of amyl nitrite in cyanide poisoning are limited, and their results are inconsistent, which makes their extrapolation to humans questionable.

EFFICACY

HUMAN STUDIES

Clinical reports are limited in number and the part played by amyl nitrite relative to the other treatments administered (e.g. life support, sodium nitrite, and sodium thiosulfate) is unclear.

ADVERSE EFFECTS

Amyl nitrite can be associated with potentially serious adverse reactions such as hypotension, syncope, excessive methemoglobinemia, and hemolysis in G6PD deficient patients. These effects are more pronounced in young children, in the elderly, and in patients with cardiac and pulmonary disorders. Dose regimen. The method of administration of amyl nitrite (breaking pearls into gauze or a handkerchief and applying it intermittently to the victim's nose and mouth for a few minutes) is not easily controlled, might result in under- or over-dosing, can prevent the caregiver from administering life support, and possibly expose him/her to amyl nitrite's adverse effects.

CONCLUSIONS

Administration of amyl nitrite in mass casualty cyanide poisoning can result in unnecessary morbidity and may interfere with the proper management of the incident and the required supportive treatment and rapid evacuation. In the authors' opinion these drawbacks make the use of amyl nitrite in pre-hospital mass casualty cyanide poisoning unwarranted.

Inhaled amyl nitrite effectively reverses acute catastrophic thromboxane-mediated pulmonary hypertension in pigs

Acute catastrophic pulmonary vasoconstriction frequently leads to cardiovascular collapse. Rapid and selective pulmonary vasodilation is desired in order to restore haemodynamic stability. This pilot study examined the effectiveness of inhaled amyl nitrite as a selective pulmonary vasodilator. Nine adult swine were anaesthetized. Acute pulmonary hypertension with haemodynamic collapse was induced with a bolus administration of a thromboxane analogue, U46619. Six animals then received a capsule of amyl nitrite. The administration of inhaled amyl nitrite decreased mean pulmonary artery pressure from 42 +/- 3 to 22 +/ 3 mmHg at five minutes (p < 0.05), with a concomitant increase in cardiac output and mean arterial pressure. Pulmonary vascular resistance decreased from 4889 +/- 1338 to 380 +/- 195 dyne. sec. cm(-5) (by 92% from the maximal pulmonary hypertension change), with significant improvement in systemic haemodynamics. During acute thromboxane-mediated pulmonary hypertension with cardiovascular collapse, prompt administration of inhaled amyl nitrite was effective in restoring pulmonary and systemic haemodynamics within five minutes.

Bedside cardiac examination: constancy in a sea of change

The general trend in the recent literature has been to highlight the difficulties and shortcomings of the physical examination and to attribute these difficulties to deficiencies in training rather than to intrinsic weaknesses in auscultation itself. The call is for better training. Given the advice of the authors mentioned above, individual training may be warranted at the postgraduate level and in the large community of practicing internists and cardiologists. Although not proven, it is likely that individual training with computer technology, audiotape instruction, or simulator technology such as described in the following paragraphs would be effective at improving bedside clinical diagnosis and cost-effective patient care in the postgraduate, continuing medical education setting. The advances in auscultation during the last few years have been more incremental than fundamental. There is ongoing research into the mechanism of production of S3 and S4, and mathematical modeling techniques have recently been used with some success in evaluating the vibrations of S3 and S4 as forced, damped oscillations of a viscoelastic system. Analysis of sound energy with the technique of spectral waveform analysis, which investigates the frequency content of sound signals, has been used for many years in the study of cardiovascular sound. By the use of various methods of mathematical analysis, investigators have found potentially useful information in spectral sound patterns of prosthetic valves, murmur characteristics, and even potentially hemodynamic information from heart sounds. Despite the mathematical advances, there are still disturbing drawbacks to some of the analytic techniques, such as the production of mathematical terms containing "negative energy." Although the potential of obtaining significant clinical information from spectral analysis of heart sound recordings is attractive, the clinical usefulness of such techniques remains virtually nonexistent. Similar to the recent advances in auscultation, the technical advances in the design of the stethoscope have also been more incremental than fundamental. There are at least 3 recently introduced electronic stethoscopes that have the capability of amplification and filtration and that claim noise reduction. Because their introduction is recent, no information is available in the peer-review literature regarding their clinical performance; therefore their place in the clinical arena remains to be elucidated--perhaps a boon for patient care providers with specific hearing defects and perhaps useful in noisy clinical environments. Peer-review literature has not shown clear superiority of one type of acoustic stethoscope over another. The teaching of auscultation has been an area of recognized importance in patient care since the inception of auscultation as a medical art. Attempts to facilitate practitioner learning in the performance and interpretation of auscultation have advanced through the decades limited only by the technical infrastructure of the day. The availability of recorded heart sounds and murmurs appeared shortly after the availability of recording and playback devices, with first vinyl and later tape recordings. In 1974, technology was employed to create a virtual patient named "Harvey," an engineered cardiology patient simulator that reproduces many of the physical findings of the cardiology examination. Later, with the advent of commercially available CD-ROM devices, newer, better-integrated teaching devices have been developed, some of them outstanding in their clarity and quality. Despite the obvious value of such instructional aids that are best used in the individual setting, there is evidence that the classroom is still of significant value in teaching auscultation. However, nowhere else in the practice of medicine is a mentor approach more valuable than in learning auscultation. (ABSTRACT TRUNCATED)

