Sprint interval training (SIT) increases peak oxygen uptake (VO2peak) but the mechanistic basis is unclear. The Fick principle broadly attributes increases in VO2peak to changes in peak cardiac output (Qpeak) and/or peak arteriovenous oxygen difference (peak a-vO2diff). The main purpose of this thesis was to investigate the role of Qpeak, measured non-invasively using inert gas rebreathing (IGR), on SIT-induced changes in VO2peak. It also considered the time course of these responses and the influence of biological sex. The SIT protocol involved 3 x 20-s “all out” sprints performed within a 10-min session of low-intensity cycling. Study 1 measured Qpeak after 2, 6, and 12 weeks of SIT and found it was increased after 12 weeks and associated with the change in VO2peak. Peak a-vO2diff, estimated based on the Fick equation (peak a-vO2diff = VO2peak/Qpeak), was also increased after SIT and associated with the change in VO2peak. Study 2 found that a novel constant-load protocol elicited Qpeak values that were non-inferior to an established step protocol, within a margin of 0.5 L/min. Both protocols elicited VO2 values at Qpeak that were similar to VO2peak. The constant load protocol had similar day-to-day repeatability as the VO2peak test (typical error = 6.6 and 6.4%, respectively). Study 3 investigated an exploratory finding from Study 1 that suggested Qpeak was increased in male but not female participants. The design was similar, but Study 3 employed suggested best practices for making sex-based comparisons. Contrary to our hypothesis, Qpeak was unchanged after 12 weeks of SIT and there was no sex-based difference. Like Study 1, peak a-vO2diff was increased and correlated with VO2peak. This thesis advances knowledge regarding the influence of SIT on Qpeak determined non-invasively and highlights the need for more mechanistic work to comprehensively assess the basis for the increase in VO2peak. / Thesis / Candidate in Philosophy / Sprint interval training (SIT) is a form of exercise that involves brief bursts of near-maximal to “all out” efforts separated by short recovery periods. The method improves cardiorespiratory fitness — an important health marker that is quantified as the highest amount of oxygen used by the body during strenuous exercise (VO2peak) — but the mechanisms are not well understood. This thesis examined the effect of SIT on peak cardiac output (Qpeak), which is the highest rate of blood pumped by the heart each minute, and the relationship to changes in VO2peak. Qpeak was measured non-invasively by having participants breathe an inert gas mixture. Two separate 12-week training studies confirmed that SIT increased VO2peak but yielded conflicting results regarding the role of Qpeak. The findings also suggest that the capacity of skeletal muscles to extract oxygen is increased after SIT. Biological sex does not appear to influence SIT-induced changes in Qpeak or VO2peak.

No description available.

No description available.

Computer-assisted technology for noninvasive determination of cardiac output (Q̇, Lmin⁻¹) offers potential to re-examine measurement stability with established methods involving CO₂ rebreathing. Reproducibility of Q̇ was examined at rest and during steady-state cycle ergometer exercise (x̄ = 51% of V̇O₂pk; SD = ± 7%) using Defares’ exponential method (MedGraphics™ CPX/D system). Characteristics of the 19 female subjects were: (Mean and SD) age = 34.6 yr and 6.6 and V̇O₂pk= 32.8 mlkg⁻¹min⁻¹ and 9.5. The Q̇ trial consisted of 15 min of seated rest and 15 min of exercise. Values were determined in triplicate at each of minutes, 6, 10, 14. Stability of subjects responses before each rebreathing maneuver was verified by inspection of V̇O₂, Two-way ANOVA’s were performed on the Q̇ data for differences; means (Lmin⁻¹), SD’s, and reliability coefficients were: Rest Q̇-6 min Q̇-10 min Q̇-14 min R Day 1 5.1 ± 0.8 5.0 ± 1.1 4.8 ± 0.9 0.81 Day 2 4.7 ± 0.8 4.6 ± 0.8 4.6 ± 0.8 0.82 Exercise Q̇-6 min Q̇-10 min Q̇-14 min R Day 1 9.14 ± 1.2 9.3 ± 1.4 9.2 ± 1.1 0.93 Day 2 8.6 ± 1.5 8.7 ± 1.4 8.7 ± 1.3 0.97 Day 1 Day 2 R Rest 5.0 ± 0.8 4.7 ± 0.7 0.05 Exercise 9.2 ± 1.2 8.7 ± 1.4 0.84 Although these findings demonstrated within-test and between-test reproducibility of Q̇ to be comparable to that described in the published literature (Kirby TE. J Cardiac Rehabil 5:97-101, 1985) the new semi-automated computerized system (MedGraphics CPX/D) offers a quick, easy, and efficient assessment of cardiac output. / Master of Science

