What is the ratio of respirations to pulse beats?
Front Physiol.
2019; 10: 371. A specific and unique aspect of cardiorespiratory activity can be captured by dividing the heart rate (HR) by the respiration rate (RR), giving the pulse-respiration quotient (PRQ = HR/RR). In this review article, we summarize the main findings of studies using and investigating the PRQ. We describe why the PRQ is a powerful parameter that captures complex
regulatory states of the cardiorespiratory system, and we highlight the need to re-introduce the use of this parameter into modern studies about human physiology and pathophysiology. In particular, we show that the PRQ (i) changes during human development, (ii) is time-dependent (ultradian, circadian, and infradian rhythms), (iii) shows specific patterns during sleep, (iv) changes with physical activity and body posture, (v) is linked with psychophysical and cognitive activity, (vi) is
sex-dependent, and (vii) is determined by the individual physiological constitution. Furthermore, we discuss the medical aspects of the PRQ in terms of applications for disease classification and monitoring. Finally, we explain why there should be a revival in the use of the PRQ for basic research about human physiology and for applications in medicine, and we give recommendations for the use of the PRQ in studies and medical applications. Keywords:
pulse-respiration quotient, PRQ, cardio-respiratory coupling, integrative human physiology, chronobiology Cardiac activity and respiration can been seen as constituting a complex non-linear dynamical system based on two weakly coupled oscillators (heart beating and breathing movement) coupled by several structural and functional types of cardiorespiratory interaction (CRI) These CRI types
involve complex interplay between the activity of the brainstem (with the autonomic nervous system and the central respiratory drive as main elements), the heart and lungs in the thoracic cavity, and the vascular system (Valenza et al., 2016; Benarroch,
2018; Elstad et al., 2018). The CRI gives rise to several types of emergent cardiorespiratory coupling (CRC) phenomena characterized by specific relationships between cardiac activity and respiration with respect to frequency, phase, and time differences between the heart beats and the respiration events
(Lotrič and Stefanovska, 2000; Moser et al., 2008; Schulz et al., 2013;
Dick et al., 2014; Krause et al., 2017; Elstad et al., 2018). One specific type of CRC is the
relationship between the heart rate (HR), colloquially the “pulse,” and the respiration rate (RR). This relationship can be analyzed in the framework of the cardiorespiratory synchrogram (i.e., analyzing the relative phases of the heart rate and respiration rate as a function of time (Schäfer et al., 1998) or simply as the ratio of the HR to the RR, termed the
pulse-respiration quotient (PRQ = HR/RR).1 The aim of this review article is to summarize the main findings and studies using and investigating the PRQ. In addition, we describe why the PRQ is a powerful parameter that captures complex regulatory states of the cardiorespiratory system, and we highlight the need to re-introduce the use of
this parameter into modern studies about human physiology and pathophysiology. The PRQ was used in many studies in the second half of the last century, mainly in papers published in German scientific journals. This explains why the many findings about the PRQ are currently not appreciated by the non-German-speaking scientific community. With this review article, we want to make the knowledge also available to the English-speaking scientific community, and draw a special
attention to the fact that the PRQ has great potential for use in future studies to give additional and novel insights into the functioning of the human body. The PRQ can be used as the ratio of the instantaneous HR to the instantaneous RR, or as the ratio of the HR to the RR measured in a defined time
interval, e.g., over 1 min. For a clear discussion of the published findings regarding the PRQ and its relationship to human physiology and pathophysiology, the following new terminology is introduced that distinguishes three basic types of PRQ parameters: rs-PRQ: resting-state PRQ, i.e., the PRQ measured during a resting state of the body. Normally, a specific protocol is used for standardized testing, e.g., the PRQ measured in the supine body
posture of the first minute after 10 min of resting to establish a baseline state. If a person is sleeping (during the day or night), then the PRQ is automatically a rs-PRQ. We regard the supine body posture as strictly required in order to determine the rs-PRQ. It is well known that the body posture has a significant impact on the cardiorespiratory state (Iida
et al., 1999; Gordon et al., 2011; Silvani et al., 2017). f-PRQ: functional PRQ, i.e., the PRQ measured during a functional task that the subject has to perform or a specific stimulation the subject is faced
with. This enables the regulatory capacity of human physiology to be determined. u-PRQ: unrestricted PRQ, i.e., the PRQ measured during normal daily activities without changing the cardiovascular system especially for the PRQ measurements. This type of PRQ is normally measured when performing 24 h PRQ measurements. Furthermore, it is necessary to distinguish when the PRQ was determined, during night or day. This can
be indicated by adding a “D” (for day, i.e., the time between sunrise and sunset) or an “N” (for night, i.e., the time between sunset and sunrise) in front of the term, leading to 2 × 3 PRQ sub-classes (for a visualization, see Figure 4G), i.e., D-rs-PRQ, D-u-PRQ, D-f-PRQ, N-rs-PRQ, N-u-PRQ, and N-f-PRQ.
