12.1 Cardiac Rehabilitation Phases & Metabolic Equivalents

Overview of Cardiac Rehabilitation

Cardiac rehabilitation is a comprehensive, evidence-based secondary prevention intervention designed to optimize physical, psychological, and social functioning in individuals recovering from acute cardiovascular events or cardiac surgery. Key candidates include patients post-myocardial infarction (MI), coronary artery bypass graft (CABG) surgery, percutaneous coronary intervention (PCI), stable angina, heart valve repair/replacement, heart transplantation, or stable chronic heart failure ($LVEF \le 35%$). The physiatrist leads the multidisciplinary team in risk stratification, exercise prescription, and monitoring for cardiac complications.

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Phases of Cardiac Rehabilitation

Cardiac rehabilitation is structured into four sequential phases, progressing from acute inpatient recovery to lifelong independent maintenance.

Phase I: Inpatient Acute / Early Recovery Phase

• Setting & Duration: Initiated in the inpatient acute care setting post-event or surgery, lasting 2 to 5 days for uncomplicated cases.
• Core Goals: Prevent bed-rest complications (e.g., muscle deconditioning, orthostatic hypotension, deep vein thrombosis, atelectasis), facilitate early ambulation, provide risk-factor education, and prepare for discharge.
• Hemodynamic Limits & Target Heart Rate:
  • Post-MI Target Heart Rate: Resting HR + 20 bpm (upper limit $\le 120\text{ bpm}$).
  • Post-CABG Target Heart Rate: Resting HR + 30 bpm (upper limit $\le 120\text{ bpm}$).
  • Blood Pressure Limits: Resting SBP $< 200\text{ mmHg}$ and DBP $< 110\text{ mmHg}$.
• Permissible Intensity: Restricted to low-level exertion (1.0 to 3.0 METs), including bedside hygiene, sitting in a chair, active range-of-motion, and short hallway ambulation.
• Sternotomy Precautions: For post-CABG or open cardiac surgery via median sternotomy, strict sternal precautions are maintained for 6 to 8 weeks to ensure bone healing and prevent sternal dehiscence:
  1. Restrict lifting, carrying, pushing, or pulling to 5 to 10 lbs.
  2. Avoid bilateral upper extremity elevation above 90 degrees.
  3. Avoid pushing up from a chair using upper arms during transfers.
  4. Hug a sternal chest pillow tightly against the sternum during coughing or transfers.

Phase II: Outpatient ECG-Monitored Aerobic Conditioning

• Setting & Duration: Initiated 1 to 3 weeks post-discharge in an outpatient center, spanning 12 to 36 sessions (3 sessions/week over 4 to 12 weeks).
• Telemetry ECG Monitoring: Employs continuous or intermittent telemetry ECG monitoring to detect exercise-induced ST-segment shifts, angina, or complex arrhythmias.
• Exercise Prescription (FITT Principle):
  • Frequency: 3 to 5 days per week.
  • Intensity: 40% to 80% of Heart Rate Reserve (HRR) or $VO_2\text{ peak}$, or target Borg RPE 12 to 14 ('somewhat hard').
  • Time: 20 to 60 minutes of continuous or interval aerobic exercise per session (with 5–10 min warm-up/cool-down).
  • Type: Aerobic activities (treadmill walking, stationary cycling, arm ergometry, elliptical training).
• Progressive Resistance Training: Low-weight resistance training (1–5 lbs, 10–15 repetitions at RPE 11–13) can be introduced after 2 to 4 weeks of stable aerobic training in Phase II (or 5–8 weeks post-CABG with a stable sternum).
• Risk Stratification:
  • Low Risk: LVEF $> 50%$, no exercise-induced angina or ST depression, no complex arrhythmias.
  • Moderate Risk: LVEF $40%–49%$, mild-to-moderate angina or ST depression ($> 1\text{ mm}$) at moderate workloads ($> 5–7\text{ METs}$).
  • High Risk: LVEF $< 40%$, severe exertional ischemia, post-infarct angina, or complex ventricular arrhythmias at low workloads ($< 5\text{ METs}$). High-risk patients require continuous ECG telemetry for 18 to 36 sessions.

Phase III: Outpatient Unmonitored Maintenance

• Setting & Duration: Outpatient facility setting lasting 6 to 12 months.
• Features: ECG telemetry is discontinued (unmonitored phase). Patients exercise independently with periodic supervision, focusing on maintaining aerobic capacity and solidifying permanent health behavior changes.

Phase IV: Long-Term Community Maintenance & Wellness

• Setting & Duration: Lifelong, self-directed program in community fitness facilities (e.g., YMCA) or home environments.
• Features: Completely unmonitored physical activity, focusing on long-term adherence to aerobic exercise, Mediterranean/DASH diet, smoking cessation, and stress management.

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Metabolic Equivalents (METs) in Rehabilitation

A Metabolic Equivalent (MET) represents resting basal oxygen consumption ($VO_2$), defined as 3.5 mL O2/kg/min in a resting adult.

Clinical Application & Activity Spectrum

• Baseline Living Requirement: A capacity of 3.0 to 4.0 METs is required for basic independent community living and personal self-care.
• Work Capacity Clearance: Sedentary desk work requires 1.5 to 2.0 METs, whereas heavy manual labor requires 7.0 to 8.0+ METs.

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Borg Rating of Perceived Exertion (RPE)

The Borg Rating of Perceived Exertion (RPE) scale is a validated numerical tool (6 to 20) constructed to correlate linearly with heart rate in healthy adults ($RPE \times 10 \approx \text{Heart Rate in bpm}$).

Clinical Utility in Patients on Beta-Blockers

Patients taking beta-adrenergic blockers (e.g., metoprolol, carvedilol) or those with chronotropic incompetence exhibit a blunted heart rate response during exercise, rendering age-predicted target heart rate formulas inaccurate. In these patients, the Borg RPE scale serves as the gold standard for prescribing exercise intensity.

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Exercise Precautions & Termination Criteria

Absolute Contraindications to Exercise Rehabilitation

1. Acute myocardial infarction within 2 days.
2. Unstable angina pectoris.
3. Uncontrolled cardiac arrhythmias causing symptoms or hemodynamic instability.
4. Symptomatic severe aortic stenosis.
5. Uncontrolled symptomatic heart failure.
6. Acute pulmonary embolism, pulmonary infarction, or deep vein thrombosis.
7. Acute myocarditis, pericarditis, or aortic dissection.

Criteria for Immediate Exercise Termination

1. Drop in SBP $> 10\text{ mmHg}$ below baseline with increasing workload accompanied by ischemic symptoms.
2. Moderate to severe angina pectoris (Grade 3 or 4).
3. Central nervous system symptoms (ataxia, dizziness, presyncope).
4. Sustained ventricular tachycardia or ST elevation $> 1.0\text{ mm}$ in non-Q wave leads.