1. An explicit mathematical description of PEP in terms of DBP was proposed, which in the first time quantitatively clarified the ventricular and arterial effects on PEP timing. / 2. A nonlinear pressure-volume relationship which reflected the natural arterial wall properties was introduced into the asymmetric T-tube arterial model, which effectively and quantitatively described the effect of pulsatile BP on arterial parameters, e.g., compliance, PTT etc. / 3. A mathematical relationship between PAT and BP was firstly proposed as a result of the heart-arterial interaction, which simulated a significantly strong and negative relationship between PAT and SBP and between PAT and MBP but a much weaker negative relationship between PAT and DBP during exercise. The hypothesis was supported by the experiment data. To our knowledge, it is the first study describing the quantitative relation of PAT and BP by both model-based study and experimental data. / 4. A novel wearable measurable CO parameter, PTRR, was proposed and it successfully showed a significantly high and positive correlation with CO during exercise both in model simulation and in the experiments. / 5. Linear prediction models using PAT to estimate MBP and using PTRR to estimate CO were proposed and evaluated in two exercise experiments conducted on 84 subjects with different ages and cardiovascular diseases. Results showed the proposed method could achieve the accuracy required for medical diagnosis. / 6. Taken the findings in 3, 4 and 5 together, this study in the first time provided both the theoretical basis and experimental verifications of developing a wearable and direct measurement technique of CPO in dynamic exercise using multiple physiological signals measured on body surface. / Cardiac power output (CPO) is defmed as the product of mean arterial blood pressure (MBP) and cardiac output (CO), and CPO measured during peak dynamic exercise (i.e. peak CPO) has been shown as a powerful predictor of death for heart failure patients. However, so far there has been no existing device which directly measures CPO, and CPO is acquired from simultaneous measurement of MBP and CO. Further, simultaneous MBP and CO measurement during dynamic exercise is a challenge for current BP and CO methods. Therefore, there is an urgent need to develop new devices which are fully wearable and unobtrusive for monitoring of CPO during dynamic exercise. Since the core problem in most wearable devices is how to estimate the target cardiovascular parameter, e.g. CPO in this study, through physiological signals measured from body surface, this thesis focus on developing a direct measurement technique of CPO in dynamic exercise using multiple physiological signals measured on body surface, specifically, electrocardiogram (ECG) and photoplehtysmogram (PPG). / Finally, based on the theoretical and experimental verifications, linear prediction models were proposed to estimate MBP from PAT and estimate CO from PTRR. The results showed that PAT can estimate MBP with a standard deviation of 7.42 mmHg, indicating PAT model has the potential to achieve the accuracy required by AMMI standard (mean error within +/- 5 mmHg and SD less than 8 mmHg). The results also showed that PTRR can estimate CO with a percent error of 22.57%, showing an accuracy which was considered as clinically acceptable (percent error less than 30%). / Heart failure is the end stage of many cardiovascular diseases, such as hypertension, coronary heart disease, diabetes mellitus, etc. Around 5.8 million people in the United States have heart failure and about 670,000 people are diagnosed with it each year. In 2010, heart failure will cost the United States $30.2 billion, and the cost of healthcare services is a major component of this total. With the resultant burden on health care resources it is imperative that heart failure patients with different risk stages are identified, ideally with objective indicators of cardiac dysfunction, in order that appropriate and effective treatment can be instituted. / In order to verify the theoretical findings, two experiments were carried out. One was incremental supine bicycle exercise conducted on 19 young healthy subjects and the other