Physiology II
Cardiovascular Case
Respiratory Case A

The Patient: a 76-year-old woman

Principal Complaint: dyspnea

History:  The patient reported difficulty in breathing especially when she was walking, even if for a very short distance. She had also complained of muscle weakness to her physician several times over the last year, but the physician had ascribed this complaint to the combination of obesity and a respiratory disorder that he thought was emphysema.

Clinical Examination:  The patient’s heart rate was 86 beats/min, her arterial blood pressure was 128/72 mm Hg, and her respiratory frequency was 24 breaths/min. A chest X-ray film showed that both hemidiaphragms were greatly elevated (contracting poorly) and diaphragmatic movements during ventilation were minimal. Her serum bicarbonate concentration was 38 mEq/liter, her pH was 7.37, her arterial PO2 was 60 mm Hg, and her arterial PCO2 was 69 mm Hg. She was asked to hyperventilate for several minutes to see if her arterial PCO2 could be decreased. She tried hard to do this by using her accessory muscles of respiration to their fullest extent, but the PaCO2 was decreased by only 25% to 51 mm Hg.

Pulmonary function tests gave the following results: FEV1.0, 1240 ml (48% of predicted); FVC, 1400 ml (40% of predicted); FEV1.0/FVC, 0.89; FRC, 1850 ml (58% of predicted); RV, 1100 ml (65% of predicted); TLC, 2460 ml (50% of predicted); MVV, 52 l/min (56% of predicted). Maximum static inspiratory and expiratory pressures were greatly reduced (25% and 35% of normal, respectively). Neurological examination and electromyograms revealed diffuse muscle fasciculation, general muscle weakness and some atrophy, but no sensory or cerebellar changes. A clinical diagnosis of amyotrophic lateral sclerosis was made.

FIO2

PAO2

PaO2

PaCO2

A-a O2
difference

FIO2
helpful?
21% 63 60 69 3 No

STUDY QUESTIONS:

1. Based on the information provided in the REFERENCE CASE above, what is the most likely cause of this woman’s arterial hypoxemia and hypercapnia? Support your answer with data from the physical examination and the pulmonary function tests.  Her FEV1 and FVC are both very low.  Her ALS, a neuromuscular disease,  has decreased her respiratory drive.  This would be considered a restrictive disorder.  The low A-a difference shows that there is no diffusion or shunting problem.  It is just that the lady can't pull the air in or get it out very well.

2. Is the arterial hydrogen ion concentration of this patient normal or abnormal? Why or why not? Support your answer with calculations based on data provided in the REFERENCE CASE.  Though her PCO2 was elevated, her body has compensated renally.  Her pH is 7.37 so it is still within normal limits though near the low end so her H+ ion concentration must be considered normal.

3. What is the rationale behind asking this patient to hyperventilate voluntarily?  The hope was to decrease her PCO2 level by increasing her frequency.  

Last Updated 04/10/00 12:27:10 PM
Return To The MNA 2001 Homepage