Sub-classes of PRQ parameters can be also formed by employing the state (rs-, u-, f-) or the time (N-, D-) indices only, i.e., rs-PRQ or N-PRQ. (A) Dependence of nightly resting-state PRQ (N-rs-PRQ) values as a function of time after a cardiac attack (n = 6–17, dependent on data point, mean ± standard error of the mean; data extracted from Kümmell and Heckmann, 1987). The PRQ normalization toward 4:1 is evident during the recovery phase after the incident. (B) The effect of a health intervention [12 spa (balneology) treatments] on the circadian PRQ oscillation amplitude is shown exemplarily for one subject. A less pronounced morning and evening PRQ peak is visible, indicating a normalization of the physiological state. Mean and standard error of the mean values are shown based on averages over 3 days. It was not possible to infer from the publication whether the PRQ values represent rs-PRQ or u-PRQ. Data extracted from Hildebrandt (1955). (C) N-rs-PRQ values for one subject (female) with anorexia nervosa that obtained two treatment periods (red and blue curves) over several weeks. Data extracted from Heckmann et al. (1990). The PRQ values represent monthly daily resting-state PRQ (D-rs-PRQ) values. A clear trend of PRQ normalization toward 4:1 is visible during the course of the treatments. (D) Changes of functional PRQ (f-PRQ) values during an experiment with three types of speaking tasks: 1: baseline; 2: speaking, style 1: recitation; 3: speaking, style 2: declamation; 4: speaking, type 3: conversation; 5-7: recovery after 2; recovery after 3; recovery after 4. Tasks 1 and 4–6 were performed while sitting, tasks 2–4 while standing. The changes of f-PRQ are therefore due to speaking and changes in the body position in parallel. Data extracted from Von Bonin et al. (2001). (E) Effects of choir singing on the f-PRQ (Marti, 2012). (F) PRQ differences between two groups of breast cancer patients: with (M, n = 11) and without (n = 26) metastasis. The PRQ values there represent mean unrestricted PRQ (u-PRQ) ones calculated for the night (1:00–5:00 am, N-u-PRQ) and the day (5:00–1:00 am, D-u-PRQ). Data extracted from Bettermann et al. (2001). (G) Schematic representation of the PRQ terminology proposed and employed in this manuscript. Distinguishing different types of PRQ parameters is important since the PRQ behavior is substantially determined also by the time of measurement. In many published papers, it is difficult to infer the PRQ subtype since only the umbrella term “PRQ” is used. In the following, the umbrella term “PRQ” is only used when referring generally to the PRQ. Historical Development, Measurement Methods, Basic Characteristics, and Link to Cardiorespiratory InteractionsThe fact that the HR of humans is approximately four times faster than the RR has been known for millennia, but it was the Austrian scientist Rudolf Steiner, who emphasized the great potential of the PRQ for our understanding of human physiology. This is especially true with respect to the significance of the PRQ value of 4 (i.e., a 4:1 ratio of the cardiac activity to respiration) (Steiner, 1989). Since then, the PRQ has been mainly investigated in physiological or medical studies by German research groups (e.g., Hildebrandt, 1955, 1960; Weckenmann, 1975; Weckenmann et al., 1988; Moser et al., 2008; Von Bonin et al., 2014). Capturing several aspects of the CRC at once, the PRQ contains information about several specific forms of CRC, i.e., cardiorespiratory frequency locking (constant PRQ over a specific time interval) or cardioventilatory coupling (CVC) (causing the occurrence of specific PRQ values). The CRC is the result of bidirectional CRI effects that adjust the cardiac and respiratory activities due to the current physiological state and needs. Both the cardiac and respiratory activities can be regarded as linear and non-linear oscillators with intrinsic (autonomous) oscillations of specific frequencies that are constantly adjusted, resulting in the actual (measurable) frequencies. Linked to this phenomenon is the fact that the intrinsic HR is larger than the actual one (Katona and Jih, 1975), and the intrinsic RR is smaller (Karczewski and Widdicombe, 1969). The complex CRIs result in several observable phenomena (see Figure 1), e.g., the skewed distributions of HR and RR values at rest (due to the interplay between the intrinsic and actual frequencies), a lognormal distribution of the PRQ (with a higher likelihood of smaller values than larger ones), and a quantization of the PRQ values with preferred values of the harmonic ratios n:m with n = 3–6 and m = 1. The rs-PRQ values normally show distribution peaks at 3 and 4, corresponding to 3:1 and 4:1 HR and RR ratio values. The cardiorespiratory state where the PRQ exhibits a value around 4 was termed “PRQ normalization” (i.e., a state of an “optimal” PRQ with respect