was incremental to maximum supine bicycle exercise conducted on 65 subjects, including heart failure patients, cardiovascular patients and healthy elderly. PAT showed significantly high and negative correlation with SBP and MBP, but lower correlation with DBP. PTRR showed significantly high and positive correlation with CO. / In this thesis, a model based study is conducted to address the above problem. Firstly, we deduced the mathematical expression of PEP as a function of DBP by introducing the arbitrary heart rate into the exponential mathematical description of a pressure-source model. Secondly, an asymmetric T-tube model was modified by introducing a nonlinear pressure-volume relationship where PTT was expressed as a dependent variant of BP. Thirdly, we proposed the mathematical equation between PAT and BP by coupling the modified ventricular and arterial models. Then, the relationships between PAT with systolic blood pressure (SBP), MBP and DBP were simulated under changing heart contractility, preload, heart rate, peripheral resistance, arterial stiffness and a mimic exercise condition. The simulation results indicated significantly high and negative correlations between PAT and SBP and between PAT and MBP whereas the correlation between DBP and PAT was low. / Next, we developed a novel CO index, namely pulse time reflection ratio (PTRR), expressed in terms of MBP and mean aortic reflection coefficient (Gamma(0)), from the modified asymmetric T-tube model. PTRR was further expressed in terms of PAT and inflection point area (IPA), a surrogate of Gamma(0) from the shape feature of PPG. The simulation results suggested significantly and positive relationship between PTRR and CO and between IPA and Gamma(0) during dynamic exercise. / Recently, a wearable measurable parameter, pulse arrival time or PAT, has been developed for BP measurement. PAT is the time delay from the R peak of ECG to the systolic foot of PPG. PAT consists of two timing components, the pre-ejection period (PEP) of the heart and pulse transit time (PTT). PTT is related to BP by an arterial elastic model and thus can be used to estimate beat-to-beat BP. However, PTT is difficult to be measured through a wearable device, and thus PAT is usually used as a surrogate of PTT for BP estimation, under the assumption of a constant PEP. However, PEP is not a constant but changing with physiological conditions, which may alter the PAT-BP relationship. Thus, it is important to clarify the PAT-BP relationship and address the feasibility of MBP estimation using PAT during dynamic exercise. / To summarize, the original contributions of this thesis are: / Wang, Ling. / Adviser: Y.T. Zhang. / Source: Dissertation Abstracts International, Volume: 73-03, Section: B, page: . / Thesis (Ph.D.)--Chinese University of Hong Kong, 2010. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [201-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese.

The purpose of this study was to explore the efficacy of two existing pulse contour analysis (PCA) models for estimating cardiac stroke volume from the arterial pressure waveform during kicking ergometer exercise and head-up tilt manoeuvres. Secondly, one of the existing models was modified in an attempt to enhance its performance. In part I, seven healthy young adults repeated two submaximal exercise sessions on a kicking ergometer, each with three different sets of steady-state cardiac output comparisons (pulsed Doppler vs. pulse contour). Across all exercise trials regression results were found to be PCA = 1. 23 x Doppler-1. 38 with an r2 = 0. 51. In part II, eight young and eight older male healthy subjects participated in a head-up tilt experiment. Cardiac output comparisons were again performed during the supine and tilt conditions using pulsed Doppler and pulse contour cardiac output. Regression results revealed that PCA performed best during supine conditions and preferentially on the older subjects. In all instances, impedance-calibrated pulse contour analysis will provide reasonable beat-by-beat cardiac output within very narrow confines and will result in a progressively more significant bias as cardiovascular dynamics change. In addition, it appears that heart rate variability negatively influences beat-by-beat pulse contour cardiac output results, further limiting application of existing models.

