Clinical significance of unexplained persistent sinus tachycardia in women with structurally normal heart during the peripartum period

Cardiovascular complications are the leading non-obstetric cause of maternal morbidity and mortality during pregnancy [1]. Adverse maternal outcomes have been well described in women with structural heart disease and have been attributed to the pre-existing cardiac conditions, as well as advanced maternal age [2‐5]. Physiologic changes during pregnancy predispose women to arrhythmias, notably supraventricular tachycardia, atrial fibrillation, and ventricular arrhythmias and women with arrhythmias during pregnancy have greater incidence of in-hospital death as well as maternal or fetal complications [6] especially for those women with underlying heart disease [7‐9].

Sinus tachycardia (ST) is often encountered in women in pregnancy and particularly in labor. Although ST is often secondary to hypovolemia, hypoxemia, anemia or infection; it commonly occurs without an identifiable pathologic condition. Unexplained ST is generally believed to be a benign finding, representing physiologic response to neurohormonal changes, volume distribution, anxiety and pain. As such, reassurance and conservative management is the recommended management for unexplained ST [10]. Nevertheless, the association between unexplained ST with obstetric outcomes has not been examined. Through our analysis, we investigate the prevalence and etiology of persistent ST in a large cohort of women with structurally normal heart admitted in labor and explore association with adverse maternal outcomes.

With this analysis we found persistent sinus tachycardia during the peripartum period was significantly associated with increased length of stay and c-section. The etiology of persistent ST was attributable to infection or hypovolemia due to blood loss in the minority of cases (29.5%) while the etiology was unclear for the remainder of the cohort (70.5%). Both categories were found to be associated with increased rates of c-section, while only those with infection or hypovolemia had lengthier hospital stays. The focus of our analysis was on the primary outcome of c-section, it is not surprising those who had a c-section also had secondary outcomes of hypovolemia or infection and therefore a prolonged length of stay. It is important to identify occult reasons for sinus tachycardia such as hypovolemia or infection, as was the case in 29.5% of our cohort. However, these findings suggest there is poor understanding of the etiology of sinus tachycardia, and its clinical significance during the peripartum period may be often underestimated.

During pregnancy, systemic and pulmonary vascular resistance are decreased through the release of progesterone and estrogen to allow for increased maternal cardiac output. Stroke volume increases during the first semester through increased cardiac preload facilitated by the renin–angiotensin–aldosterone system [11]. Increased heart rate mediated by the parasympathetic and sympathetic nervous system becomes the compensatory mechanism during the second and third trimesters for increased cardiac output [12]. In addition to plasma volume expansion, there is an increase in red blood cell production up to 40% via erythropoiesis. Plasma volume increases proportionally more than the red blood cell mass, resulting in a “physiological anemia” from hemodilution, with hemoglobin levels as low as 11 g/dL considered physiological [11]. Distinguishing between physiologic or pathologic sinus tachycardia due to these inherent alterations in the cardiovascular system of pregnant women can be challenging [13]. Fluctuations in normal physiological parameters makes determining a heart rate threshold to diagnose persistent sinus tachycardia difficult for even experienced clinicians [14]. Clinicians may be unaware that epidemiological studies have demonstrated that the expected median heart rate during pregnancy is 91 bpm (3^rd centile 68 to 97^th centile 115) at 34.1 weeks. Heart rates over 105 bpm were only seen in 10% of the 1041 women included in the study after 28 weeks. The American College of Obstetricians and Gynecologists recommends investigating heart rates over 115 bpm or over 110 bpm in conjunction with other findings or symptoms [15]. These heart rates are known to vary across diverse populations. Additionally, Savu’s et al. analysis of the morphological and functional adaptation of the maternal heart during pregnancy found maximum heart rate rarely exceed 100bpm [16].

Sinus tachycardia is generally considered a benign arrhythmia, subsequently management recommendations are absent from newly published guidelines [17]. Therefore, the question remains: if persistent sinus tachycardia is considered a benign occurrence, with the only intervention to the patient being reassurance [18], why do women with persistent ST of unclear etiology experience higher rates of c-sections Answering this question may be of vital significance as it may shorten then LOS but also prevent unnecessary C-sections which can be associated with short- and long-term effects for both mother and child [19].

In almost 70% of the patients in our cohort, the etiology of persistent ST could not be elucidated, and could be deemed as either inappropriate tachycardia or sinus tachycardia of unknown etiology. Inappropriate sinus tachycardia, defined by the 2015 HRS consensus as “sinus heart rate more than 100 bpm at rest with mean 24‐hour heart rate more than 90 bpm, in absence of secondary causes”, has been shown to predominately affect young females [20]. Shabtae et al. found pregnancy to be the most common precipitating factor for inappropriate ST [21]. Dysregulation of the compensatory mechanisms to increase cardiac output, namely sympathetic and renin–angiotensin–aldosterone system activation create autonomic dysfunction, is a known etiology for inappropriate ST [22, 23]. Only small case series in the literature report women who experience inappropriate ST during the peripartum may be at risk for adverse maternal outcomes. Sharp et al., found an increased rate of induction of labor among the 19 patients they identified, although they did not find elevated rates of c-section in this study conducted in the England [24, 25].

As shown in our study, sinus tachycardia cannot be explained solely by indolent infection or hypovolemia. Autonomic dysfunction, hormonal fluctuations, catecholamine surge, and medications have been proposed as explanations for sinus tachycardia of unknown etiology [26, 27]. Although clinicians must balance between extensive diagnostic investigation to determine the etiology of inappropriate ST and reassurance when it is a benign incident, reflecting physiologic changes, distinguishing between the two may have an impact on preventing adverse maternal outcomes. Further evidence is needed to understand the clinical significance and to deliver evidence-based care.

Through this analysis we emphasize the importance of not dismissing ST in the obstetric population and seek to raise awareness of its significance in predicating complications and poor maternal outcomes. ST is one of the earliest and most insidious signs of a poor prognosis, however it is frequently considered “physiologic”, even in the absence of a clear etiology. ST could be a surrogate of underlying pathology leading to poor outcomes and should prompt further investigation in obstetric patients.

Occult reasons for sinus tachycardia such as hypovolemia or infection were identified in approximately a third of our cohort. Among pregnant patients, patients with unexplained ST have higher rates of C-Sections. Although potential explanations for this trend include catecholamine surge, indolent infection, hormonal fluctuations, and medications, further studies are needed to determine the clinical significance of ST and help physicians distinguish between an innocent physiologic response and a potential threat for the mother and the fetus.