Clinical Testing Insights: Threshold Identification

Recap: Aerobic to Anaerobic Metabolism

• Lawrence will cover this in more detail in following lectures.

• Question: What measure of expired air is an indicator of the transition from aerobic to anaerobic metabolism?

• Question: What has increased to initiate a change in ventilation/breathing rate?

• Question: What detects the change?

Cardiorespiratory Responses to Aerobic Exercise

• Coupling of muscle pulmonary to cellular activity.

• Gaseous exchange at the airway is a consequence of Q and pulmonary blood flow.

• Peripheral O2 extraction coupled with ventilation (QO2 = O_2 utilisation).

• Heart, with circulation, couples gas exchanges of muscle respiration with the lungs.

• Physiological Responses:

  • \uparrow QCO_2

  • \uparrow SV

  • Recruit

  • \downarrow VT

  • 100% Recruit

  • \uparrow HR

  • Dilate

  • \uparrow f

  • \uparrow VCO_2

• Homeostatic control in action:

  • Chemoreceptors detect changes in and (amongst other things!).

  • Baroreceptors detect changes in __.

CPET: Cardio-Pulmonary Exercise Testing

• Non-invasive.

• Simultaneous/integrated assessment of CV and pulmonary (or ventilatory) function under stress of exercise.

• Can be submaximal or maximal depending on the setting and the patient.

• Incremental exercise to exhaustion or symptom-limited.

• Focus on gaseous exchange variables.

• Usually completed with ECG, HR, BP, RPE and SpO_2.

CPET Protocols

• Which mode? Treadmill or Cycle?

• Step: Incremental increase in external work. Standardised protocols available (Bruce, Astrand etc.).

• Ramp: Constant and continuous increase in external work.

  • Work (ramp rate) can be individualised for patient’s capabilities.

  • Linear increase in VO_2, such that the range of metabolic capacity is distributed evenly over an appropriate duration.

Clinical vs Sport Science CPET

• Sport science VO_{2max} testing is basically CPET but they don’t pay attention to all the ventilatory markers/thresholds as they will often directly measure blood lactate.

• Clinical environments.

Clinical Exercise Physiologists (CEPs)

• CEPs specialise in exercise testing and assessment, alongside the design, delivery and evaluation of evidence-based exercise interventions.

• CEP scope of practice encompasses apparently healthy individuals to those with chronic and complex conditions, along the care pathway from primary prevention, through acute management, to rehabilitation and maintenance.

• Interventions are exercise or physical activity-based and also include health and physical activity education, advice and support for lifestyle modification and behaviour change.

• CEPs work in a range of primary, secondary and tertiary care settings as part of a multidisciplinary team of health care and rehabilitation providers and in community settings.

• Undertake and record a thorough, appropriate and detailed assessment of health status and history to guide exercise risk stratification using evidence-based methods.

• A newly HCPC profession with opportunities.

Why is CPET used?

• Differential diagnosis: CPET can provide answers to why someone is breathless/unwell in a way that other functional tests cannot

• Disability evaluation: CPET can objectively evaluate how unwell someone is, using CPET as an alternative marker of function

• Intervention assessment: CPET can be used to assess efficacy of therapeutic interventions (training, drugs, surgery)

• Rehabilitation prescription: CPET can be used to personalise exercise training/rehabilitation programmes at an appropriate intensity

• Medical intervention suitability: CPET can be used to determine if someone is fit for surgery

• Pre-operative risk: CPET can objectively risk assess cardiopulmonary reserve during surgery

CPET Utility in CVD

• Key CPET variables hold powerful diagnostic and prognostic utility in patients with CVD (Chaudry et al., 2018).

• CPET also holds considerable promise in gauging response to a broad range of therapies, including pharmacologic, surgical and lifestyle interventions (Chaudry et al., 2018).

• Improving CPET response (VO_2 peak) may evolve into a primary treatment goal in CVD patients (Chaudry et al., 2018).

Information Gained from CPET Data

• CPET data can be used to identify if a patient is limited due to:

  • Circulatory.

  • Ventilatory.

  • Coronary disease.

  • Low fitness.

  • Poor effort.

The 9-Panel Plots

• Oxygen Uptake (VO_2).

• Carbon Dioxide Production (VCO_2).

• Ventilation (VE).

• Heart Rate.

• Fuel Usage (RER).

• Oxygen Saturation (SaO_2).

Peak Oxygen Uptake (VO_2 peak)

• In clinical populations, they may not reach VO2 max as symptom-limited, therefore typically refer to as VO2 peak.

• Used to help determine the %VO_2 peak achieved at the point of the clinical thresholds.

• Status Cut-off.

  • Normal: ≥ 85% of predicted VO_2 peak.

  • Abnormal: < 85% of predicted VO_2 peak.

Anaerobic Threshold

• When VCO2 increases excessively in relation to VO2.

• The deflection point between VO2 and VCO2 via the V-slope method.

• The point above which further increases in intensity/work rate are increasingly sustained through anaerobic metabolism.

• The physiological dividing line between moderate and heavy intensity exercise.

• Status Cut-off.

  • Normal: ≥ 40% of predicted VO_2 peak.

  • Abnormal: < 40% of predicted VO_2 peak.

Ventilatory Threshold (VT1 and VT2)

• Deflection between minute ventilation (Ve) and intensity.

• VT1 = CO_2 begins to accumulate alongside increases in H^+ (lower pH).

• VT2 = CO2 rapidly increases so more hyperventilation to remove excess CO2 (and H^+).

  • Associated with Lactate Threshold.

  • Also referred to as the Respiratory Compensation Point.

  • Low pH stimulates carotid bodies to increase Ve.

Ventilation (Ve)

• Minute ventilation (VE) adapts to energy needs, meeting demands for VO2 and CO2 elimination (VCO_2).

• Links with the venti.

• Increases in respiratory rate (fB) and tidal volume (VT) occur, such that VE may increase ~25-fold with exercise.

• Inability to increase VE due to lung disease may limit exercise capacity.

Ventilatory Equivalents

• Ve/VO2 – At the transition point (VT or AT), Ve increases exceeds the rate of rise in VO2 so Ve/VO2 increases.

• Ve/VCO2Ve increases in proportion to increases in VCO2 but remains constant or falls slightly.

• Gives a measure of instantaneous ventilatory and gas exchange efficiency: How many L does the patient have to breath in to uptake 1L oxygen or to produce 1L carbon dioxide?

Ventilatory Efficiency Slope (Ve/VCO2)

• Status Cut-off.

  • Normal: ≤ 34

  • Abnormal: >34 = POOR PROGNOSIS

• Mismatch of ventilation in/out of the lungs WITH perfusion of O_2 in the heart/tissues = physiological dead space.

• Potential issues with peripheral chemoreceptor sensitivity.

Respiratory Function

• How is it measured?

• What would suggest restrictive lung disease?

• What would suggest obstructive lung disease?

• Potential clinical skills PDO and/or year 3 health module for more.

• We will briefly cover in the practical.