O envelhecimento promove o aparecimento de doenças crônicodegenerativas, entre elas a osteoartrose. Artroplastia Total de Quadril (ATQ) tem sido procedimento comumente frequentemente com uso de cimento ósseo metilmetacrilato (MTC). Por outro lado, importantes alterações hemodinâmicas são descritas durante o procedimento de cimentação. A raquianestesia é um procedimento anestésico que produz menores efeitos hemodinâmicos e melhor analgesia pós-operatória em comparação a anestesia geral para a ATQ. Entretanto, especificamente neste procedimento cirúrgico tais alterações permanecem a ser investigadas. Buscamos avaliar os efeitos hemodinâmicos através da técnica de cardioimpedância em pacientes idosos submetidos à cirurgia de ATQ com o uso de MTC e raquianestesia. Utilizamos um estudo transversal, observacional, comparativo, inter-metodológico e com intervenção ativa. Método: Doze pacientes de ambos os sexos, > 65 anos, com indicação ortopédica para ATQ foram estudados. Os sujeitos receberam avaliação pré-anestésica e realizaram exames laboratoriais préoperatórios. Foram submetidos à cirurgia de ATQ com utilização de raquianestesia e implante de cimento de MTC. Os pacientes foram monitorados hemodinamicamente por meio da cardiografia de impedância, da saturação de oxigênio e pressão arterial, no intra-operatório, no momento acordado, durante a indução anestésica, na colocação do MTC e após este procedimento. Após a expansão volêmica observamos que houve aumento do DC quando comparado a raquianestesia, cimentação, inserção, redução da prótese e ao final da cirurgia (P<0,05). Tais resultados foram de certa forma, atenuados na presença de efedrina. No entanto, o VS sofreu alterações nos eventos supracitados na presença ou não da droga. Por outro lado, a FC, o IC e a RVS sofreu poucas alterações durante os eventos estudados. O DC, IC e o VS se correlacionaram com a massa corporal (r = 0,81, 0,60 e 0,67 respectivamente), o VS se correlacionou negativamente com a idade (r = -0.67), o IC se correlacionou negativamente com a classe funcional pela NYHA (r = -0,61) e o DC se correlacionou positivamente com a quantidade de efedrina administrada (r = 0,70). Conclusão: O procedimento cirúrgico de ATQ com MTC sob raquianestesia produz alterações hemodinâmicas e a cardioimpedância foi um instrumento útil para guiar os procedimentos e conduta do intraoperatório, configurando-se como uma técnica interessante para monitorização não invasiva. / Aging promotes the onset of chronic diseases, including osteoarthritis. Hip arthroplasty procedure has been commonly often the use of methylmethacrylate bone cement (MMA) has been used in the surgical procedure. On the other hand, significant hemodynamic changes are described in the cementation procedure. Spinal anesthesia is an anesthetic that produces less hemodynamic effects and better postoperative analgesia compared to general anesthesia for surgery arthroplasty. However, this particular surgical procedure (hip arthroplasty) such changes remain to be investigated. To evaluate the hemodynamic effects through cardioimpedance technique in elderly patients undergoing surgery for hip arthroplasty with the use of MMA for prosthetic hip and spinal. Twelve patients of both sexes, > 65 years, with orthopedic indication for hip arthroplasty were studied. The subjects received preanesthetic evaluation and underwent preoperative laboratory. From then underwent surgery for hip arthroplasty with use of spinal anesthesia and MMA cement. Patients were haemodynamically monitored by impedance cardiography, oxygen saturation and blood pressure intraoperative, when awake, during anesthesia induction, installing the MMA cement and after this procedure. After volume expansion, we observed that there was increase in CO when compared to spinal anesthesia, cementing, insertion, and reduction of the prosthesis at surgery (P<0.05). These results were somewhat attenuated in the presence of ephedrine. However, the SV underwent changes at aforementioned event on the presence or not of drugs. On the other hand, HR, CI and SVR has a few changed during the studied event. The CO, CI and the SV was correlated with body mass (r = 0.81, 0.60 and 0.67 respectively), SV was negatively correlated with age (r = -0.67), the CI was negatively correlated with NYHA functional class (r = -0.61) and CO was positively correlated with the amount of ephedrine administered (r = 0.70).Conclusion: The surgical procedure for hip arthroplasty under spinal anesthesia with MMA produces haemodynamic changes and cardioimpedance was a useful tool to guide the procedures and conduct of the intraoperative considering as interesting technique for noninvasive monitoring.