to the functioning of the cardiovascular system) to highlight the significance of this state for human physiology (Hildebrandt, 1960). (A) Illustration of the principle of cardiorespiratory coupling (CRC) and exemplary time series of heart rate (HR), respiration rate (RR), and the pulse-respiration quotient (PRQ) (data extracted from Lotrič and Stefanovska, 2000). On the right side of each time series, the histograms are shown exhibiting clearly the phenomenon of the complex interplay between the intrinsic oscillations and coupling of the oscillators causing a deviation of the intrinsic frequencies as evident by the skewed distributions of HR and RR. The directions of deviations are marked with arrows. (B–D) The relationship of the PRQ with HR and RR [simulation; normal ranges of human HR and RR values are used (from resting state to intense physical activity), i.e., HR = 45–170 1/min, RR = 6–31 1/min]. (D) PRQ-HR-RR space where the extreme PRQ values (PRQ > 10 and < 2) are mainly due to pathophysiological states. (E) The quantization phenomenon of the PRQ (n:m ratio with n = 3–6 and m = 1 is shown as often observed in human PRQ studies) (data extracted from Moser et al., 2008). The PRQ can be regarded as a parameter incorporating the information about the HR and RR, while both determine the PRQ differentially (the HR in a linear way and the RR in a non-linear way). For a visualization of this aspect, see Figures 1B–D. The PRQ thus represents an emergent behavior that cannot be obtained when analyzing the HR and RR separately (as normally done in studies about human physiology and pathophysiology). As a further indication of the significance of the PRQ, it can be noted that almost all physiological variables of biological organisms (mammals) are body mass-dependent, i.e., they follow allometric scaling laws (Schmidt-Nielsen, 1984; West et al., 1999), but there are a few exceptions and the PRQ is one of them. The PRQ is independent of the mass of the organism and approximately 4.5 for all mammals (Stahl, 1967; Schmidt-Nielsen, 1984) – an interesting fact that should be subject to further investigations. As a parameter that is sensitive to the current state of the human cardiorespiratory system (CRS), as well as linked to the cardiovascular system (CVS) and the autonomic nervous system (ANS), the PRQ is influenced by internal and external factors. Eight main factors have been identified: age, sex, chronobiology, body posture, behavior, physiological constitution, environmental influences, and the health/disease state. The PRQ is related to the CRS and CVS in parallel since both the HR and the RR are a result of the structural (anatomical) and functional (physiological) state of cardiac activity and respiration, which in turn are determined and modulated by the current state of the vascular system (Elstad et al., 2018). Since the state of the CRS and CVS is influenced by various other systems’ states (e.g., immune system, endocrine system, and nervous system) and the state of the structural and functional relationships within and between these systems (as described, e.g., by chronobiology and systems medicine), the PRQ is naturally a useful parameter to capture the overall current state of human physiology in general. Measurement of the PRQThere are different ways to measure the PRQ, depending on how the HR and RR are determined (see Figure 2A). One way is to determine the HR and RR using only one single measurement method, i.e., electrocardiography (ECG) or photoplethysmography (PPT). In this case, the HR (or the pulse rate, PR, in case of PPT) can be extracted as well as the RR. There are several algorithms available to determine the RR based on ECG or PPT data (e.g., Schäfer and Kratky, 2008; Nilsson, 2013; Charlton et al., 2018). The RR can also be measured directly using a thermistor, a respiration belt, an audio analysis, thermal imaging or by analyzing the end-tidal CO2 (PETCO2) waveform obtained by capnography (Al-Khalidi et al., 2011). In several of the published PRQ studies (especially the older ones), the PRQ was simply calculated by manually determining the PR and HR (by palpation of the pulse and visual observation of the respiration-related movements of the chest). Since each HR and RR determination method has its own unique accuracy and precision, the resulting PRQ is always to some degree dependent on the methods applied. Studies performing a comparison of different methods to determine the PRQ have not been performed so far, but it has already been noted that the way the PRQ is determined can have a significant impact on the results (e.g., the calculated numerical PRQ value depends on the method used) (Heckmann, 2001). It is therefore recommended to use the most direct and up-to-date approach for measuring the HR and RR and to report precisely how the PRQ was determined when publishing