Inhalation of amyl nitrite and the measurement of left ventricular outflow velocity: studies in normal, young adults

Amyl nitrite inhalation is useful in the identification of patients with provocable left ventricular (LV) outflow tract obstruction. However, there are no prospective studies that assess the normal change in LV outflow velocity during this intervention. Eighteen normal subjects (mean age, 34+/-5 years; 9 men and 9 women) inhaled amyl nitrite during measurement of LV outflow velocity. Peak velocity increased from 109+/-16 cm/s to 144+/-24 cm/s (P<0.001). There were no significant gender differences in velocity measurements at baseline or at peak. Our study provides prospective data that may be useful when evaluating young adults for LV outflow tract obstruction with Doppler echocardiography during amyl nitrite inhalation.

Resting echocardiographic features of latent left ventricular outflow obstruction in hypertrophic cardiomyopathy

We determined resting echocardiographic features predictive of latent left ventricular (LV) outflow obstruction in 50 consecutive patients with nonobstructive hypertrophic cardiomyopathy (26 provocable, 24 nonprovocable with amyl nitrite inhalation) to have a better understanding of the pathophysiology of this condition and to identify such patients without pharmacologic provocation. Measurements included wall thickness, type of hypertrophy, LV outflow tract diameter, degree of mitral systolic anterior motion, outflow pressure gradient, and ventricular volume. The direction of the ejection streamline was measured to assess the magnitude of the drag force acting on the mitral valve. Thirteen of 16 patients (81%) with proximal septal bulge were provocable, whereas only 3 of 8 patients (38%) with asymmetric septal hypertrophy and 10 of 26 (38%) with concentric hypertrophy were provocable (p < 0.05). LV outflow tract was significantly narrower and the angle between the ejection flow and the mitral valve was larger in provocable patients. The sensitivity for predicting provocable patients by a combination of a narrow outflow tract (< or = 2 cm) and a large angle (> or = 35 degrees) was 65%, with a specificity of 80% and a positive predictive value of 79%. When these criteria were combined with the presence of septal bulge, the sensitivity was 35%, but the specificity and the positive predictive value were both 100%. Patients with nonobstructive hypertrophic cardiomyopathy with proximal septal bulge, a narrow LV outflow tract, and an oblique angle between the ejection flow and the mitral valve appeared to be predisposed for latent outflow obstruction. These features are consistent with the presence of the large Venturi and drag forces. Thus, the left ventricle, which is capable of increasing both the Venturi and the drog forces on the basis of the morphologic change, contributes to the development of outflow obstruction with amyl nitrite inhalation.

Comparison of velocity and volumetric indexes of left ventricular filling during increased heart rate with exercise and amyl nitrite

Physiologic variables, such as heart rate, affect the noninvasive indexes of left ventricular filling, complicating the interpretation of these indexes for clinical assessment of diastolic function. We compared the effect in normal subjects of increased heart rate provoked by both exercise and amyl nitrite on noninvasive velocity and volumetric indexes of left ventricular filling. Velocity indexes were affected in a different pattern with exercise compared with amyl nitrite because peak E wave velocity and relative atrial contribution to filling increased with exercise. In contrast, the volumetric index of rapid left ventricular filling increased similarly with both mechanisms. These findings demonstrate the importance of recognizing the different effects on indexes of left ventricular filling when heart rate is increased by different methods.