A hipotensão trans-anestésica é frequente em equinos submetidos a anestesia geral inalatória e pode desencadear graves complicações pós-anestésicas. Consequentemente, o tratamento adequado deve ser instituído rapidamente, entretanto, há poucos estudos na espécie que auxiliem na escolha do fármaco simpatomimético quando o agente inalatório empregado é o isofluorano. Dessa forma, o presente estudo teve como objetivo comparar os efeitos cardiovasculares da efedrina, com os da dobutamina. O experimento foi dividido em duas fases: na Fase I foram utilizados 13 cavalos monitorados com cateter em artéria pulmonar o que possibilitou avaliação hemodinâmica, além da obtenção de sangue venoso misto. Ademais, foi realizada a mensuração do lactato sérico e troponina I e de parâmetros de ventilação e oxigenação, sendo que os animais não foram submetidos a procedimento cirúrgico. Na Fase II foram utilizados 22 equinos da rotina do serviço de cirurgia de grandes animais, submetidos a avaliação cardiovascular (frequência cardíaca e pressão arterial sistêmica), parâmetros de ventilação e oxigenação, avaliação de eletrólitos e lactato sérico. Em ambas as fases os animais foram randomizados entre o grupo dobutamina e grupo efedrina, e o tratamento teve início após detecção de hipotensão arterial (PAM <60 mmHg). O grupo dobutamina recebeu taxa de infusão inicial de 1 &micro;g/kg/min, aumentada conforme necessário até atingir a PAM igual a 70mmHg ou taxa máxima de 5 &micro;g/kg/min. Já o grupo efedrina foi tratado com infusão contínua na dose de 20 &micro;g/kg/min até atingir PAM de 60mmHg, depois reduzida para 10 &micro;g/kg/min até obter PAM igual 70mmHg e posteriormente mantida em 5 &micro;g/kg/min. Após determinação da normalidade, os dados foram submetidos à ANOVA com duas variáveis independentes e pós teste de Bonferroni. Na Fase I os dois grupos resgataram a pressão arterial em tempos semelhantes, diferindo entre si quanto a frequência cardíaca, que foi superior no grupo efedrina (p=0,0098), porém com a ocorrência de arritimias apenas no grupo dobutamina; e quanto a pressão de oclusão da artéria pulmonar, a qual se elevou apenas no grupo dobutamina (p&lt;0,0001). Os dois protocolos experimentais elevaram de forma significativa as pressões arteriais, o débito cardíaco (p=0,0012), índice cardíaco (p=0,0013), resistência vascular sistêmica (p=0,008), índice de resistência vascular sistêmica (p=0,0001), enquanto o índice sistólico elevou-se significativamente apenas no grupo dobutamina (p=0,003). Dentre os índices de oxigenação e gases sanguíneos TEO2 (p=0,0008), IDO2 (p&lt;0,0001), SvO2 (p= 0,0005) e SpO2 (p=0,006) diferiram entre tempos nos dois grupos, sem diferir entre grupos. Apenas no grupo efedrina verificou-se redução da C(a-v)O2 (p=0,02), porém sem diferir entre grupos. O lactato, utilizado como indicador de perfusão tecidual não diferiu entre tempos, nem entre grupos, assim como a Troponina I, biomarcador de lesão miocárdica. O K reduziu significativamente em ambos os grupos nessa fase, enquanto o cálcio reduziu apenas no grupo efedrina. Na fase II verificou-se que a pressão arterial se elevou em tempos semelhantes em ambos os grupos, porém não foram observadas alterações na FC. Parâmetros de ventilação e oxigenação também não sofreram alterações, tampouco diferiram entre os grupos, com exceção da PaCO2 que se elevou significativamente (p=0,006) e a concentração sérica de Ca que diminuiu (p=0,01) apenas no grupo efedrina. Conclui-se que ambos os fármacos são