a study involving the PRQ. It is expected that the precision and accuracy are not really an issue when working with averaged PRQ values over relatively long-time intervals (minutes or hours), but a precise PRQ is necessary when an accurate time resolution of the PRQ changes is relevant (often the case when determining f-PRQ values). (A) Visualization of the main methods of measuring the pulse-respiration quotient (PRQ). (B) PRQ during human development [n = 1,820, age range: 5.5–15 years, daily resting-state PRQ (D-rs-PRQ) measurements] as a function of age, sex, and body position (data extracted from Matthiolius and Hildebrandt, 1995). CV: coefficient of variation, error bars: standard error of the mean. (C) Typical changes of heart rate (HR), respiration rate (RR), and PRQ during sleeping (data extracted from Pöllmann and Hildebrandt, 1970; n = 7). The PRQ normalization tendency during sleep (especially during the last hours before awakening) is clearly visible. Error bars: standard error of the mean. PRQ in the Context of Human Physiological Development, Chronobiology, and SomnologyThe PRQ Changes During Human DevelopmentAll three basic types of PRQs (rs-PRQ, u-PRQ, and f-PRQ) depend on the age of the subject. The following studies investigated this topic:
The PRQ is Time-Dependent: Ultradian, Circadian, and Infradian RhythmsThe PRQ of a human depends on the time when it is measured due to the chronobiological variability of human physiology. The circadian oscillation of the PRQ has been reported and investigated in several studies (Hildebrandt, 1954, 1955, 1976; Engel et al., 1969; Breithaupt et al., 1980; Heckmann, 2001). The circadian oscillation found and investigated was nycthemeral, i.e., related to the day-night cycle. The main findings about the circadian oscillation can be summarized as follows:
Regarding infradian (longer than a day) oscillations of the PRQ, we were unable to find any systematic investigation. However, a PRQ oscillation with a period of approximately a week can be inferred from a figure published by Hildebrandt (Hildebrandt, 1980) based on the data of Fechner (1980), and in one study, long-term changes of the PRQ in humans were documented along with correlated changes in atmospheric air pressure (Hildebrandt, 1955). Ultradian PRQ oscillations (shorter than a day) are also a feature of PRQ variability. During sleep, ultradian PRQ changes can be observed (for a discussion, see section “The PRQ Shows Specific Patterns During Sleep”). During the day, ultradian oscillatory features of the circadian PRQ variability can be seen in the figures of a few studies (e.g., Figures 3A–F, 4B), but no specific investigation of them has been performed to data to the best of our knowledge. The PRQ Shows Specific Patterns During SleepThe PRQ change during sleep has been investigated by several studies, yielding these main findings:
The PRQ Changes With Behavior and Body PostureThat the PRQ changes with behavior (e.g., physical activity) and body posture was noticed and studied already in the first phase of the PRQ research history. Several studies investigated how these two factors determine the PRQ. Concerning the impact of the physical activity on the PRQ, the following main findings were obtained:
Another factor determining PRQ values is the body posture. It is not only related to behavior and physical activity but also independent of it. While physical activity normally involves changes in the body posture, a specific body posture does not automatically involve physical activity. The following insights about the relationship between the PRQ and the body posture have been gathered so far:
The PRQ Is Linked With Psychophysical and Cognitive ActivityThere are only a few published studies reporting investigations into how psychophysical parameters or cognitive activity is linked to the PRQ. Marti (2013) observed PRQ changes during cognitive tasks (concentrating alertness on an abstraction vs. listening to a spoken text), and Hildebrandt and Engel (1963) found a statistically significant positive correlation (r = 0.45) between fluctuations in auditory reaction times of humans with fluctuations of the PRQ. Interestingly, the HR and RR fluctuations correlated to a lesser extent than the PRQ fluctuations. The PRQ Is Sex-DependentA further aspect of the PRQ is that it depends on sex. This fact, however, was unfortunately not considered in several PRQ studies published. There are two main studies that showed the sex-dependence of the PRQ:
