Carotid sinus hypersensitivity: block of the sternocleidomastoid muscle does not affect responses to carotid sinus massage in healthy young adults

Abstract

The arterial baroreflex is crucial for short-term blood pressure control - abnormal baroreflex function predisposes to syncope and falling. Hypersensitive responses to carotid baroreflex stimulation using carotid sinus massage (CSM) are common in older adults and may be associated with syncope. The pathophysiology of this hypersensitivity is unknown, but chronic denervation of the sternocleidomastoid muscles is common in elderly patients with carotid sinus hypersensitivity (CSH), and is proposed to interfere with normal integration of afferent firing from the carotid baroreceptors with proprioceptive feedback from the sternocleidomastoids, producing large responses to CSM. We hypothesized that simulation of sternocleidomastoid "denervation" using pharmacological blockade would increase cardiovascular responses to CSM. Thirteen participants received supine and tilted CSM prior to intramuscular injections (6-8 mL distributed over four sites) of 2% lidocaine hydrochloride, and 0.9% saline (placebo) in contralateral sternocleidomastoid muscles. Muscle activation was recorded with electromyography (EMG) during maximal unilateral sternocleidomastoid contraction both pre- and postinjection. Supine and tilted CSM were repeated following injections and responses compared to preinjection. Following lidocaine injection, the muscle activation fell to 23 ± 0.04% of the preinjection value (P < 0.001), confirming neural block of the sternocleidomastoid muscles. Cardiac (RRI, RR interval), forearm vascular resistance (FVR), and systolic arterial pressure (SAP) responses to CSM did not increase after lidocaine injection in either supine or tilted positions (supine: ΔRRI -72 ± 31 ms, ΔSAP +2 ± 1 mmHg, ΔFVR +4 ± 4%; tilted: ΔRRI -20 ± 13 ms, ΔSAP +2 ± 2 mmHg, ΔFVR +2 ± 4%; all P > 0.05). Neural block of the sternocleidomastoid muscles does not increase cardiovascular responses to CSM. The pathophysiology of CSH remains unknown.

Keywords: Arterial baroreflex; carotid sinus hypersensitivity; sternocleidomastoid.

Figure 1

A representative example tracing from one individual showing cardiac, blood pressure, and forearm vascular resistance responses to carotid sinus massage. The period of massage is indicated by the grey box. Note the modest bradycardia and vasodilation induced by the massage. Responses were calculated as the appropriate minimum (Min) or maximum (Max) value (indicated) compared to the mean value for 5‐sec prior to the onset of massage (baseline). CSM, carotid sinus massage; ECG, electrocardiogram; RRI, RR interval; SAP, systolic arterial pressure; FVR, forearm vascular resistance.

Figure 2

Effect of lidocaine administration on sternocleidomastoid electromyography (EMG). (A) Drawing showing the anatomical locations of the key structures in the neck. Carotid sinus massage was performed at the location of the carotid bifurcation determined using ultrasound (white circle). Lidocaine or placebo injections (2 mL per site) were conducted on contralateral sternocleidomastoid muscles at four injection sites encompassing the full length of the muscle (yellow stars). (B) Representative EMG recorded from one participant before and after lidocaine and placebo injections. Traces show EMG recordings for the last 5 sec of a 7‐sec maximal voluntary contraction. There was a near‐abolition in EMG following lidocaine injection. EMG is expressed as percent of maximum root mean squared (RMS) during preinjection contraction. (C) Group data showing RMS EMG after injection with lidocaine and placebo (expressed as percent of maximum preinjection contractions). The horizontal dashed lines represent group means, with solid horizontal lines representing the 95% confidence interval; *Denotes statistical significance (P < 0.0001).

Figure 3

Effect of lidocaine and placebo injection on the supine cardiovascular responses to carotid sinus massage (CSM). Responses are expressed as the change in response to CSM in the postinjection condition compared to the preinjection condition for: (A) maximum RRI prolongation; (B) change in RRI; (C) change in SAP; and (D) change in FVR. Horizontal dashed lines represent group means, solid horizontal lines represent 95% confidence interval. RRI, RR interval; SAP, systolic arterial pressure; FVR, forearm vascular resistance.

Figure 4

Effect of lidocaine and placebo injection on the tilted cardiovascular responses to carotid sinus massage (CSM). Responses are expressed as the change in response to CSM in the postinjection condition compared to the preinjection condition for: (A) maximum RRI prolongation; (B) change in RRI; (C) change in SAP; and (D) change in FVR. Horizontal dashed lines represent group means, solid horizontal lines represent 95% confidence interval. RRI, RR interval; SAP, systolic arterial pressure; FVR, forearm vascular resistance.