efetivos para o tratamento de hipotensão em equinos anestesiados com isofluorano. / Trans-anesthetic hypotension is frequent in horses submitted to general inhalation anesthesia and may lead to severe post-anesthetic complications. Consequently, adequate treatment should be instituted rapidly, however, there are few studies in the species that assist in the choice of sympathomimetic drug when the inhalant agent employed is isoflurane. Thus, the present study aimed to compare the cardiovascular effects of ephedrine with those of dobutamine. The experiment was divided into two phases: in Phase I, 13 horses were monitored with a pulmonary artery catheter, which allowed a hemodynamic evaluation, in addition to obtaining venous mixed blood. In addition, measurement of serum lactate, troponin I and ventilation and oxygenation parameters were performed, the animals were not submitted to a surgical procedure. In Phase II, 22 horses from large animals surgery routine were submitted to cardiovascular evaluation (heart rate and systemic arterial pressure), ventilation and oxygenation parameters, electrolyte evaluation and serum lactate. In both phases the animals were randomized in two groups, the treatment started after detection of hypotension (MAP &lt;60 mmHg). The dobutamine group received an initial infusion rate of 1 &micro;g/kg/min, increased as needed until the MAP was equal to 70 mmHg or a maximum rate of 5 &micro;g/kg/min. On the other hand, the ephedrine group was treated with continuous infusion at a dose of 20 g/kg/min until it achieved a PAM of 60 mmHg, then reduced to 10 &micro;g/kg/min until the PAM was 70 mmHg and subsequently maintained at 5 &micro;g/kg/min. After determination of normality, the data were submitted to ANOVA with two independent variables and post Bonferroni test. In Phase I, the two groups recovered blood pressure at similar times, differing in heart rate, which was higher in the ephedrine group (p = 0.0098), but with the occurrence of arrhythmias only in the dobutamine group; and the pulmonary artery occlusion pressure, which only increased in the dobutamine group (p &lt;0.0001). The two experimental protocols significantly elevated arterial pressures, cardiac output (p = 0.0012), cardiac index (p = 0.0013), systemic vascular resistance (p = 0.008), systemic vascular resistance index (p = 0.0001), whereas the systolic index increased significantly only in the dobutamine group (p = 0.003). Among the rates of oxygenation and blood gases TEO2 (p = 0.0008), IDO2 (p &lt;0.0001), SvO2 (p = 0.0005) and SpO2 (p = 0.006) differed between times in the two groups, without differing between groups. Only in the ephedrine group C(a-v)O2 decrease signficantly (p = 0.02), but did not differ between groups. Lactate, used as an indicator of tissue perfusion did not differ between times, niether between groups, as did Troponin I, a biomarker of myocardial injury. Potassium significantly reduced in both groups in this phase, while calcium reduced only in the ephedrine group. In phase II, it was found that blood pressure increased at similar times in both groups, but no changes were observed in HR. Ventilation and oxygenation parameters also did not change, neither differ between groups, with the exception of PaCO2 that increased significantly (p = 0.006) and the Ca concentration that decreased (p = 0.01) only in the ephedrine group. It is concluded that both drugs are effective for the treatment of hypotension in horses anesthetized with isoflurane.

