Heart Rate Variability and Hemodynamic Alterations in Canines with Normal Cardiac Function during Exposure to Pressure Support, Continuous Positive Airway Pressure, and a Combination of Pressure Support and Continuous Positive Airway Pressure

Abstract

Variations in intrathoracic pressure generated by different ventilator weaning modes may significantly affect intrathoracic hemodynamics and cardiovascular stability. Although several investigators have attributed cardiovascular alterations during ventilator weaning to augmented sympathetic tone, there is limited investigation of changes in autonomic tone during ventilator weaning. Heart rate variability (HRV), the analysis of beat-to-beat changes in heart rate, is a noninvasive indicator of autonomic tone that might be useful in the identification of patients who are at risk for weaning difficulty due to underlying cardiac dysfunction. The authors describe HRV and hemodynamics in response to 3 ventilatory conditions: pressure support (PS) 10 cmH2O, continuous positive airway pressure (CPAP) 10 cmH2O, and a combination of PS 10 cmH2O and CPAP 10 cmH2O (PS + CPAP) in a group of canines with normal ventricular function. Six canines were studied in the laboratory. Continuous 3-lead electrocardiographic data were collected during baseline (controlled mechanical ventilation) and following transition to each of the ventilatory conditions (PS, CPAP, PS + CPAP) for analysis of HRV. HRV was evaluated using power spectral analysis to define the power under the curve in a very low frequency range (0.0033 to < 0.04 Hz, sympathetic tone), a low frequency range (0.04 to < 0.15 Hz, primarily sympathetic tone), and a high frequency range (0.15 to < 0.40 Hz, parasympathetic tone). A thermodilution pulmonary artery catheter measured cardiac output and right ventricular end-diastolic volume to describe global hemodynamics. There were significant increases in very low frequency power (sympathetic tone) with a concomitant significant reduction in high-frequency power (parasympathetic tone) with exposure to PS + CPAP. These alterations in HRV were associated with significantly increased heart rate and reduced right ventricular end-diastolic volume. Although there was a small but significant increase in cardiac output with exposure to PS, HRV was unchanged. These data indicate that there was a relative shift in autonomic balance to increased sympathetic and decreased parasympathetic tone with exposure to PS + CPAP. The increase in intrathoracic pressure reduced right ventricular end-diastolic volume (preload). This hemodynamic alteration generated a change in autonomic tone, so that cardiac output could be maintained. Individuals with autonomic and/or cardiovascular dysfunction may not be capable of this type of response and may fail to successfully wean from mechanical ventilation.

Heart Rate Variability as a Predictor of Cardiac Dysrhythmias During Weaning From Mechanical Ventilation

“…11,14 Using frequency domain measures of HRV in a group of 6 healthy anesthetized canines with normal ventricular function, Frazier et al 11 found a significant increase in activity of the sympathetic nervous system and a significant decrease in activity of the parasympathetic nervous system with exposure to a combination of pressure support 10 cm H 2 O and continuous positive airway pressure 10 cm H 2 O. In our study, almost half of our patients (49%) had a history of cardiac disease.…”

“…during weaning, changes in the ANs with a shift to sympathovagal imbalance (ie, increased sympathetic and/or reduced vagal tone) 11,14 may occur in patients who have underlying cardiovascular dysfunction and impaired compensatory mechanisms, resulting in decreased HRV. [11][12][13][14] sympathetic dominance is associated with decreased duration of myocardial action potential, and increased myocardial excitability, which may enhance abnormal automaticity, triggered activity, and reentry and produce cardiac dysrhythmias.…”

“…during weaning, changes in the ANs with a shift to sympathovagal imbalance (ie, increased sympathetic and/or reduced vagal tone) 11,14 may occur in patients who have underlying cardiovascular dysfunction and impaired compensatory mechanisms, resulting in decreased HRV. [11][12][13][14] sympathetic dominance is associated with decreased duration of myocardial action potential, and increased myocardial excitability, which may enhance abnormal automaticity, triggered activity, and reentry and produce cardiac dysrhythmias. [15][16][17] Numerous experimental and clinical studies have revealed a relationship between reduced HRV and the genesis of malignant ventricular dysrhythmias, paroxysmal atrial fibrillation, and atrial flutter, especially in patients with heart failure, myocardial ischemia, or myocardial infarction.…”

“…[11][12][13] In response, the autonomic nervous system (ANs) is activated and induces alterations in ventricular afterload, contractility, and heart rate to maintain cardiac output and tissue perfusion. [11][12][13] The balance between the 2 components of autonomic tone, sympathetic and parasympathetic innervation, can be evaluated by measuring heart rate variability (HRV).…”

Predicted nine-year risk of diabetes among professional divers: a prospective study

“…Not only do deep-sea divers often have difficult tasks to perform, but their body also has to deal with considerable stress caused by this pressurised environment. We can observe this stress, in particular, on the cardiovascular system [1][2][3]: accelerating heart beat due to hypercapnia, impact of pressure variations in the airways on chest venous return, changing myocardial contractile function observed in animal experiments etc.…”