M. de Burgh Daly

ROLE OF CAROTID-BODY CHEMORECEPTORS AND THEIR REFLEX INTERACTIONS IN BRADYCARDIA AND CARDIAC ARREST

Stimulation of the carotid-body chemo-receptors by asphyxia during an apnoeic episode may contribute to the vagally mediated cardiac arrest and sudden death that sometimes occurs in man. Apnoeic asphyxia may be induced centrally or reflexly by stimulation of upper airways receptors. Conditions associated with apnoeic asphyxia and in which the risk is likely to be greatest include intubation, laryngoscopy and bronchoscopy; accidents involving underwater swimming; inhalation of sympathomimetic amines in aerosols by asthmatic patients; and chronic hypoventilation syndromes. These reflexes may be responsible for some victims of sudden infant death syndrome. Stimulation of the carotid bodies normally produces hyperventilation and bradycardia. When apnoea occurs centrally or reflexly, carotid chemoreceptor excitation resulting from asphyxia now causes a much enhanced bradycardia and even cardiac arrest, but paradoxically does not usually affect respiration. These reflexes and their interactions normally serve protective and purposeful functions, but may under certain circumstances become exaggerated and put the patients life at risk.

Some Aspects of Upper Respiratory Tract Reflexes

Respiratory and cardiovascular reflexes have been elicited from receptors in the nose and larynx in the anaesthetized dog. Cigarette smoke in the nose causes reflex apnoea, bradycardia and vasoconstriction, probably due to systemic absorption through the nose. Stimulation of laryngeal nerve also results in reflex apnoea, bradycardia, and limb vasoconstriction. When asphyxia supervenes due to apnoea, stimulation of the carotid body chemoreceptors occurs which normally cause, as primary effects, hyperpnoea and bradycardia. However, it has been shown that stimulation of the laryngeal receptors inhibits the carotid body respiratory reflex and facilitates the carotid body cardio-inhibitory reflex, the latter leading to temporary cardiac arrest. The clinical implications of this finding are discussed.

The effects of artificial lung inflation on reflexly induced bradycardia associated with apnoea in the dog

1. The cardiac effects of artificial inflation of the lungs were studied during reflexly induced apnoea and bradycardia in anaesthetized dogs.

Comparison of the Size of the Vascular Compartment of the Carotid Body of the Fetal, Neonatal and Adult Cat

The carotid bodies from full-term fetal cats, 3- to 4-day-old neonates and adult cats were perfusion-fixed at normal arterial blood pressure with 3% phosphate-buffered glutaraldehyde. Serial 5-microns sections were cut and stained by the MSB method. Using an interactive image analysis system, determinations were made of the volumes of the carotid body and of its vascular and extravascular compartments. Compared to the fetus, the carotid body of the neonate increased in volume by 51% and by 286% in the adult cat. There was a proportional increase in the volumes of the vascular compartment and of the small vessels (5-12 microns diameter) in that compartment. The volume of the small vessels, expressed as a ratio of the total volume of the organ, remained constant in the three animal groups at 5-7%. The small vessel endothelial surface area, expressed as a ratio of the extravascular volume (which was assumed to consist largely of type 1 and type 2 cells), was the same in the neonate as in the full-term fetus. Thus, there were no apparent quantifiable morphological features of the carotid body and its vasculature which would account for the resetting of the hypoxic sensitivity of the organ from the fetal to the adult range within a few days of birth.

A comparative study of the distribution of carotid body type-I cells and periadventitial type-I cells in the carotid bifurcation regions of the rabbit, rat, guinea-pig and mouse

SummaryThe bilateral distribution of carotid body type-I cells was investigated in five rabbits, rats, guinea-pigs and mice by serially sectioning the carotid bifurcation regions. Carotid body type-I cells occurred bilaterally in close proximity to the wall of the internal carotid artery in the rabbit, rat and mouse and to the wall of the ascending pharyngeal artery in the guinea-pig. The rat carotid body was sometimes recessed into the lateral aspect of the superior cervical ganglion and was the most easily defined organ in the four animals studied. Caudally, and separate from the principal mass of carotid body type I cells, isolated groups of periadventitial type-I cells were observed in the connective tissues around the internal carotid artery and adjacent to the carotid bifurcation and common carotid artery in the rabbits only. An overall picture of the carotid body in the four animals was constructed. In all specimens rostral-caudal dimensions were recorded and compared bilaterally.

