Plasticizer Interaction With the Heart: Chemicals Used in Plastic Medical Devices Can Interfere With Cardiac Electrophysiology

Abstract

Background: Phthalates are used as plasticizers in the manufacturing of flexible, plastic medical products. Patients can be subjected to high phthalate exposure through contact with plastic medical devices. We aimed to investigate the cardiac safety and biocompatibility of mono-2-ethylhexyl phthalate (MEHP), a phthalate with documented exposure in intensive care patients.

Methods: Optical mapping of transmembrane voltage and pacing studies were performed on isolated, Langendorff-perfused rat hearts to assess cardiac electrophysiology after MEHP exposure compared with controls. MEHP dose was chosen based on reported blood concentrations after an exchange transfusion procedure.

Results: Thirty-minute exposure to MEHP increased the atrioventricular node (147 versus 107 ms) and ventricular (117 versus 77.5 ms) effective refractory periods, compared with controls. Optical mapping revealed prolonged action potential duration at slower pacing cycle lengths, akin to reverse use dependence. The plateau phase of the action potential duration restitution curve steepened and became monophasic in MEHP-exposed hearts (0.18 versus 0.06 slope). Action potential duration lengthening occurred during late-phase repolarization resulting in triangulation (70.3 versus 56.6 ms). MEHP exposure also slowed epicardial conduction velocity (35 versus 60 cm/s), which may be partly explained by inhibition of Na_v1.5 (874 and 231 µmol/L half-maximal inhibitory concentration, fast and late sodium current).

Conclusions: This study highlights the impact of acute MEHP exposure, using a clinically relevant dose, on cardiac electrophysiology in the intact heart. Heightened clinical exposure to plasticized medical products may have cardiac safety implications-given that action potential triangulation and electrical restitution modifications are a risk factor for early after depolarizations and cardiac arrhythmias.

Keywords: action potentials; electrophysiology; heart; plasticizer; plastics.

Figure 1.

Effect of mono-2-ethylhexyl phthalate (MEHP) on ECG characteristics during sinus rhythm. A, Experimental protocol timeline; all ECG recordings were performed during sinus rhythm. B, ECG signals collected from excised, intact rat hearts with PR interval denoted. C, Heart rate (HR) decreased with time of perfusion and after application of blebbistatin for mechanical uncoupling, with no difference between treatment groups. D, Similar rate slowing was observed in the RR interval, and coincided with heart rate variability changes in E SD of the normal RR intervals (SDNN) and (F) root means successive square difference (rMSSD) after 30-min perfusion. G and H, Prolonged depolarization and repolarization in MEHP-exposed hearts as measured from R to J inflection point (RJ_point) and R to T-wave (RT) (G) PR interval time was significantly lengthened in MEHP-exposed hearts compared to both baseline and control (Ctrl) hearts after the same duration of perfusion (P<0.05). n≥7 per group. BPM indicates beats per minute; and EP, electrophysiology protocol.

Figure 2.

Sinus node function and atrioventricular conduction altered after mono-2-ethylhexyl phthalate (MEHP) treatment. A, ECG trace corresponding to a 200 ms pacing cycle length (PCL) train (indicated by arrows) on the right atrium in a control (Ctrl) heart. Sinus node recovery time (SNRT) is indicated as the delay between the cessation of pacing and the resumption of sinus node activity. B, SNRT measured in MEHP-treated heart shows marked prolongation compared with Ctrl. C, Aggregate SNRT analysis for MEHP-treated hearts and time-matched Ctrls (P<0.05). D, Wenckebach cycle length (WBCL) measured as the earliest PCL in which Wenckebach phenomenon was visible (P=0.07). E, Atrioventricular nodal refractory period (AVNERP) assessed by S1-S2 pacing on the right atrium was significantly lengthened in MEHP hearts compared with Ctrls (P<0.05). n=5–8 per group.

Figure 3.

Mono-2-ethylhexyl phthalate (MEHP) causes action potential morphology changes with corresponding modification of the electrical restitution curve. A, MEHP exposure caused the restitution curve to change from a biphasic form to a monophasic form with no plateau phase. The slope of the plateau on the control (Ctrl) heart was 0.06 compared with 0.18 in the MEHP hearts. B, Despite the similarities in the action potential duration at faster cycle lengths, the ventricular effective refractory period (VERP) was markedly prolonged in the MEHP-treated group (P<0.05). C, With rapid pacing (pacing cycle length [PCL]=140 ms), the action potential duration (APD) appeared similar between the Ctrl and MEHP-treated groups: there were no difference between (D) APD30, (E) APD90, or (F) AP triangulation. G, At slower cycle lengths (PCL=240 ms), the action potential morphology was quite different: while (H) APD30 was similar, (I) lengthening was apparent in the late phase as shown by APD90 (P<0.05). J, The effect on APD90 with no change on APD30 caused an increase in the triangulation shape of the AP after MEHP treatment (P<0.05). N=7 for each group. AP Tri indicates action potential triangulation; and Vm, transmembrane voltage.

Figure 4.

Action potential alternans and arrhythmia susceptibility from mono-2-ethylhexyl phthalate (MEHP) exposure. A, MEHP-treated hearts displayed increased susceptibility to alternating action potential morphology at the same pacing cycle length (PCL) compared with baseline as predicted by the change in electrical restitution (See Figure 3A), PCL=110 ms this example. B, The longest PCL which resulted in alternating action potentials (alternans threshold) was longer in the MEHP-treated hearts (P=0.06). C, Hearts after MEHP exposure became more susceptible to spontaneous arrhythmia as shown by a higher incidence of both premature ventricular contractions (PVC) and ventricular tachycardia or ventricular fibrillation (VT/VF) following electrophysiological study (P<0.0001, χ² test). N=8 for each group. AUF indicates arbitrary units of fluorescence; and Ctrl, control.

Figure 5.

Activation maps and measured conduction velocity (CV) after mono-2-ethylhexyl phthalate (MEHP) exposure. A, Each pixel on the heart was assigned an activation time based on the maximum upstroke velocity in the fluorescent signal to create an isochrone map. Wavefronts are visible emanating from the center of the left ventricular epicardium where the cathodal pacing electrode was placed. B, After 30-min control (Ctrl)-media perfusion, the activation maps did not show significant differences. C, In Ctrl hearts, analysis of the pacing site (R1) and a distal site (R2) did not show significant differences in conduction time when compared between baseline and 30-min perfusion. D, Pre-MEHP baseline showed similar CV as the Ctrl baseline. E, After 30-min of MEHP exposure, the activation maps changed in morphology. F, Analysis of the pacing site (R1) and distal site (R2) indicated a decreased upstroke velocity after 30-min of MEHP exposure. G, CV was significantly slower in MEHP-treated hearts compared with Ctrls after 30 min at both pacing cycle length (PCL)=240 ms (P<0.05) and (H) PCL=140 ms (P<0.05), with N=5 per group. I, Sodium current (I_Na) was measured with HEK293 (human embryonic kidney 293) cells which stably express Na_v1.5. Fast sodium current (I_Na) was measured in the absence of anemone toxin, and half-maximal inhibition concentration (IC50) was determined at 874 µmol/L. Late sodium current (I_NaL) was activated with anemone toxin, and corresponding IC50 was determined at 231 µmol/L (N=5 for each group).