by Yu Chung Wah. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1998. / Includes bibliographical references (leaves 85-102). / Abstract also in Chinese. / Chapter Section 1 --- Literature review / Chapter 1.1 --- Physiology of anemia in the prematurity --- p.1 / Chapter 1.11 --- Erythropoiesis and Erythropoietin --- p.2 / Chapter 1.12 --- Postnatal changes in oxygen transport and delivery --- p.3-6 / Chapter 1.2 --- The concept of oxygen supply and demand --- p.6-7 / Chapter 1.3 --- Compensatory response and adverse effects of anemia --- p.8 -9 / Chapter 1.4 --- Treatments of anemia --- p.9 / Chapter 1.41 --- Red blood cell transfusion --- p.9-13 / Chapter 1.42 --- Recombinant human erythropoietin (rHuEpo) --- p.13 -14 / Chapter 1.5 --- Methods of cardiac output measurements --- p.15 / Chapter 1.51 --- Determination of cardiac output by invasive method --- p.15 -19 / Chapter 1.52 --- Determination of cardiac output by non-invasive methods --- p.20 -30 / Chapter 1.6 --- Methods of oxygen consumption measurements --- p.31 -37 / Chapter 1.7 --- Haemodynamic effects of red blood cell transfusion in preterm infants . --- p.38 -39 / Chapter Section 2 --- Introduction --- p.40 -42 / Chapter Section 3 --- Methodology --- p.43-47 / Chapter Section 4 --- Results --- p.48-51 / Chapter Section 5 --- Discussion --- p.52 -56 / Chapter Section 6 --- Conclusion --- p.57 / Chapter Section 7 --- Future Direction --- p.58 / Chapter Section 7 --- Tables and Figures --- p.59-84 / Chapter Section 8 --- References --- p.85 -102

Slow and deep breathing (SDB) has long been regarded as a nonpharmacological method for dealing with several physiological and emotional imbalances, and widely used for relaxation purposes. There is, however, limited understanding of the putative mechanisms by which SDB acutely impacts the cardiovascular and autonomic systems to elicit chronic adaptations. The present thesis explored how the manipulation of breathing pattern and intrathoracic pressure during SDB could further the understanding of the regulatory mechanisms that underpin the acute cardiovascular response to SDB. This thesis makes an original contribution to the existing knowledge by reporting a previously undescribed inversion of normal within-breath (inspiration vs. expiration) left ventricular stroke volume (LVSV) pattern for breathing frequencies < 8 breaths∙min-1. This finding might reflect the influence of a lag between enhanced right atrial filling and right ventricular stroke volume during inspiration, and its expression in left ventricular stroke volume; this lag results from the time required for blood to transit the pulmonary circulation. Furthermore, blood pressure variability (BPV) was reduced significantly at the lowest breathing frequencies, likely due to the involvement of baroreflex mediated responses. The pattern of responses was consistent with the buffering of respiratory-driven fluctuations in left ventricular cardiac output (Q̇) and arterial blood pressure (ABP) by within breath fluctuations in heart rate (fc), i.e., respiratory sinus arrhythmia (RSA) (Chapter 4). Chapter 5 demonstrated that magnifying negative intrathoracic pressure with inspiratory loading during SDB increased inspiratory pressure-driven fluctuations in LVSV and fc, and enhanced Q̇, independently of changes in VT and fR. The data support an important contribution to the amplification of RSA, during SDB, of previously underappreciated reflex, and/or 'myogenic', cardiac response mechanisms. The findings in Chapter 6 confirmed that inspiratory loading during SDB amplified the effects observed with un-loaded SDB (reported in chapter 5). In contrast, expiratory loading increased ABP and attenuated RSA, LVSV and Q̇ during SDB. A lower RSA for higher ABP, supports the presence of a formerly underappreciated contribution of sinoatrial node stretch to RSA, and throws into question the clinical benefits of expiratory resisted SDB, particularly in hypertensive populations. In conclusion, the findings of the present thesis provide novel information regarding the mechanisms contributing to acute cardiovascular response to SDB. These new insights may contribute to the development of more effective SDB interventions, geared towards maximising the perturbation to the cardiovascular control systems.