The volume of the carotid body and periadventitial type I and type II cells in the carotid bifurcation region of the fetal cat and kitten

SummaryThe bilateral distribution of carotid body type I cells was investigated in 6 fetuses (gestational age 95%) and 9 newborn kittens (aged 1 day to 4 days) by serially sectioning the carotid bifurcation regions. In most specimens type I cells occurred in close proximity to the wall of the occipital artery or one of its small proximal branches within a division of connective tissue with defineable but irregular borders. This combination of type I cells and connective tissue constituted the principal mass of the carotid body. Using an interacting image analysis system, the area of the carotid body in each serial section was measured by accurately contouring its perimeter. The volume of the carotid body was calculated by multiplying the sum of the areas of the serial sections by the thickness of the section. The volume of the carotid body was 0.052±0.018 mm3 in the fetuses and 0.025–0.117 mm3 in the 1–4 day old kittens. A degree of symmetry in the values for the volume of the right and left carotid body was found. Caudally, and separate from the principal mass of carotid body type I cells, isolated groups of periadventitial type I cells were noted in the connective tissues around the occipito-ascending pharyngeal trunk, origin of the occipital artery and rostral end of the common carotid artery in 7 out of 12 specimens from fetal cats and 11 out of 18 specimens in newborn kittens. The volumes of the periadventitial groups of cells ranged between 25–1,365 μm3 in fetuses and 10–1,351 μm3 in kittens.

Distribution of Carotid Body Type I Cells and Periadventitial Type I Cells in the Carotid Bifurcation Regions of the Dog

SummaryThe distribution of carotid body type I and periadventitial type I cells in the carotid bifurcation regions was investigated unilaterally in seven and bilaterally in two New Zealand White rabbits. Carotid body type I cells occurred in close proximity to the wall of the internal carotid artery immediately rostral to the carotid bifurcation, within a division of connective tissue with defineable but irregular borders. Caudally, and separate from the main mass of carotid body type I cells, isolated groups of periadventitial type I cells lay freely in the connective tissue around the internal carotid artery and alongside the carotid bifurcation and common carotid artery. A overall picture of the carotid body in the rabbit was reconstructed and the occurrence and significance of periadventitial type I cells discussed.

Dimensions and volume of the carotid body in the adult cat, and their relation to the specific blood flow through the organ: a histological and morphometric study

In perfusion-fixed preparations of the carotid body, morphological measurements were made on serial histological sections using an interactive image analysis system. The volume of the organ was found to be 0.247 +/- 0.092 mm3. This is considerably smaller than the previous estimates based on the measured postmortem wet weight. This means that the specific blood flow to the carotid body of 2,000 ml/min/100 g using the previously obtained values for the organs total blood flow and postmortem wet weight may be an underestimate of the true value.

The carotid body of the harbour seal (Phoca vitulina richardsi)

SummaryThe bilateral distribution of carotid body type 1 and 11 cells was investigated in five harbour seals (Phoca vitulina richardsi), by serially sectioning the carotid bifurcation regions. The cells occurred bilaterally in the animals and were also present in one specimen from a sixth animal available for study. The type 1 and 11 cells were located in the space between the internal and external carotid arteries and had a varied relationship to the occipital and condyloid arteries. They lay within a division of connective tissue with irregular but defineable borders and this combination of connective tissue and type 1 and 11 cells constituted the principal mass of the carotid body. The carotid body occurred in a variety of forms: wedge-shaped, crescentic or horse-shoe shaped, or as a discrete oval structure. In some specimens the carotid body had a central ‘neurovascular’ core of small blood vesels and nerves. The artery to the organ originated from either the external carotid, internal carotid or common carotid arteries. Using an interactive image analysis system in eight specimens, which had been perfusion-fixed at a normal arterial pressure, the mean volume of the carotid body was 1.666±0.45 (SD) mm3. Caudally and separate from the principal mass of the carotid body periadyentitial type 1 and 11 cells were noted in 4 out of 11 specimens in the connective tissues adjacent to the external carotid artery, origin of the occipital, and the rostral part of the common carotid artery and its bifurcation.

DEFENSIVE REFLEXES FROM THE NOSE, INCLUDING THE DIVING RESPONSE

Publisher Summary This chapter focuses on defensive reflexes from the nose, including the diving response. The term diving response is used to describe the triad of effects that occurs while holding the breath during immersion in water. It is preferable to the term diving reflex because this implies that the reflex effects are because of a single reflex, but they are in fact the result of a number of reflexes occurring simultaneously together with various interactions between these reflexes, which also involve mechanisms because of changes in respiration. The direction and the magnitude of the responses occurring during diving or through elective stimulation of the nasal mucosa depend to a large extent on the accompanying change in breathing and on the degree of involvement of other receptors. This is the result of the interactions of a number of autonomic responses through stimulation of facial, nasal or laryngeal receptors, carotid body chemoreceptors, and arterial baroreceptors in which mechanisms initiated by the concomitant changes in breathing play a central role.