These findings are not unexpected since there are differences in physiology between both the sexes (Legato et al., 2016; Boese et al., 2017; Kerkhof and Miller, 2018). A sex-dependence was found by many other studies investigating the CRS and CVS in humans. For example, parameters of heart rate variability (HRV) (Sinnreich et al., 1998; Antelmi et al., 2004; Park et al., 2009; Boettger et al., 2010; Nemati et al., 2013; Hernandez et al., 2015), blood pressure (Kuo et al., 1999; Harrap et al., 2000; Ellis et al., 2004), and respiration (Snieder et al., 1997; Becklake and Kauffmann, 1999; Carey et al., 2007) depend on sex. Cardiac activity is more dominantly regulated by the parasympathetic nervous system in women, while for middle-aged men, the regulation is more due to the sympathetic nervous system (Kuo et al., 1999). Also, the exercise vasodilator response is different between the sexes with a greater one in females (Kellawan et al., 2015). The aging of the CVS is also sex-dependent (Kane and Howlett, 2018). These factors are clearly associated with the individual PRQ. The Individual Physiological Constitution Is Linked to the PRQDespite the fact that (age- and sex-adjusted) normal or reference ranges for physiological variables are widely used in human physiology and medicine, it is known that the variable values from a large population follow multimodal distributions (i.e., distributions with several peaks), indicating a subgrouping of the subjects (Zaitseva and Son’kin, 2005). One factor explaining this is the existence of subgroups according to the individual physiological constitution, i.e., the specific inborn integrated physiological (and anatomical) phenotypic traits determined by genetic inheritance and epigenetics.2 While the existence of constitution types is well supported by empirical evidence, the classification (number and defining characteristics) is still a matter of research and debate, with several different models posited to classify the proposed constitutional types (Günther, 1922; Ciocco, 1936; Conrad, 1941; Portmann, 1962, 1969; Kim et al., 2009; Wang, 2012; Pham et al., 2013; Metzger, 2016; Gruber, 2017; Yu et al., 2017). That the physiological constitution partially may contribute to the disposition for disease was a finding of numerous studies (e.g., Feigenbaum and Howat, 1934; Koleva et al., 2002). A specific aspect of the physiological constitution is the prevailing state of chronobiological rhythms of a subject, termed “chronotype” (Bender and Wellbery, 1991). The current, actual, physiological constitution of a human being can be regarded as a combination (and interaction) of the specific inborn constitution type combined with the anatomical and physiological states and changes experienced during that individual’s life until now. A term encompassing both these aspects of intersubject variability is simply the “phenotype.” The phenotype-based classification of human physiological states is an intensive research field in the context of personalized medicine. A research field is generally termed “phenomics” (Houle et al., 2010). That the physiological constitution also plays a role in the PRQ of humans has been shown by several studies:
Medical Aspects of the PRQ: Application for Disease Classification and MonitoringThe PRQ as an Indicator for Health and DiseaseIt has been obvious since PRQ research was first conducted in the 1920s that the PRQ is relevant in assessing the status of human health and disease. Studies showed that a person’s pathophysiological state is indicated by deviations of the PRQ from normal values during static (e.g., resting) or dynamic states (e.g., body posture transitions). Besides this, interpreting PRQ based on its chronobiological variability has been shown to offer new insights into the health and disease states of humans. In particular, the following insights have been gained with regard to the PRQ as an indicator for health and disease:
The PRQ as a Useful Tool for Disease and Treatment MonitoringTesting how the PRQ behaves as a marker for disease and treatment monitoring was an aim of several studies so far, with the following main findings:
Case for Reviving the Use of the PRQ for Basic Research About Human Physiology and for Applications in MedicineBased on the literature reviewed, we believe the PRQ has a great potential for use in future studies about human physiology as well as for applications in medicine. The case for its revival as an informative biological marker can be summarized by these three reasons:
Recommendations for the Use of the PRQ in Studies and ApplicationsBased on the literature reviewed, we make the following recommendations to ensure proper use of the PRQ in studies and for medical applications:
