# CPET

Ref: DOI: 10.1002/cphy.c110048

Indications: 
- Mechanisms of residual DOE: https://www.atsjournals.org/doi/full/10.1513/AnnalsATS.202004-398FR

![alt](https://photos.collectednotes.com/photos/5187/8607dc22-d92d-4356-be22-a4f23e56ba8c)
from
https://journal.chestnet.org/article/S0012-3692(20)31415-X/pdf

2 ways to calc vo2 that correspond to the 2 systems of bulk flow / convective O2 transport: the respiratory system (measuring directly via douglas bag) and the CV system (measure via blood gasses and Fick Equation). 

VO2 = VE * (PIO2 - PEO2) = Qt * (CaO2 - CvO2)

CPET measures both

- Grading severity of heart failure (<14L = risk of dying of HF > transplant)
- Lung resection stratification

## Adequate

- HR 80%+
- RER > 1.15
- BR < 15%
- Borg > 9
- HCO3 drop of ____


##RER (or equiv at cellular level = RQ)
 
RER = amount expelled by lungs (measured at mouth or nose). VCO2 / VO2
RQ = metabolism at the cellular level. Proportion of CO2 volume generated to the volume of consumption. 

RER estimates RQ. 

Over 1.0 implies anaerobic metabolism

## Oxygenation
Normally, oxygenation is roughly maintained during exercise. However, A-a Gradient (A-aDO2) increases due to decreased V/Q matching and development of diffusion limitation of oxygen. 

![alt](https://photos.collectednotes.com/photos/5187/c04b089a-785b-4a2c-8fa6-508649d21ea0)

V/Q: matching - 
![alt](https://photos.collectednotes.com/photos/5187/2ee92559-9087-479c-a6fe-a1f4514232b9)

The combination of these effects leads to very fit athletes achieving EIAH (Excercise-induced arterial hypoxemia). However, this is near universal at very high altitudes

Why do we have an A-a gradient at all (seems like there would be a lot of selection pressure to maximize inefficiency)
=> perhaps selection is on exercise physiology, which would be when it matters. 

#### Exertional Hypoxemia in COPD

Dynamic hyperinflation: May have dynamic hyperinflation -> increased deadspace (higher pressures in diseased lung -> less capillary blood flow). -> corrects easily with O2. Also, would push more Q to bases and possibly increase shunting. 

Also, if ventilation limitation is present, VO2 might increase more than DO2 => decreased SVo2 

#### Exertional Hypoxemia in ILD / pHTN

Blood flow through lung units is faster => less time for diffusion to occur => may not reach equilibrium

[ ] time course of PO2 in the capillary slide

(The above also explains hypoxemia with altitude, which can occur in normals - Normal people become diffusion limited at high enough)

Pulmonary Vascular pattern = same? Probably reduced effective area for which oxygen diffuses through. 


##Ventilatory Thresholds

Determining ET

VCO2 - VO2 plot - spot where the slope increases above 1:1 = AT. 

Other ways to check
- VE/VO2 - time plot, where that number starts to go up
- PET O2 starts to increase

Isocapnic buffering - VCO2 increases

## Ventilatory Indices

VE / VCO2 - VD/VT should decrease with exercise -> more efficient ventilation (04 -> 0.3). If this doesn't occur (meaning the lungs can't become more efficient), the VD/VT remains high and the VE/VCO2 is high (high in pulmonary vascular disease, ILD, and CHF)

VE / VO2 is less important.

##Cardiac Indices

- Q increases 5-6L for each 1L of O2 consumption

O2 pulse = VO2 / HR = SV * C(a-v)O2
- relatively conserved between AT and max exercise. 
- treated as a non-invasive surrogate of stroke volume (assuming you don't have an extraction of O2 problem)


Change in VO2 / for change in WR - roughly 10 in everyone. Decreases in problems with O2 delivery or utilization (most often cardiac disease).  Obesity doesn't effect this relationship.

## Patterns 

1 approach - 

1. Is study maximal effort? (VO2 plateau, ratings of exertion, RER 1.15+, HR > 80%)
2. Is VO2 peak normal or abnormal? (85% or higher)
3. Is VO2 at anaerobic threshold normal? (should be ~40%. Identify on VCO2 to VO2 panel at inflection point, or VECO2 panel)
4. Is CV response to exercise normal? (Peak HR vs predicted HR, Peak O2 pulse VO2/HR, EKG without signs of ischemia)
5. Is ventilatory response to exercise normal (calculate MVV vs Peak Ve - look at UCLA panel 1)
6. Is gas exchange normal? (SpO2 drop, VE/VCO2 = efficiency normal?, pattern of EtCO2 (should rise slightly)

![alt](https://photos.collectednotes.com/photos/5187/65d70d99-2e34-492f-9502-cecd019b13f8)
from: 
https://journal.chestnet.org/article/S0012-3692(20)31415-X/pdf