How do you interpret an ambulatory blood pressure (ABP) record?

Interpreting Ambulatory Blood Pressure Records

Interpret ambulatory BP records by calculating mean daytime, nighttime, and 24-hour values, then comparing these to established thresholds: daytime <135/85 mmHg, nighttime <120/70 mmHg, and 24-hour <130/80 mmHg are normal, with values exceeding 140/90,125/75, and 135/85 mmHg respectively indicating hypertension. 1

Data Quality Requirements

Before interpretation, verify data adequacy:

• Minimum 70% of readings must be usable for valid interpretation 1
• Daytime requires >14 systolic and diastolic measurements 1
• Nighttime requires >7 measurements (though emerging data suggests ≥4 sleep readings may suffice) 1
• Review raw data for outliers and physiologically impossible values (e.g., diastolic equals systolic) before calculating means 1
• Delete only grossly incorrect readings; if sufficient measurements exist, extensive editing is unnecessary 1

Defining Day and Night Periods

Two acceptable methods exist for separating daytime from nighttime 1:

1. Patient diary method: Use actual bedtime and wake time recorded by the patient 1
2. Fixed-time method: Define daytime as 0900-2100 hours and nighttime as 0100-0600 hours, eliminating variable retiring (2101-0059) and rising (0601-0859) periods 1

The fixed-time method reduces variability between patients and across cultures 1.

Normal Values and Diagnostic Thresholds

Most Recent Guidelines (2024 ESC)

The 2024 ESC guidelines provide the most current thresholds 1:

• Daytime: Normal <135/85 mmHg; Hypertension ≥135/85 mmHg
• Nighttime: Normal <120/70 mmHg; Hypertension ≥120/70 mmHg
• 24-hour: Normal <130/80 mmHg; Hypertension ≥130/80 mmHg
• Elevated BP: Daytime 120/70 to <135/85 mmHg; Nighttime 110/60 to <120/70 mmHg; 24-hour 115/65 to <130/80 mmHg

Earlier Guidelines (British Hypertension Society, 2000)

These slightly more conservative values remain widely referenced 1:

• Daytime: Normal <135/85 mmHg; Abnormal >140/90 mmHg
• Nighttime: Normal <120/70 mmHg; Abnormal >125/75 mmHg
• 24-hour: Normal <130/80 mmHg; Abnormal >135/85 mmHg

Use the 2024 ESC thresholds for current practice, as they represent the most recent consensus 1.

Key Clinical Patterns to Identify

White Coat Hypertension

• Occurs in 15-30% of the general population, particularly common in elderly and pregnant women 1
• Diagnosed when office BP ≥140/90 mmHg but daytime ambulatory BP <135/85 mmHg 1
• Carries lower cardiovascular risk than sustained hypertension but may be a precursor to true hypertension 1
• Should prompt consideration before initiating drug therapy, as it can prevent unnecessary treatment 1

Nocturnal Dipping Pattern

• Normal "dippers" show ≥10% BP reduction from day to night 2
• "Non-dippers" fail to achieve this 10% reduction and have significantly increased cardiovascular risk 2
• Absence of nocturnal dipping associates with target organ damage and may suggest secondary hypertension 1
• Nighttime BP may be the strongest independent predictor of cardiovascular risk, even exceeding daytime values 1, 2

Masked Hypertension

• Office BP appears normal (<140/90 mmHg) but ambulatory BP is elevated 1
• Requires ambulatory monitoring for detection as office measurements miss this high-risk phenotype 1

Resistant Hypertension

• Defined as office BP >150/90 mmHg despite three or more drugs 1
• Ambulatory monitoring distinguishes true resistance from white coat effect 1
• Non-dipping pattern may indicate secondary hypertension requiring further workup 1

Statistical Presentation

Essential statistics to report 1:

• Mean daytime systolic and diastolic BP with heart rate
• Mean nighttime systolic and diastolic BP with heart rate
• Mean 24-hour systolic and diastolic BP with heart rate
• Graphical plot with BP on vertical axis, time on horizontal axis, ideally with normal reference ranges displayed 1

Special Populations

Elderly Patients

• Office BP averages 20 mmHg higher than daytime ambulatory BP in elderly with isolated systolic hypertension 1
• Ambulatory monitoring prevents overdiagnosis and excessive treatment 1
• Multiple hypotensive patterns occur: postural, postprandial, drug-induced, and autonomic failure 1
• Identifying hypotension is critical as elderly are particularly susceptible to adverse drug effects 1

Pregnancy

• White coat hypertension occurs in nearly 30% of pregnant women 1
• Ambulatory BP correlates better with proteinuria than office measurements 1
• Better predictor of complications and infant birth weight than conventional measurement 1
• Can prevent unnecessary hospital admissions and cesarean sections 1

Hypotension

• Useful for identifying symptomatic hypotensive episodes in young patients 1
• Detects drug-induced BP drops that may harm patients with coronary or cerebrovascular disease 1

Guiding Treatment Decisions

Ambulatory monitoring provides superior assessment of treatment response compared to office measurements 1:

• Determines drug efficacy without white coat effect 1
• Identifies excessive drug effects and associated symptoms 1
• Demonstrates duration of drug action over 24 hours 1
• Studies show significantly less antihypertensive medication prescribed when treatment decisions based on ambulatory rather than office BP 1

Common Pitfalls

• Ignoring data quality: Always verify ≥70% usable readings before interpretation 1
• Failing to review raw data: Outliers may indicate arrhythmias, poor technique, or device malfunction 1
• Using only 24-hour averages: Daytime and nighttime values provide distinct prognostic information 1, 2
• Overlooking non-dipping: This pattern carries independent cardiovascular risk 1, 2
• Not considering patient diary: Activities, medications, and sleep times assist interpretation 1
• Applying office BP thresholds: Ambulatory values are consistently 5-10 mmHg lower than office readings 1