Summary, Conclusions, and OutlookAs we summarized in the previous sections, the PRQ is a powerful, easy-to-measure, easy-to-calculate, and useful parameter that captures basic properties of the complex interaction between the cardiac and respiratory systems, reflecting fundamental properties of human physiology (e.g., chronobiological state, reactivity and adaptability of the CVS and CRS). The PRQ contains information that cannot be obtained when analyzing the HR or the RR separately. The reason for this is that the PRQ reflects emergent properties of the complex interplay between the cardiac and respiratory activities. Even the HRV, which contains also information about the respiration (Brown et al., 1993; Aysin and Aysin, 2006; Lenis et al., 2017), is linked to the PRQ in such a way that the HRV cannot be used as a surrogate for the PRQ. For example, the correlation strength between the PRQ and the HR as well as the ln LF/HF (low frequency/high frequency power ratio) HRV parameter depends on the daytime (higher correlations during day), while the correlation strength between the PRQ and ln LF/HF is generally weak (day: r = 0.31 ± 0.09; night: r = 0.21 ± 0.06, r: Pearson’s correlation coefficient) (Cysarz et al., 2008). One novel information the PRQ contains, which is not available from the HR, RR, and HRV, is, e.g., the already discussed (section “Historical Development, Measurement Methods, Basic Characteristics, and Link to Cardiorespiratory Interactions”) quantization of the PRQ values with preferred values of the harmonic ratios n:m (with n = 3–6 and m = 1) and the tendency of the preferred 4:1 coupling (PRQ = 4). The absolute PRQ values thus have already a meaning due to the emergent dynamical order of cardiorespiratory interactions. In other words, the cardiorespiratory system behaves as a self-organized system with the emergence of order, reflected in the PRQ properties. In our review, there are two factors explaining why the PRQ is not widely applied in current studies about human physiology and why it is normally not used for medical applications at present. First, the concept of the PRQ is not well known in the scientific community (since modern studies only use it rarely). Second, since much of the knowledge accumulated about the PRQ over the last decades was published in German, it is not readily accessible to the English-speaking scientific community. With the present review article, we address both aspects by giving a concise introduction to concept of the PRQ and summaries of the German publications and findings. Due to its ability to capture the individual cardio-respiratory state of a person in a unique integrative way, the PRQ is especially suited well to assess the subject-specific physiological or pathophysiological state in general. Such an assessment is much more being considered as relevant for modern physiological studies, which increasingly also focus on the individual physiology, as well as for a subject-specific medical diagnosis, disease monitoring, and treatment in the context of the paradigm of a “predictive, preventive, personalized, and participatory (P4)” medicine (Hood and Friend, 2011; Hood and Flores, 2012; Sagner et al., 2017). The PRQ also fits neatly into physiological and medical concepts that view human physiology and medicine in a more integrated and systemic way like the approaches of “systems biology” (Kitano, 2002; Somvanshi and Venkatesh, 2013), “integrative human physiology” (Coleman et al., 2011; Mackay et al., 2016), “functional medicine” (Jones and Quinn, 2010; Bland, 2015), “integrative medicine” (Barrett et al., 2003; Mcewen, 2017), and “personalized medicine” (Hamburg and Collins, 2010; Goetz and Schork, 2018). The PRQ might be also a useful parameter to be included in multimodal neuroscientific studies that incorporate the measurement of systemic physiological variability alongside the changes in brain activity. While the measurement of HR and RR alongside detections of brain activity with functional magnetic resonance imaging (fMRI) has been conducted in numerous studies and is considered beneficial for a correct and integrated interpretation of fMRI data (Bulte and Wartolowska, 2017), this approach has only recently also been introduced for optical human neuroimaging studies with functional near-infrared spectroscopy (fNIRS). Our group pioneered this approach termed “systemic physiology augmented functional near-infrared spectroscopy” (SPA-fNIRS) (Metz et al., 2017; Scholkmann et al., 2017) and was the first to measure stimulus-evoked changes of the PRQ with simultaneous assessment of the changes in cerebral tissue oxygenation and blood perfusion (Scholkmann et al., 2017). To the best of our knowledge, we were also the first to investigate the relationship between absolute cerebral tissue oxygenation and rs-PRQ values (Scholkmann et al., 2019). The application of the PRQ in neuroscientific research is only just starting and harbors great potential for future insights into the coupling between neuronal activity and systemic physiological activity, and the cardiorespiratory system in particular. Understanding human physiology and pathophysiology by analyzing the PRQ is a paradigm that should be studied in detail with new investigations and studies. According to our view, the following topics and research questions are of particular relevance for future studies:
Besides these new studies needed in the field of human physiology and medical applications, there is also potential to extend the concept of the PRQ by developing methods that capture specific PRQ states or dynamics automatically. Such extensions have been used already in the previous studies. For example, novel ways of quantifying the PRQ distributions or capturing PRQ frequency locking time intervals have been developed and used already. There might also be the possibility to define pulse-respiration rate variability (PRQV), analog to the HRV; pulse-respiration rate complexity (PRQC), analog to the heart rate complexity (HRC) (Batchinsky et al., 2010; Brindle et al., 2016); or pulse-respiration rate fragmentation (PRQF), analog to the heart rate fragmentation (HRF) (Costa et al., 2017). PRQV, PRQC, and PRQF could then be quantified by the already existing methods developed for HRV, HRC, and HRF analyses. Furthermore, since standard clinical patient monitors already measure the HR and RR, it would make sense to calculate and display the PRQ. This would automatically introduce the PRQ to clinicians, which could help spread the use of the parameter also for new studies and standard clinical monitoring. We encourage manufacturers of clinical patient monitors to update their device software to include the PRQ in the parameters determined and displayed. It is important that manufacturers and reaching hospitals also take measures to raise awareness of the new parameter. Since the PRQ is strongly modulated by chronobiology, using clinical monitors showing the PRQ might also encourage clinicians to take into account the chronobiological variability of physiological variables. As was pointed out recently by Mckenna et al. (2018), “critical care traditionally focuses on the ‘normalisation’ of physiological indices despite a limited evidence base (Holst et al., 2014; Gotts and Matthay, 2016), but preservation of circadian physiology is not part of clinical practice.” According to the authors, it is therefore possible that “neglecting the influence of circadian rhythmicity could contribute to the apparent lack of benefit from the majority of critical care targets tested in randomised controlled trials.” In conclusion, our review article lists and summarizes key aspects of the PRQ and provides a detailed discussion of the PRQ’s role in future research about human physiology and pathophysiology, including in the medical context. We hope that we motivate the readers to use the PRQ in own studies and to participate in the further research about the PRQ in general. The time is ripe for re-discovery of the PRQ with respect to human physiology and medicine. Author ContributionsFS and UW contributed to the conception and design of the work and revised the first draft. FS conducted the literature research, organized the material, drafted the first version of the manuscript, both created the final version. Conflict of Interest StatementThe authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. AcknowledgmentsWe thank Timo Hafner (University of Bern) for performing initial literature research about the PRQ for his medical thesis at our institute. This research was partly supported by grant SAGST PN9715. Footnotes1To the best of our knowledge, the first time the parameter was termed “pulse-respiration quotient” was in the 1970s by the group of Hildebrandt. In the German literature, the parameter was named differently depending on the research group, or researcher, e.g., “Puls-Atem-Quotient” (PAQ), or “Quotient Puls Atem” (QPA or QP/A). Recently, the English terms “heart respiratory rate quotient” and “heart respiration ratio (HRR)” have been also used (Von Bonin et al., 2014). 2Also the term “physical constitution” is used in the literature. In our review paper, we use the term “physiological constitution” as an umbrella term for both physiological and anatomical aspects, without including psychological ones. References
Articles from Frontiers in Physiology are provided here courtesy of Frontiers Media SA What is the ratio of respirations to pulse beats in a healthy adult at rest?In healthy adults at rest, normal values are as follows: Heart rate (pulse): 60-100 bpm. Respiratory rate: 16-20 breaths per minute.
What is the normal ratio of pulse?A normal resting heart rate for adults ranges from 60 to 100 beats per minute. Generally, a lower heart rate at rest implies more efficient heart function and better cardiovascular fitness. For example, a well-trained athlete might have a normal resting heart rate closer to 40 beats per minute.
What is the usual ratio of pulse rate to respiration rate quizlet?What is the usual ratio of pulse rate to respiration rate? A person's pulse rate is ordinarily about four times the respiration rate.
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